Monday, November 21, 2011

Color Theory - where Science/Technology and Visual Arts meet

Have you ever wonder what the world would be like if we can only see black and white, and shades of gray?  Have you ever wondered what do human’s animal friends and foes see?  Many of us played with crayons when young and learned the basics of theory of color in the introductory science class.  But if you want to tweak colors in a photo using software like Photoshop, or want to mix color paints to create a new color for desired effects, what works and what doesn’t, and why? What is color and how much do we know about color anyway?  Below is the first installment of my report from my current study of Color Theory that is a required basic course for most Fine Arts degrees.

Color begins with light; if there is no (visible) light, there will be no color perceived or to speak of.  Everyone has been awed when young by rainbows but it took quite some time before scientists figured out what is rainbow, how they are formed and why they look the way they do.  Isaac Newton, one of the greatest physicists, published his seminal work Opticks in 1704 in which he discussed, among other things, how (white) light can be split (by refraction) into color lights with a prism.  He identified the seven basic colors of light seen after splitting - Red, Orange, Yellow, Green, Blue, Indigo, and Violet, or “ROY G. BIV” as every American kid was taught in grade school.  Of course, light spectrum is continuous and there just aren’t enough names to describe them all with arbitrary granularity.   Sir Newton also showed that white light can be created with overlapping lights of colors.

We now know that technically speaking, humans can only detect different “colors” in a narrow spectrum of electromagnetic (EM) radiation between roughly 390 to 750 nanometers (nm) in wavelength, or 400 to 790 Tera Hertz in frequency (see the reference chart below for the whole range of EM radiation). Given such a wide possibility, one should not be surprised that different species of animals and insects “see” the world differently.  It turns out that many birds, bees and insects can see ultraviolet (UV) wavelength in 300-400 nm And some snakes, fishes, and mosquitoes can see infrared (IR) of wavelength in 700-3,000 nm range (where humans would need help with a night vision device to “see”).   Note that regardless of the color, there are many animals and insects that can also detect objects under much dimmer light compared to humans.  

While evolutionary biologists can tell you and I why and how such distinct visual sensory systems came about, scientists now have a pretty good idea how human vision works.  In particular, humans have a huge number of neurons (photoreceptor cells) in the retinas of our eyes that can detect and convert the light information to signals and transmit them to the brain for further processing.  There are about 120 millions of photoreceptors (called rods) for detection of light beyond some intensity and 5 millions of photoreceptors (called cones) for detecting lights of certain wavelengths (see the diagram to the right of the structure of eye).  It so happens that humans have three types of cones that are capable of detection of color Red, Green, and Blue (RGB) that determines pretty much what colors we see.   

In contrast, other species of animals and insects have distinct ability in color detection depending on how many types of cones they have and what wavelengths of light they can detect.  For example, cats and dogs have only two types of cones and can only detect Yellow and Blue colors but not the red.  Some mammals have none and are color blind. On the other hand, some birds and fish have four types of color-sensitive cone cells, giving them greater sophistication in distinguishing colors.  And bees have three types of cones like humans but they can detect shorter wavelengths and can see the ultraviolet as mentioned earlier. They use their better color vision to search for nectars and can distinguish flower colors invisible to human.  At the other end of spectrum, mantis shrimps have ten types of rods and presumably see a lot finer colorful world.  The bottom line is even when some animals, insects can detect radiation in overlapping range of the EM wave, they don’t necessarily see the same colors.  For more detailed discussions, you can read the excellent article Color in Nature by Philip Ball. 

Astute readers by now have probably noticed that when discussing how colors are detected by our visual system, there was no mentioning of the source of the light.  What Sir Newton has focused on was the white light itself as emitted from a light source and its color as seen by the eyes.  The source could be the sun, a light ball, a flash light, a TV screen or a computer monitor.  And it is an additive process, i.e., our eyes receive all the component lights of various wavelengths combined together.  For the rest and majority of the colors of objects we see, eyes receive lights coming from the reflections of the light shined on the objects.   Thus if the object absorbs all lights completely, we would see and consider it has color black.  In other words, the perceived color of the object (e.g. on a printed page) is the result of subtraction: our eyes receive original light minus whatever wavelengths and amounts of light spectrum that were absorbed by the object.   For more detailed discussions, one can start with the Wikipedia article pigment and the references therein. 
Artists (and other –ists) before and after Newton have been well aware of the distinctions between color of light and color of an object empirically.  However it wasn’t until 100 years later in early 1800s when the German writer/scientist/artist Johann Goethe challenged and addressed the limitation of Newton’s theory of light and laid the foundation of a practical theory that are used in our daily life now.  Color theory is in fact still an active area of research as it touches upon and calls for contributions and inventions from artists and scientists ranging from biologist to cognition psychologist to understand and make use of our visual ability fully. 

With the introduction above, we are now ready to talk about color models.  A good place to start for any theory is a good model – something that provides us the structure and means to approach and discuss an otherwise intractable and complex phenomenon like our perception (and illusion) of color.  For historical and technical reasons, there are unfortunately many different color models for different applications (while sharing some of the same vocabulary) that cause a lot of confusion for beginners.

Most of us have heard of the popular RGB color model which is based on the use and additive properties of the additive primary colors Red, Green, and Blue lights to generate a broad range of colors.  It is most often found in TV, mobile phone, and computer displays, scanners, camcorders and digital cameras.   There is also the CMYK (Cyan, Magenta, Yellow, black/Key) color model used in color printing that relies on the subtractive properties of the primary colors of Yellow, Magenta and Cyan.  Alternatively, artists typically use the RYB color model and refer them to 12 colors Color Wheel – three (Red, Yellow, and Blue) subtractive primaries along with nine secondary and tertiary colors that are placed around a circle uniformly.  The cool thing with the color wheel is that the pair of diagonally opposite colors – the so called complementary colors, when mixed, neutralizes each other and produces eventually a gray.  See the figure to the right that I did in the class using only Yellow, Blue and Red acrylic paints. 

Since both RGB and CYMK color models are device-dependent (i.e.: different devices/material necessarily detect or reproduce a given RGB value differently), there is no fixed representation of the color and they do vary from manufacturer to manufacturer and difficult to convert and match. Continuing efforts have been made to create device independent models that can describe all the colors visible to the human eye and are perceptually uniform (i.e., a change of the same amount in a color value should produce a change of about the same visual importance). Of particular interest is the most recent Color Appearance Model standard - CIECAM02, ratified by CIE (Internationale de l´Eclairage or The International Commission on Illumination, an international non-profit organization).  The model has been quantified with rigorous subjecting testing in lab and careful measurements of perceptual responses.  It is based on the widely used Munsell color system that separates color-making attributes into three independent dimensions - hue, value, and chroma.

A 3-D diagram illustrating the notion of Munsell color system is shown to the right.  Technically, Hue codes the distinctive color from red, blue, green and yellow.  Value (or lightness vs. darkness) indicates the relative brightness to the brightness of white under similar viewing conditions whereby brightness is the perceived amount of light emanating from the observed object (just think of the brightness associated with different wattage of of common household light bulbs).   Chroma is the colorfulness relative to the brightness of white under similar viewing conditions whereby colorfulness is the degree of difference between a color and gray which is neutral in color.  Note in this definition, chroma is different from saturation which is the degree of purity of a hue.  That is, chroma is the purity relative to gray and thus to other colors while it is only meaningful to talk about (relative) degree of saturation of a color by itself.  

So far we have been focusing on aspects of color theory from scientific and technological perspectives. Scientists seek to understand, explain, and to predict the phenomena.  Technologists strive for creating methods and tools that allow people to perform tasks which are otherwise difficult.  Artists on the other hand express themselves and communicate to people via their sensory system plus imagination.   Indeed, artists have been creating visual languages in color intuitively and successfully long before science was even developed.  In my next installment of color theory, I will show some illustrative examples of what one can do with colors.   Talk to you soon!

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