How to Protect Your Kidney Function

Give your kidneys a bit of a break by using the tips below:

• Drink lots of water to flush out wastes. Drinking water also helps lower the chances of kidney stones and infections.
• Keep your blood pressure in the target range. Weight control, exercise, and drugs can control blood pressure—and prevent or slow the risk of kidney failure. Also, blood pressure drugs in the ACE-inhibitor and ARB class help slow down a loss of protein in the urine and protect the kidneys. Blood pressure puts a lot of stress on your kidneys. If you have high blood pressure and any other kidney problem, treating your blood pressure will help protect your kidneys.
• If you have diabetes, keep your blood sugar in the target range. Weight control, exercise, medication, and tight control of blood sugar can prevent or slow the risk of kidney failure.
• Have blockages (e.g., narrowed arteries) treated. Sometimes blockages can be opened to help save function in a blocked kidney. If you think you have a blockage, ask your doctor what can be done about it.
• Take steps to prevent kidney stones if you are prone to them.
• Take steps to prevent infections if you are prone to them.
• Check with your doctor to see if there is a special diet you should be on, such as low protein, low salt (sodium), and low phosphorus.

Avoid Known Kidney Toxins

• Limit use of over-the-counter or prescription painkillers that contain ibuprofen (Advil®, Motrin®), naproxen (Aleve®), or acetaminophen (Tylenol®). These non-steroidal anti-inflammatory drugs (NSAIDS) cause blood vessels in the kidneys to shrink, so less blood flow comes through. If you take any of these drugs often, be sure to tell your doctor. Taking these drugs with caffeine can also further harm the kidneys. Drinking a large glass of water when you take an NSAID can help your kidneys flush the drug out.
• Know your own drug allergies. Sometimes taking a drug that you are allergic to can damage the kidneys.
• Ask about a drug’s effects on the kidneys any time you take a new medication. Some antibiotics and chemotherapy drugs are known to be hard on the kidneys. If you know that your kidney function is less than normal, avoid these drugs if you can and see if your doctor can prescribe something else.
• Avoid X-ray dye tests or have your doctor take steps to protect your kidneys. Less toxic dye can be used (this costs more), the dye can be diluted, and it can be flushed out of your body with extra fluid. Some doctors prescribe a drug called Mucomyst ® to help protect the kidneys from the dye.
• Get a throat culture if you have a sore throat—and treatment if it is caused by Strep bacteria.
• Avoid use of street drugs. If you use them, know that they can harm your health and seek help to stop. Be honest with your doctor about what you are using so he or she can help treat you.
• Quit smoking. In people with CKD, research has linked smoking to an increase in the amount of protein in the urine. In smokers with diabetes, kidney disease may progress twice as fast . Get help to quit smoking and prolong your kidney function.

Other steps you take do to keep your kidneys working well for as long as possible include:

• Visit your doctor for check ups.
• Take all drugs as prescribed—in the right amount, at the right time.
• Tell your doctor about any herbs, supplements, or over-the-counter drugs you take. Just because some products are sold without a doctor’s prescription does not mean they are safe for people with less than normal kidney function.
• Follow any dietary limits as prescribed.
• Know your blood or urine test names and what the results mean.

Do Your Part

If you are at risk for kidney disease, there are many things you can do to help keep your kidneys working. With knowledge of the signs and symptoms of CKD and early treatment you can protect your kidneys and live long and well with CKD.

Are You At Risk?

• Calculate Your Kidney Function
• Take the CKD Quiz
• 10 Reasons to Get Tested for CKD

About CKD

• Kidney Diagnosis Guide
• Symptoms of CKD
• CKD - A Public Health Crisis
• Kidney Disease and Your Heart: The Hidden Link
• Other Resources on CKD
• Kidney Disease Glossary

What You Can Do

• How To Protect Your Kidneys
• How to Talk to Your Doctor

Checking Your Kidney Function

Since kidneys clean the blood and make urine, blood and urine tests are a good way to check whether the kidneys are doing their job. If your blood or urine has high levels of substances the kidneys should filter out, something may be wrong. Knowing the names of your tests and what the results mean is a way for you to track your kidney function and see how you are doing over time. The two most common kidney function tests are creatinine (blood) and albumin (urine).

Chronic Kidney Disease

CKD means the kidneys are at least 40% less able to filter out wastes and water from the blood, and the damage is permanent. In time, CKD may lead to kidney failure, in which case dialysis (blood cleaning) or a kidney transplant is needed to support life.

The two most common causes of CKD are type 2 diabetes and high blood pressure. Among Americans with kidney failure, 75% have one or both of these problems. The rest have genetic diseases, infections, birth defects, kidney stones, or other causes.

If CKD is caught early, it may be possible to slow it down or even stop it. The National Kidney Foundation lists five stages of CKD. Each stage is based on the percent that the kidneys can filter, or “glomerular filtration rate” (GFR). GFR is not a blood test; it’s a formula used to figure out how well your kidneys are filtering your blood.

The stages of CKD are:

• Stage 1 – Kidney damage (protein in the urine) and normal GFR (>90)
• Stage 2 – Kidney damage and mild drop in GFR (60-89)
• Stage 3 – Moderate drop in GFR (30-59)
• Stage 4 – Severe drop in GFR (15-29)
• Stage 5 – Kidney failure: dialysis or kidney transplant needed (GFR <15)

how to protect your kidneys

If you have type 2 diabetes, high blood pressure, or other risk factors, you are at a higher risk of having kidney problems in the future. The good news is, if you’re at risk for kidney disease you can protect your kidneys.

• Chronic Kidney Disease
• Checking Your Kidney Function
• Symptoms of CKD
• How to Protect Your Kidney Function

The list of advice includes for protecting kidney

• strict control of any tendency towards hypertension. The blood pressure should be kept well under the usual guideline of 140 (systolic) over 90 (diastolic).
• Use of a type of prescription medication for hypertension that's called an "ACE-inhibitor" There's one ACE-inhibitor that's been approved by the US FDA(1) for use to protect the kidneys in people with diabetes: its brand name is Capoten (the generic name is captopril). Probably all the ACE-inhibitors work about the same to protect the kidneys, and should be as good as captopril.
• Use of a modified meal plan, with a fairly small amount of protein. A Registered Dietitian should be consulted to develop a workable meal plan for people with the dual disorders of diabetes and kidney disease.
• Avoidance of certain medications that aggrevate kidney disorders. The list to avoid includes some over-the-counter pain medications such as ibuprofen (Motrin and others) and Alleve, but does not include aspirin and Tylenol (acetominophen). Certain prescription drugs that are used for the treatment of arthritis and pain, called "non-steroidal anti-inflammatory drugs" (NSAID's) are all to be avoided.
• Avoidance of dehydration. If there's vomiting or severe diarrhea, go to an Emergency Room or other facility, and get "tanked up" with intravenous fluids. You'll feel much better, and avoid any risk of kidney shutdown from dehydration.
• Avoidance of X-rays that involve the injection into your blood vessels of liquids called "radioopaque contrast agents" (a frequent slang term for these agents is "X-ray dyes") unless there's a nephrologist (kidney specialist) assisting with the case. These contrast agents are eliminated from the body through the kidneys, and can clog up the kidneys if there's already some kidney damage.
• Prompt treatment of any problems with urination, such as painful urination or blood in the urine.
• Cessation of smoking.
• Control of blood sugar level (see comments about the results of the DCCT).
• Referral to a nephrologist (kidney specialist) for additional advice when the urine protein level exceeds 1000 mg (the normal amount should be below 150) or the creatinine level in the blood is over 3.0 (creatinine normally should be below 1.5), or upon the request of the patient.

This is now a standard list of recommendations that should increase the time between the initial diagnosis of diabetic kidney damage, and the inevitable dialysis, transplant, or death.

How To Protect Yourself Against Liver Damage

Medical researchers who co-ordinated this study had 106 participants take 4 grams of acetaminophen (the equivalent of eight 500 mg tablets of extra-strength Tylenol) every day for two weeks. Thirty-nine participants received placebo pills.

The key results were as follows:

• Among the particpants who took acetaminophen, almost 40 percent showed signs of acute liver damage
• Those who showed signs of acute liver damage continued to show signs of liver stress four days after they stopped taking acetaminophen; it took a full eleven days for their liver enzymes to return to normal levels

Why is your liver so susceptible to damage by acetaminophen? Because almost everything that enters your mouth, travels down your digestive tract, and gets absorbed into your blood stream must travel directly to and through your liver before it travels through the rest of your circulatory system.

Your liver acts as a processing plant. It receives everything that you put into your mouth and that ends up in your blood stream, does its best to sort out useful nutrients and harmful substances, and then packages these nutrients and harmful substances to be delivered to your cells and eliminated from your body, respectively.

When certain drugs like those that contain acetaminophen reach your liver for processing, they can cause direct injury to your liver cells, sparking an inflammatory reaction that leads to increased production of liver enzymes, which a standard blood test can detect. If your liver cells are injured repeatedly, they can go through a series of degenerative changes, the last ones being cirrhosis (hardening) and cancer of the liver.

Medical drugs that list liver damage as a potential "side" effect and all forms of alcohol are the two groups of substances that can most efficiently and predictably cause liver damage in the fashion described above.

Protecting yourself against liver damage must begin with avoidance of said medical drugs (whenever possible and under the guidance of your doctor) and alcohol. It's true that not everyone who takes medical drugs and drinks alcohol regularly over many years ends up with irreversible liver damage. Genetics and lifestyle changes can help to ward off liver damage despite a history of alcohol and drug use. Still, you should be aware that any amount of these two substances can and do heap unnecessary stress onto your liver cells.

Beyond doing your best to avoid drugs and alcohol, here are some additional steps that you can take to protect yourself against liver damage:

1. Stay away from foods that are high in unhealthy fats and/or sugar. Donuts, deep fried fast food, and soda are three of the worst foods for your liver. And yes, chicken McNuggets and McChicken sandwiches are deep fried.
2. Eat mainly fresh, minimally processed foods. Focus on green vegetables and produce that comes in rich colors like field tomatoes, carrots, avocado, mango, blueberries, and blackberries. These richly colored fruits and vegetables contain naturally occurring antioxidants that can help to protect your liver cells against physical injury.
3. Use extreme caution and common sense when it comes to being physically intimate with another person. Hepatitis B and C are viruses that can cause significant liver damage. These viruses live in all body fluids, including blood and seminal fluids.
4. Be aware that hepatitis C is most commonly spread via blood, and can be acquired through contaminated needles used for IV drug injections, body piercing, and tattooing.
5. Strive to stay away from chemicals of all types, particularly pesticide and insecticide sprays, which can inflict direct damage to your liver cells once they make their way into your blood stream. Many chemicals are capable of entering your blood stream through your skin, so try to use on your skin only those products that you feel comfortable eating.

One final note: if you rely on regular intake of acetaminophen to deal with intolerable pain, I encourage you to try taking a high quality cod liver oil or fish oil for their omega-3 fatty acids, which can be extremely effective at decreasing inflammation.

Do children need sunglasses?

Yes. Children are at special risk from the harmful effects of UV, since their eyes do not have the same ability as adults to protect from UV radiation.

Here are some helpful suggestions for choosing sunglasses for children:

• Check to make sure the sunglasses fit well and are not damaged,
  
  
• Choose sunglasses that fit your child's lifestyle-the lenses should be impact resistant and should not pop out of the frames,
  
  
• Choose lenses that are large enough to shield the eyes from most angles, Find a wide-brimmed hat for your child to wear along with the sunglasses.

You should choose sunglasses that

• reduce glare
  
  
• filter out 99-100% of UV rays
  
  
• protect your eyes
  
  
• are comfortable to wear
  
  
• do not distort colors.

Protect Your Eyes from the Sun!

Sunglasses help you in two important ways. They filter light and they protect your eyes from damaging ultra-violet (UV) rays. Mounting evidence shows that exposure to UV rays can damage your eyes. Long-term exposure to UV rays can lead to cataracts, macular degeneration, or skin cancer around the eyelids. Sunglasses should be worn when you are outdoors so you can protect your eyes.

1.4.3 Motion Illusions and Gestalt

In [Creative computing I, vol. 1, section 7.2], several principles of
Gestalt psychology were introduced. The Gestalt school of
psychology was largely initiated by experiments performed by
Wertheimer14, researching into the perception of motion. He and his 14Max Wertheimer (1880–1943),
Czech psychologist. His paper
Experimentelle Studien uber das
Sehen von Bewegung (1912),
Zeitschrift fur Psychologie 61,
161–265 is credited with launching
the Gestalt revolution.
colleagues used very simple stimuli, arrangements of dots switched
on and off in particular patterns, to elicit perceptual responses from
25
CC227 Creative Computing II Perception and Information Retrieval
subjects. Two effects in particular were described by Wertheimer:
beta motion and the phi phenomenon.
Beta Motion
Beta motion is believed to be the perceptual illusion responsible for
converting a succession of still images (as projected in a cinema, for
example) into the perception of motion.
Learning activity
Type the following Processing code into a sketch, and run it. What do you observe?
boolean left;
void setup() {
left = false;
smooth(); frameRate(3); size(200,200);
}
void draw() {
background(0); left = !left;
if(left) {
ellipse(50,100,30,30);
} else {
ellipse(150,100,30,30);
}
}
Alter the sketch so that different stimuli are used instead of the circles: for example,
squares, text or an image. Does this affect your perception of the scene? What
happens if you use different colours?
If one of the stimuli is made smaller than the other, some people report that the motion
is backwards and forwards in addition to sidetoside.
Implement this, and see what
your perception is.
Comments on the activity
A typical response to this stimulus is along the lines of: “the circle moves from the left
side to the right and back again”
Phi Phenomenon
A separate peceptual illusion of motion was described by
Wertheimer, who called it the phi phenomenon or pure motion:
distinguishable from beta motion by the fact that it was objectless
motion: the motion perceived in the phi phenomenon is not that of
any of the stimuli, unlike beta motion. It is this observation which
was truly novel and serendipitous in 1912, and which kickstarted
the Gestalt movement in psychology.
Learning activity
Type the following Processing code into a sketch, and run it. What do you observe?
26
Motion
int NSTIMULI = 6;
int pos;
void setup() {
pos = 0;
smooth(); frameRate(15); size(200,200);
ellipseMode(CENTER);
}
void draw() {
background(0);
int i;
for (i = 0; i < NSTIMULI; i++) {
float phase = 2*PI*i / NSTIMULI;
if (!(i == pos)) {
ellipse(100+50*cos(phase), 100+50*sin(phase), 30, 30);
}
}
pos = (pos + 1) % NSTIMULI;
}
Alter the sketch so that different stimuli are used instead of circles: text, images or
different shapes. Does this affect your perception of the scene?
Observe the sketch with different numbers of stimuli: is the effect always clear? How
does the number of stimuli affect the Processing frame rate necessary to see the
effect?
Comments on the activity
Typical response: “something with the same colour as the background is moving
round, obscuring the dots”.
The original phi phenomenon was described in terms of just two dots; however, many
people find the phenomenon much clearer with more stimuli.
Many textbooks are unclear over the distinction between beta motion and the phi
phenomenon; beta motion is the perceived motion of an object, elicited by successive
showing of two related stimuli in different spatial locations, while the phi phenomenon
is perceived motion of an unseen object with the visual properties of the scene’s
background, elicited by implied occlusion of stimuli at a relatively high frequency.
More information about the phi phenomenon can be found at the Magniphi
website15. 15http:
//www2.psych.purdue.edu/
Magniphi/index.html. See
also the paper referenced there:
Steinman, R. M., Pizlo, Z. and Pizlo,
F. J. (2000) Phi is not beta, and why
Wertheimer’s discovery launched
the Gestalt revolution: a minireview.
Vision Research, 40, 2257–2264.
Figure 1.10: Illustration of the Gestalt principles of proximity (left)
and closure (right): in each case, grouping into a structure or inference
of a shape occurs without there being a direct stimulus for that
inference.
There is no single authoritative list of Gestalt principles of
27
CC227 Creative Computing II Perception and Information Retrieval
perception; the theory of Gestalt perception has been in existence
for about a century, and many distinct effects have acquired the
label of ‘Gestalt’. However, a conservative list of principles would
probably include the effects of proximity, closure, similarity,
continuity and common fate.
The common thread underlying these perceptual principles is the
grouping or inference of an entity without having a direct associated
stimulus: in figure 1.10, the grouping into units by proximity and
the inference of a triangle’s presence over three circles is performed
by the perceptual system without any direct evidence. It is exactly
this effect – the inference of an object or grouping without any
direct stimulus – that is present in the phi phenomenon’s objectless
motion.
In this course and others we have presented Gestalt perceptual
phenomena using visual examples. However, we will find in the next
chapter, on the subject of Sound, Hearing and Music, that many of
the same principles carry over to the auditory domain.
Exercises
1. Create an interactive, 3D Processing sketch to illustrate
schematically the structures in the eye: information that you
should impart to the viewer should include:
two distinct shapes to represent rod and cone cells;
three different pigments for the cone cells;
the spatial distribution of rods and cones over the retina;
the presence of the blind spot.
2. Convert the following XYZ colours into the CIE LAB colour
space, and compute the distances between each pair. Which
colours are closest together, and which are further apart?
Comment on your answer.
XYZ sRGB
{38.76,42.87,67.20} (140,180,210)
{40.05,43.39,73.97} (140,180,220)
{40.86,47.06,67.90} (140,190,210)
{40.53,43.78,67.29} (150,180,210)
3. (a) Write a Processing function implementing the
transformation given in equations (1.15) and (1.16), to
convert sRGB values between 0 and 255 into XYZ
tristimulus values.
(b) Write a Processing function transforming XYZ tristimulus
values to xyY chromaticity coordinates.
(c) Using these two functions, display the colours available to
you in Processing on a chromaticity diagram. You may wish
to include some animated or interactive component to your
sketch, to allow the viewer of your sketch better to
understand the chromaticity available to them on an sRGB
display.
4. Because of the nonlinear
transfer function in sRGB, a na¨ıve
approach to image scaling does not work (see for example
http://www.4p8.com/eric.brasseur/gamma.html for
discussion of this point). Implement a Processing sketch which
28
Motion
correctly scales down images of even dimension by a factor of 2,
by converting each pixel’s colour into CIE XYZ space, averaging
blocks of four pixels, and converting back into sRGB. Compare
the results of your scaling on some test images with the results
from commercial or free image manipulation programmes.
5. Design Processing sketches illustrating the Gestalt principles of
proximity, closure, similarity, continuity and common fate. In
each case, illustrate in a manner of your choosing the inferred
structure that does not correspond to a direct stimulus.
6. Select a website with a large number of users, and examine the
use of colour from the perspective of accessibility: are the
colours used sufficiently distinct even for those with anomalous
vision? Are there any mitigating factors, such as alternate
stylesheets, which could improve the accessibility of such a
website?

1.4.2 Persistence of Vision

Persistence of vision, mentioned in [Creative computing I, vol. 1,
section 8.3], can mean one of two things. The strict meaning relates
to the fact that the response to a visual stimulus can persist after that
stimulus has disappeared; a simple example of this can be seen by
moving a luminous object (such as a torch or sparkler) in the dark:
the light leaves an apparent ‘trail’ everywhere that the stimulus has
been observed in the previous second or so. This occurs because, as
discussed in section 1.1.1, the photoreceptors in the eye have a high
response time; the chemical reactions involved in detection of light
are not instantaneous, but happen over a period of time.
The second meaning of “persistence of vision” is the overall
processing by the visual system – eye and brain – which allows the
perception of motion from a series of still images shown in rapid
succession. The persistence of visual response to a momentary
stimulus is only one small part of the overall effect of inducing
motion where none is present; some other perceptual effects playing
their part in the perception of motion, and in particular beta motion,
are introduced in the next section.
Persistence of vision has implications for the design of projection
apparatus, for example for use in cinemas. There are two rates of
importance in such an apparatus. The first is the frame rate: the rate
at which different images are displayed on the screen; for smooth
motion to be perceived, the frame rate should be above about 16
hertz (16 frames per second – modern film runs at 24 frames per
second); frame rates lower than this do not give a convincing
illusion of smooth motion when rapid movements are being
conveyed.
The second rate is the flicker rate: the visual system can perceive a
regular flicker (between frames) as a distraction even at relatively
high frequencies. The flicker rate is at least as high as the frame
rate, as there must be one interruption between frames; however,
modern projectors are designed to add additional interruptions, to
increase the flicker rate (and so decrease its detectability). Typically,
the projector shutter is made doublebladed,
so that one of the
blades obscures the light beam when the projected image is being
updated, and the other at the halfway point; for a frame rate of
24Hz, this would give a flicker rate of 48Hz; a triplebladed
apparatus would give a flicker rate of 72Hz.

1.3.5 Colour Profiles

Now that we have seen a representation of the entire colour gamut
perceivable by the eye, it is straightforward to understand the
statement that it is not possible to reproduce all colours perceivable
by the eye using three primaries of red, green and blue light; the
visible gamut does not form a triangle, and so no additive mixture
of three real primary colours can possibly span the gamut.
Nevertheless, digital displays do use three real primary colours. In
order to ensure that these displays reproduce the intended colours
from image files, a standard colour profile, sRGB, was developed by
HewlettPackard
and Microsoft.
Converting from CIE XYZ tristimulus values to sRGB involves two
transformations. The first is an axis transformation, to convert from
the CIE XYZ primaries into specific red, green and blue primaries;
we can express this transformation as a matrix multiplication:


Rl
Gl
Bl


= 

3.2410 −1.5374 −0.4986
−0.9692 1.8760 0.0416
0.0556 −0.2040 1.0570 



X
Y
Z 

(1.13)
where the tristimulus values are scaled so that Y = 1.0 corresponds
to the maximum brightness of the display hardware.
The second transformation adjusts (with gamma correction) for the
nonlinear nature of brightness on computer monitors and similar
digital displays; in Cathode Ray Tube monitors, for example, the
physical processes involved in the emission of electrons and the
excitation of the phosphors to produce the image on the screen give
a perceived brightness that is not linearly related to the input signal,
but related by an approximate powerlaw
instead. Liquid Crystal
Displays have a very complicated relationship between input voltage
and perceived brightness; however, they incorporate hardware to
22
Colour Spaces and Profiles
mimic the powerlaw
behaviour of a CRT. If C refers to each of R, G
and B in turn,
CsRGB = � 12.92Cl Cl < 0.00304
1.055C1/2.4
l − 0.055 otherwise
(1.14)
The CsRGB values are clamped to be between 0 and 1, and then
scaled to whatever colour resolution is required; for 8bit
colour
channels, such as are found in Processing, Cascading Style Sheet
colour specifications, and PNG images (among other uses), the
CsRGB values are multiplied by 255.
Apart from the clamping of values to between 0 and 1, the
transformation from CIE XYZ to sRGB is reversible; to convert from
sRGB coordinates to XYZ tristimulus values, first invert the transfer
function to yield linear values Cl with
Cl = � CsRGB
12.92 CsRGB < 0.04045
�CsRGB+0.055
1.055 �2.4
otherwise
(1.15)
and then invert the matrix multiplication:


X
Y
Z 

= 

0.4124 0.3576 0.1805
0.2126 0.7152 0.0722
0.0193 0.1192 0.9505 



Rl
Gl
Bl


(1.16)
y
x 0 0.5
0
0.5
Figure 1.9: A representation of sRGB (solid triangle) and Adobe RGB
(dashed triangle) colour space gamuts, relative to the CIE standard
observer chromaticity gamut of figure 1.7.
As well as sRGB, there are other standardized colour spaces,
intended for example for professional printing workflows. One such
is Adobe RGB, which has a larger gamut of colours than sRGB
(compare the dashed triangle in figure 1.9 to the solid triangle);
however, all image manipulation software and printing hardware
must be aware of the choice of colour space, otherwise colour
manipulation operations will not do what is intended.
23
CC227 Creative Computing II Perception and Information Retrieval
1.4 Motion
The perception of motion by the visual system is exploited in the
construction of video and animation artifacts, with the illusion of
motion generated from the presentation of a succession of static
images. This section describes some of the perceptual effects behind
the illusion of motion. Regarding the algorithmic detection of
motion, note that not everything is fully understood, to the extent
that it is difficult to characterise computationally the particular
motion that will be perceived from a digital video stream. We will
return to this when we discuss Multimedia Information Retrieval.
1.4.1 Philosophy of Motion
Even the concept of motion has certain philosophical difficulties, as
it is bound up in the continuity of self over time: to perceive motion,
an observer must understand that an object observed twice, once at
a later time than the other, is the same underlying object. Zeno’s
paradoxes12 illustrate those difficulties, which even if they have 12Zeno of Elea (490 BC? – 430 BC?),
Greek philosopher. mathematical resolutions might remain troubling from a
philosophical perspective. Zeno is said to have devised his paradoxes
in support of the philosophy of his teacher Parmenides, saying
amongst other things that change and motion are only illusions.
Two related paradoxes devised by Zeno – the Dichotomy paradox,
and the paradox of Achilles and the Tortoise – essentially illustrate
that an infinite process needs to be achieved before any motion can
be deemed to have taken place.
The dichotomy paradox begins by considering motion from a point
A to a different point B. In order to perform that motion, a point C
halfway between A and B needs to be visited. But to move from A
to C, a point D halfway between them must be reached; iterating
this argument leads to the conclusion that motion between any two
points requires an infinite number of steps.
The paradox of Achilles and the Tortoise is somewhat similar; the
scenario involves fleetfooted
Achilles racing a tortoise: to make the
race fair, the tortoise is given a head start. Now, by the time that
Achilles reaches the tortoise’s initial position, the tortoise will have
moved along a certain distance. For Achilles to overtake the tortoise,
he must additionally cover that distance, by which time the tortoise
will have moved forward again – and each time Achilles covers the
remaining distance, the tortoise inches forward, with the conclusion
that Achilles needs to reach the tortoise’s current position an infinite
number of times before he can overtake13. 13Achilles and the Tortoise are used
as characters in the dialogues in
Godel, Escher, Bach: an Eternal
Golden Braid by Douglas
Hofstadter, a book exploring the
relationships between creativity,
mind and computation.
A third paradox due to Zeno explores a slightly different aspect of
the difficulty of motion: the Arrow paradox considers the
relationship of motion and time. The scenario involves an arrow in
flight towards a target; consider isolating an instant of time in that
flight. In that snapshot, the arrow is not in motion – but then how
can a succession of instants lead to the arrow progressing in its
path? The resolution of this apparent paradox lies in a careful
mathematical treatment of infinitesmal quantities; however, the
24
Motion
paradox is also related to an illusion allowing the visual perception
of motion to arise from a succession of still images, discussed in the
next section.

1.3.4 Systematization of colour

Figure 1.6: A Maxwell triangle representing the colour space of red,
green and blue primaries. The secondary colours (cyan, magenta
and yellow) are on the edges of the triangle, half way between the
primaries (at points C, M and J respectively); white (‘equalenergy
white’) is at point E. The dashed line is a mixture line, and indicates
the positions of the colours in this diagram producable by mixing red
and cyan light together; P is a pink mixture of red and cyan.
Given three specific additive primary stimuli, we can represent the
colour or chromaticity of an additive mixture as a point inside a
triangle with the primaries at the corners, known as a Maxwell
triangle; the gamut of colours expressible by all possible mixtures of
these primaries lies within or on the edge of the triangle (see figure
1.6). The chromaticity that is represented in Maxwell’s triangle is
the qualitative aspect of psychophysical colour; the quantity of that
colour – corresponding to brightness or luminance – is not
represented.
To work out the position of a mixture of primaries in the Maxwell
triangle, let r, g and b be the amounts of red, green and blue in the
mixture. Then the position is at xcoordinate
r
r+g+b and ycoordinate
g
r+g+b ; for example, if r = 60, g = 80 and b = 60 (a greenish gray),
the chromaticity coordinates are (0.3,0.4) and the total luminance is
200.
Using this coordinate system, we can now specify precisely colours
as an additive mixture of specified primaries, produced perhaps in
some wellde
fined physical situation. However, as mentioned in
section 1.2.2 above, it is not possible to find any three stimuli which
can produce all visible colours by addition; the gamut of all colours
does not form a triangle – instead, to match some colours, it is
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CC227 Creative Computing II Perception and Information Retrieval
necessary to add primaries to that colour in order to produce
something that can be matched with the other primaries.
The Commission internationale de l’´eclairage or CIE is an
international authority on light, colour and illumination; it was
established in 1913, and has produced standards for describing
colours unambiguously. The colour space in question resembles
figure 1.6, with three primary stimuli, but the primaries themselves
do not correspond to visible colours – the gamut of the primaries is
larger than that of all colours.
700
600
560
520
500
490
480
380
y
x 0 0.5
0
0.5
Figure 1.7: The CIE 1931 standard observer on the xy plane. The
gamut of visible colours lies within the thick line; all other points
in the plane contain impossible negative components of some real
colour. The dotted line is the limit of the (imaginary) CIE 1931
gamut; colours from pure wavelengths lie along the the thick solid
curve (the numbers correspond to the wavelength in nanometres),
and the thick dashed curve is the ‘purple line’, nonspectral
colours
which are perceptually a mixture of red and blue.
The CIE 1931 colour space selected three imaginary primaries,
known to as the CIE 1931 standard observer. The quantity of the
imaginary primaries (or tristimulus values) X, Y and Z, then define
a specific colour; exactly as with red, green and blue, we can
construct a Maxwell triangle for the chromaticity in that colour
space, defining x = X
X+Y +Z and y = Y
X+Y +Z . The resulting
chromaticity diagram for the CIE 1931 colour space is shown in
figure 1.7; any visible colour can be specified by its position in that
chromaticity diagram, along with a luminance value. To convert
back from chromaticity and luminance (xyY values) to tristimulus
(XYZ) values, we use X = x
y Y and Z = 1−x−y
y Y .
The CIE 1931 colour space is adequate for its intended purpose,
which is to specify all possible visible colours unambiguously; as an
example of its use, we can now present the colours that people with
particular forms of anomalous vision will be unable to distinguish,
as in figure 1.8. It does not, however, represent a perceptually
uniform space; there is much more resolution in the green area
(around 500nm) than the red and blue, even allowing for the
greater sensitivity of the eye to greens. This means that the CIE
1931 space does not represent colour differences in a uniform way;

1.3.3 Other colour mixture models

In the previous sections we have covered mixture by addition,
where a compound stimulus is created through the addition at
source of two stimuli; observing the mixture produces a sensation
distinct in character from the two mixed stimuli. There is a second
way in which mixtures can be produced: instead of having a mixed
stimulus, the visual system itself produces a mixture by averaging
different stimuli arriving at the eye.
The first way in which this can happen is through colour mixing by
averaging over an area, where regions of different colour are
amalgamated into a single colour. The pointillism and divisionism
styles of painting use this feature: many small dots of varied colours
are painted, which when viewed from a distance meld into an area
of a single colour. Pointillism in particular makes an interesting use
of this averaging, by using individual dots of strongly contrasting
colours.
This technique, using the eye to mix colours, sees applications in
digital displays and in printing. Digital displays with 24bit
colour
(8 bits for each of a red, green and blue primary) are considered to
be adequate in the colour space resolution, and are now common;
displays with lower colour resolution, however, are still in existence,
and can be used to reproduce images with a higher colour resolution
by dithering: to reproduce a colour not exactly representable in the
lowerresolution
colour space, pixels in the region are coloured so
that their average is close to the desired colour.
In printing, a similar technique called halftoning is used: the dots
printed by a halftoning process can be of different sizes, and are on
grids with different orientations. The result is that different amounts
of pigments of different colours arrive on the paper, some dots
overprinting others wholly or partially and some not; the visual
system produces the desired colour by averaging. This phenomenon
is also used to make printed colours lighter; assuming that the ink is
printed onto white paper, by using less ink (smaller dots) more light
of all colours is reflected from that area, which will lead to a lighter
average colour.
Learning activity
Use the following Processing code as a basis for investigating the colours achievable
by dithering.
colorMode(RGB,1.0);
for (int x = 0; x < 100; x++) {
for (int y = 0; y < 100; y++) {
int parity = (x + y) % 2;
stroke(parity,1,(1parity));
point(x,y);
}
}

1.3.2 Subtractive Colour Models

The colour model in Processing, and indeed in the vast majority of
computer applications, is additive: colours are generated by
additively mixing different intensities of primary colours. This is a
good model to use when the emission of light can be controlled
directly; however, there are many situations where light emission is
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CC227 Creative Computing II Perception and Information Retrieval
fixed or not under direct control, and instead the reflection and
absorption of light by filters or pigments is used to generate colour.
Conceptually the simplest case of this is the use of filters to let light
through selectively; instead of having the starting state being ‘no
light’ and adding to it, filtering usually starts with white light, a
mixture of many light wavelengths (for example, from a filament
light bulb), and subtracts light components from it (by interposing a
coloured film, for instance).
Where in additive mixing the three primaries should correspond to
sources spanning as much of the colour space as possible (see
section 1.3.4 below for a more precise statement), the subtractive
primaries should correspond to filters removing one of the primaries
– or, in other words, transmitting a mixture of the other two. Thus a
set of three filters which respectively block red, green and blue light
act as the primary colours, transmitting respectively cyan, magenta
and yellow light.
Then, producing light of one of the additive primaries can be done
by applying two appropriate subtractive primary filters in
succession: we can produce red light, for example, by applying a
magenta filter (transmitting red and blue, but removing green
wavelengths from the white light) and then a yellow filter
(transmitting red and green but removing blue): the net effect of
the two filters is to allow through red light only. The other
combinations of two filters can be used to transmit green light and
blue light from a white light source.
Subtractive mixing is used in the process colour or CMYK model of
colour printing. A colour printer has four inks: one ink for each of
the subtractive primary colours, and a black ink (known in this
context as key). To create regions of other colours, the subtractive
primary colours are printed on top of each other, effectively
combining their subtractive effects; a cyan dot overprinted on a
yellow one produces a green dot, just as with the light filters.
A combination of the three subtractive primaries produces black;
however, there are a number of reasons why process colour includes
a separate black ink. Firstly, inks are not perfect subtractive
primaries, and consequently the black produced by their mixture is
not necessarily a strong, dark black; it can sometimes be a muddy
brown. This is particularly important when printing text, which has
the additional requirement of precise reproduction of fine details
(such as serifs): to produce those serifs from a mixture of three inks,
the registration (or alignment) of the three separate coloured images
would have to be extremely precise.
As well as these aesthetic aspects to the use of the key ink, there are
practical ones: a full mixture of three different inks on the paper can
either take too long to dry, or even cause the paper to fall apart.
Finally, using a black ink for black, and for darkening colours, will
be much cheaper than the mixture of three coloured inks.
While the key ink can be used to darken tones, it obviously cannot
lighten them. The way that light colours, and colours not producible
by an equal mixture of inks, are printed is through colour mixture
by averaging, discussed in the next section.

Spatial representations of colour spaces

The RGB and HSB colour spaces have natural representations as
threedimensional
shapes (‘threedimensional’
because there are
three components needed to specify a colour). The RGB space is
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CC227 Creative Computing II Perception and Information Retrieval
most clearly represented as a colour cube: each of the red, green
and blue components of the colour space corresponds to one of the
axis directions of the threedimensional
space, and so a particular
colour’s position in this space is specified by a particular amount of
red, green and blue. Given this representation, it is possible to
express the colour distance between two colours, corresponding to
the distance between points in this colour space; for colours C and
C�, if ∆r = r − r� (and similarly for the green and blue components)
then the difference in colours is
∆CRGB = �(∆r)2 + (∆g)2 + (∆b)2. (1.6)
For this to be meaningful, the range of r, g, b (e.g. [0, 1) or [0, 255])
needs to be specified.
h
b s
Figure 1.4: The HSB colour space represented as a cone; the directions
of hue, saturation and brightness are shown as h, s and b.
The HSB space is most easily represented as a cone: the hue
coordinate is arranged in a circle; the saturation of a colour
increases towards the outside of the cone; and the brightness varies
along the cone’s axis (see figure 1.4. This means that a colour
position can be represented in coordinates as {b, bs cos h, bs sin h},
which means that in this space
∆CHSB = �(∆b)2 + (bs cos h − b�s� cos h�)2 + (bs sin h − b�s� sin h�)2
(1.7)
We will discuss the distances between colours more in section 1.3.4;
for now, be aware that the distances expressed in equations 1.6 and
1.7 are neither equal to each other nor strongly related to the
perception of colour differences. Both of these spaces are useful for
computational manipulation, but they do not capture the complexity
of relationships between colours.
Learning activity
Using the builtin
support for the RGB and HSB colour spaces in Processing, construct
3D sketches illustrating the cube and cone representations discussed in this section.
Afterimages and John Sadowski’s illusion
After staring at an image for a while, looking at a plain white
surface gives the perception of a ‘negative’ version of the stimulus
14
Colour Spaces and Profiles
(where dark regions in the negative correspond to light ones in the
original, and vice versa). This is known as a negative afterimage, and
occurs because the cone cells in the retina lose their sensitivity
temporarily after being overstimulated. When the attention is
turned to a white surface after looking at the image, those cells that
previously were firing will be less responsive, while those that were
not will respond as normal (and hence will fire more).
Learning activity
Processing has support for inverting an image in the HSB colour space, using the
INVERT specification to PImage.filter(). The following code implements an
illusion due to John Sadowski: staring at an inverted image, then at a greyscale version
of the original, gives the perception of the full colour image, until the eye moves.
PImage orig, gray, invert;
boolean inverted = false;
void setup() {
orig = loadImage("/home/crhodes/tmp/foo.jpg");
size(orig.width,orig.height);
gray = new PImage(orig.width,orig.height);
gray.copy(orig,0,0,orig.width,orig.height,0,0,orig.width,orig.height);
gray.filter(GRAY);
invert = new PImage(orig.width,orig.height);
invert.copy(orig,0,0,orig.width,orig.height,0,0,orig.width,orig.height);
invert.filter(INVERT);
colorMode(HSB);
invert.loadPixels();
for (int i = 0; i < invert.pixels.length; i++) {
float h = hue(invert.pixels[i]);
float s = saturation(invert.pixels[i]);
float b = brightness(invert.pixels[i]);
invert.pixels[i] = color(h, (s+255+255)/3, (b+255)/2);
}
}
void keyPressed() {
inverted = !inverted;
}
void draw() {
image(inverted ? invert : gray,0,0);
fill(0);
ellipse(orig.width/2,orig.height/2,2,2);
}
The INVERT filter inverts the hue in HSB space, whereas the opponent process
theory of colour vision would suggest that Sadowski’s illusion should be stronger if red
is transformed to green and yellow to blue. Investigate which version is more effective
for you.

1.3.1 Colour in Processing: RGB and HSB

In working with Processing, we have already met, in
[Creative computing I, vol. 2, chapter 1], two different colour
spaces: the RGB colour space, where each colour is represented in
terms of additive composition of three primary colours; and the HSB
space, where a colour is identified by values identifying its hue,
saturation and brightness. We have also already met the
colorMode() operator in Processing, which alters how colours are
specified in Processing code10. 10Note that the description of
colorMode() is in the
Processing documentation; the
colorMode() operator does not
change the interpretation of the
colour objects themselves (the
signed 32bit
integer) but rather the
conversion of a colour specification
into such an object of type color.
The RGB and HSB colour spaces are devicedependent
colour spaces:
they do not unambiguously specify a particular colour, as the colour
resulting from a particular specification will depend on what
equipment is used to display it; other devicedependent
colour
spaces, not available directly in Processing, include subtractive
models such as CMY (CyanMagentaYellow)
and HSB variants such
as HSL (HueSaturationLightness).
We will introduce
deviceindependent
colour spaces in section 1.3.4; these spaces
provide the means to specify a particular colour sensation,
independently of the device used to display the colour, and so allow
the exact reproduction of particular perceptual stimuli. The rest of
this section describes in detail the devicedependent
colour spaces
and the relationships between them.
r
g
b
h
Figure 1.3: Diagrammatic representation of hue in relation to red,
green and blue (r, g, b) components of a colour; the hue angle h is
the angle from the red axis around a colour circle, and is computed
using equation 1.1.
Let r, g, b be the coordinates of a colour in RGB space (with the
maximum value normalized to 1 for each); let max be the maximum
value of the three coordinates and min the minimum. Then
h = 

0 max = min;
π
3 × g−b
max−min mod2π max = r;
2π
3
+ π
3 × b−r
max−min max = g;
4π
3
+ π
3 × r−g
max−min max = b;
(1.1)
gives the hue angle from the red, green and blue components (see
figure 1.3; the mod2π is there to place the angle in the range
between 0 and 2π.
The saturation of a colour in HSB space is essentially how intense
12
Colour Spaces and Profiles
the colour itself is, and is computed by
s = � 0 max = 0;
1 − min
max otherwise
(1.2)
while the brightness is a measure of the overall intensity of the light,
and in HSB space is simply given by
β = max. (1.3)
To convert back from a hue, saturation, brightness specification to
red, green and blue values is the above process in reverse. If h, s
and β are the hue, saturation and brightness values, then let i be
�3h
π � (indicating which sixth of the hue circle the hue is in) and f,
the fractional part of the hue sextant, be 3h
π − i. Then to compute
the (r, g, b) values, compute
p = β × (1 − s)
q = β × (1 − f × s)
t = β × (1 − (1 − f) × s)
(1.4)
and then assign to (r, g, b) as follows
(r, g, b) =


(β, t, p) i = 0;
(q, β, p) i = 1;
(p, β, t) i = 2;
(p, q, β) i = 3;
(t, p, β) i = 4;
(β, p, q) i = 5;
(1.5)
Learning activity
Implement a pair of Processing classes, RGBColor and HSBColor, with fields for
red, green, blue and hue, saturation, brightness respectively.
Now implement a pair of functions, RGB to HSB and HSB to RGB, which take as
argument an instance of the appropriate class and converts it to the representation of
the same colour in the other colour space. You will need to define appropriate ranges
for each of the member variables.

Spatial correlations and Grid Illusions

The Hermann8 grid illusion occurs when a dark background is 8Ludimar Hermann (1838–1914),
German speech scientist [and,
incidentally, coiner of the word
format; see chapter 2...
covered by a lightcoloured
grid. At the points of intersection
between grid lines, dark spots appear transiently, disappearing when
they are looked at directly.
A variant on Hermann’s grid, called the scintillating grid, was
discovered by Elke Lingelbach. The only alteration to Hermann’s
grid is to draw white circles at the intersections; this causes the
perception of a grid of lights flickering (scintillating) on and off.
Interestingly, while the Hermann grid illusion can appear with a
single intersection, it seems to be necessary to have at least a 3×3
grid for the scintillating effect to be perceived.

1.2.3 Colourbased

all colours from wholly positive
mixtures of colours.
As well as the uncertainty discussed above as to how colour
perception works even in the simple cases, there are many
interesting effects that can be generated by certain kinds of stimuli.
7
CC227 Creative Computing II Perception and Information Retrieval
In this section we will look at some illusions, where the perception
of colour does not correspond to what is really there.
PatternInduced
Flicker Colours and Benham’s Top
Figure 1.2: A simplified version of the design on Benham’s top. When
spun, colours are usually perceived, but different colours are seen by
different people.
One such is Benham’s6 top, which was marketed as a toy in Victorian 6Charles Edwin Benham
(1860–1929), English amateur
scientist and polymath.
England. The basic design has half of a circle completely black, and
the other half has circular arcs; a simplified version is shown in
figure 1.2. When this disk is spun (at a rate of about 3 to 5
revolutions per second), people perceive colours from the circles
described by the arcs, usually pale reds and blues; the colours
usually change places if the disk is spun in the opposite direction.
These perceived colours are known as BenhamFechner
colours (after
those who documented them) or PatternInduced
Flicker Colours
(PIFCs).

1.2.2 Colour mixture by addition

In 1853, Grassmann4 formulated a set of axioms, empirically 4Herman Gunther Grassman
(1809–1877), German
mathematician, physicist and
linguist.
validated, which are now known as Grassman’s laws of colour
perception. They are, framed in terms of lights which may or may
not be mixtures,
Additivity: adding a third light to each of two lights perceived as
equal produces equal mixtures. Algebraically, x = y ⇒ x + z = y + z.
Proportionality: altering the luminances of two equal lights by
equal factors produces two equal lights. Algebraically,
x = y ⇒ αx = αy.
Transitivity: equality of light mixtures implies that the equal lights
can replace each other in all contexts. Algebraically,
(x = y) ∧ (y = z) ⇒ x = z.
Note that these laws break down at very low luminances, where
stimulation of rods is more important than cones; they also are not
strictly true over changes in luminance even at ordinary levels,
though for practical purposes they are sufficiently accurate over a
wide range of illumination that they can be used without significant
error.
Grassman’s laws imply that if we have two coloured lights
representable as mixtures of certain primaries (whether those
primares are fundamental to the eye or simply chosen as points of
reference), then the mixture of the two colours is also representable
as the mixture of those primaries. Specifically, if light X matches
aA + bB + cC, and light Y matches a�A + b�B + c�C, then the
mixture X + Y will match (a + a�)A + (b + b�)B + (c + c�)C. This rule
of mixture by addition, coupled with the assertion that three
primaries are enough to match any colour, is the basis for much of
digital colour production.

1.2.1 Opponent theory of colour perception

The section above describes how the light signal is detected by
structures within the eye. However, vision generally and colour
vision in particular is not experienced as a sequence of random spots
of light or colour flashing on and off in rapid succession; instead, we
see whole objects, and whole areas of colour. This tells us that the
firing of nerves in response to cone and rod cell excitation is not the
end of the story by any means. While the mechanism for detection
of light stimulus by the eye is wellunderstood
and uncontroversial,
there is no similarly wellunderstood
mechanism for interpreting
those detected stimuli as complete visual sensations.
One theory accounting for some empirical observations is known as
the opponent theory of colour vision. The observation that it most
clearly explains is the commonlyexpressed
view that some colours
are opposites of each other: red and green are opposites, and so are
yellow and blue. By ‘opposite’ here is meant that there is no such
thing as a greenishred
colour or a bluishyellow,
whereas other
colours are perceived as mixtures of these opponent primaries.
The proposed mechanism for this opponent theory is that the nerves
do not transmit the firing intensities of the different kinds of cone

Colour Vision
cells directly; instead, the brain receives information about the
differences between combinations of these intensities. The
physiological effects of opposed colours was investigated in Theory
of Colours by Goethe3; more recent psychological theories use the 3Johann Wolfgang von Goethe
(1749–1832), German writer,
scientist and diplomat.
mechanism of opposition to explain not only colour vision but also
other sensations such as emotion and addiction. However, even in
the case of colour vision, there is experimental data that suggests
that the opponent theory is not precisely true: under certain
conditions, it is possible to cause people to perceive a colour which
is described as reddishgreen.

Photoreceptors: rods and cones

As mentioned above, there are two distinct kinds of photoreceptive
cells in the retina: rods and cones. Rod cells are extremely sensitive
to light, with the ability to send a signal to the brain if a single
photon (unit of light) is absorbed. On the other hand, rod cells are
slower to fire than cones, with a response time of about 100ms.
Because of the high sensitivity of rods to light, they are the primary

CC227 Creative Computing II Perception and Information Retrieval
photoreceptors used in dark conditions, such as at night. In
addition, the rods are largely absent from the centre of the retina
(completely absent from the fovea) and are common on the outside;
thus they are used for peripheral vision. The rods are sensitive to
motion, but have poor spatial discriminating power, which explains
why peripheral vision often gives the information that something is
moving, but not any detail of what that something is.
The rods have a single kind of pigment within them, which absorbs
light in the wavelength range of about 380–640nm. In particular,
they are completely insensitive to light of wavelengths above 640nm
(a shade of red). This is the cause of the Purkinje2 effect, the name 2Jan Evangelista Purkyne
(1787–1869), Czech anatomist and
physiologist.
given to the observation that blue objects appear brighter than red
ones under darker illumination conditions. Apart from this, rods
play little part in colour vision, as they cannot discriminate between
different wavelengths of light.
Cone cells, on the other hand, come in three different kinds, each
with a distinct pigment; one most sensitive to longwavelength
light,
one more towards the medium wavelengths, and one to
shortwavelength
light: respectively, their peak sensitivies are at
wavelengths around 570nm, 540nm and 430nm. The cones are able
to respond quickly to a stimulus, and have high concentration in the
fovea, giving highdetail
central vision; however, they only operate
under conditions of bright illumination.
The three different pigments present in cone cells permits colour
vision: the differentlypigmented
cells will fire in different
proportions when viewing light of particular frequencies, which
allows the brain to attribute a colour to a stimulus. The next section
discusses some details of colour vision in more depth.

Photoreceptors: rods and cones

As mentioned above, there are two distinct kinds of photoreceptive
cells in the retina: rods and cones. Rod cells are extremely sensitive
to light, with the ability to send a signal to the brain if a single
photon (unit of light) is absorbed. On the other hand, rod cells are
slower to fire than cones, with a response time of about 100ms.
Because of the high sensitivity of rods to light, they are the primary

CC227 Creative Computing II Perception and Information Retrieval
photoreceptors used in dark conditions, such as at night. In
addition, the rods are largely absent from the centre of the retina
(completely absent from the fovea) and are common on the outside;
thus they are used for peripheral vision. The rods are sensitive to
motion, but have poor spatial discriminating power, which explains
why peripheral vision often gives the information that something is
moving, but not any detail of what that something is.
The rods have a single kind of pigment within them, which absorbs
light in the wavelength range of about 380–640nm. In particular,
they are completely insensitive to light of wavelengths above 640nm
(a shade of red). This is the cause of the Purkinje2 effect, the name 2Jan Evangelista Purkyne
(1787–1869), Czech anatomist and
physiologist.
given to the observation that blue objects appear brighter than red
ones under darker illumination conditions. Apart from this, rods
play little part in colour vision, as they cannot discriminate between
different wavelengths of light.
Cone cells, on the other hand, come in three different kinds, each
with a distinct pigment; one most sensitive to longwavelength
light,
one more towards the medium wavelengths, and one to
shortwavelength
light: respectively, their peak sensitivies are at
wavelengths around 570nm, 540nm and 430nm. The cones are able
to respond quickly to a stimulus, and have high concentration in the
fovea, giving highdetail
central vision; however, they only operate
under conditions of bright illumination.
The three different pigments present in cone cells permits colour
vision: the differentlypigmented
cells will fire in different
proportions when viewing light of particular frequencies, which
allows the brain to attribute a colour to a stimulus. The next section
discusses some details of colour vision in more depth.

1.1.1 The Eye

Because of the remoteness of the scale of light waves from that of
easilyobserved
phenomena, understanding the human perception
of light is not straightforward. We begin our path to understanding
by examining the way in which light is detected in the human body.
The purpose of the focussing is to cause the light to fall onto the
retina, which contains two principal kinds of photoreceptive cells:
rods and cones. These cells are sensitive to particular kinds of light,
firing under some circumstances and not others.
There are two areas of particular interest on at the back of the eye:
one is the fovea, a pit with the highest density of cones on the retina;
it is responsible for our sharp central vision, used whenever fine
detail is required. The other is the optic disc, where the nerve cells
joined to the photoreceptors congregate before going into the optic
nerve towards the brain. It is this area which gives rise to the blind
spot in human visual perception; there are no useful photoreceptive
cells in this area, as all the nerve cells come over the front of the
retina, obscuring the light.

1.1 Light and The Eye

What we call ‘light’ is a small portion of the spectrum of
electromagnetic radiation: waves propagating through electric and
magnetic fields. The eye is the primary organ for converting those
electromagnetic waves into sensation, responding to waves with
wavelengths between 380 nanometres and 700 nanometres
(3.8×10−7m and 7×10−7m; nanometre is abbreviated as nm. The
wavelength of light is also sometimes quoted in ˚angstr¨oms1: there 1Anders Jonas Angstrom
(1814–1874), Swedish physicist. are 10˚angstr¨oms in a nanometre, and visible light has wavelengths
between 3800˚A and 7000˚A. We will use nanometres throughout this
study guide).
Like all wave phenomena, light exhibits certain characteristic
behaviours, such as reflection (from mirrors, but also from any
visible object), refraction (when the properties of the medium
changes, such as at a water/air boundary or in a desert) and
scattering (turning the sky blue). Since the wavelength of visible
light is so small compared with most everyday
phenomena, the
behaviour is less easy to predict from commonsense
than other
waves; for instance, it is easy to see that water waves can scatter
from a rock in the sea, but much less intuitive that blue light scatters
off air molecules more than red light.

Light and Vision

The irony of a chapter on Light and Vision being printed in black
and white is not lost on the author. Furthermore, as will be
discussed in the rest of this chapter, neither the perception of colour
nor accurate reproduction of colours is a straightforward topic. That
said, more than usually there will be call to visit the module website
to download additional materials, including colour illustrations.