An in-depth explanation of some interesting optical effects that are totally irrelevant to web design! Includes
& so much more
like this handy-dandy glossary of color-related terms, not all of which are covered in this article:
Objects which exhibit the following:
First, some background info
A simplistic explanation of how color is perceived involves white light, "a mixture of all the colors of the rainbow", falling on a red object, which absorbs all the colors except the red portion of the spectrum, which is reflected back to the viewer.
When white (full spectrum) light passes through a cloud of gas, the same bandwidths are absorbed rather than emitted, producing gaps in the spectrum that can also serve as identifiers for the gas composition.
The wavelengths of the complete visible spectrum, between infrared and ultraviolet, range from approximately 390 to 750 nm (nanometers, billionths of a meter). Spectral wavelengths are also frequently given in Å (angstroms, 10 nm) or °K (degrees Kelvin). While active upon the human body, infrared and ultraviolet are invisible to the human eye. These are the wavelengths for the traditional visible "seven colors of the rainbow", ROYGBIV:
These naming divisions are arbitrary, made by Newton when he used a prism to separate sunlight into its spectral components. Color can additionally be expressed in Kelvin, the temperature in degrees Kelvin of a Planckian radiator emitting at a specific temperature, as described above.
do we think we see a contiuous spectrum?
And now, what you've all been waiting for!
Metamerism, the situation where two color samples appear to match under one condition but not under another, is the result of differences in object surface composition. The dyes and pigments used to create the color of objects such as textiles, paints and so on, have different spectral reflectance curves. Color perception is a combination of the spectral reflectance of the pigment or of the dye (and its substrate) and the spectral distribution of the light source. The color you see is influenced by the emission spectrum of the source of the light available to be reflected by the object. The most common light sources are fluorescent, incandescent, ultraviolet and sunlight, which have different spectral distributions. Incandescent lighting is generally considered warm compared to cooler fluorescent lighting, famous for sucking the life out of beige. Other colors that are likely to have metameric problems include taupe, mauve, lilac, tan, celadon, gray-blues, and grays. While metamerism is normally a light-source effect, it can also arise from differences in the physiological optic structure of observers, e.g. color-blindness.
to Test for Metamerism
Instrumental Test for Metamerism
Slightly more relevant info:
Since the beginning of the twentieth century, many forms of artificial electroluminescence have become available (to include discharge tubes, neon signs, television and the video display tube). In the common discharge tube, a small quantity of gas or metal-vapour is sealed within a cylindrical glass or quartz tube. When an electrical current is passed or 'discharged' between the electrode ends, a flow of electrons is released which excites the elemental gas (which is most usually mercury vapour) to emit its characteristic spectral wavelengths. (In the high-pressure mercury-vapour lamp these combine optically to produce a bluish-white emission, commonly used in street lighting since 1933.) Other familiar discharge lamps are the yellow low-pressure sodium-vapour and pink high-pressure sodium street-lamp. (Increasing the pressure within the tube will tend to broaden the emission spectra to wavelengths beyond the spectral lines normally associated with each element.) The xenon lamp emits a combination of spectral lines which fuse in the human eye to yield a white closely similar to daylight.
Almost all solid materials capable of luminescence consist of a so-called 'host crystal' activated by an impurity, to which it usually owes its colour appearance. For example, the host crystal zinc sulphide appears yellow if its impurity is manganese, blue if silver, or green if bismuth or copper. Such crystals are stimulated to emit light by forcing an electric current through them. One arrangement utilises luminescent powder 'sandwiched' between glass and a reflective metal plate; an electrical circuit is made complete by placing a sheet of conducting glass over the metal and sealing the edges. Relatively little current is drawn, and the whole flat panel is illuminated. The intensity of the light emitted depends on the magnitude of the current and the colour on the selection of crystals used. The flat, pale turquoise nightlights popular in the 60s are an example.
A particular limitation in the use of the mercury vapor street-lamp is its absence of emission in the red region of the spectrum; in an interior setting, this would cause red objects to appear dark and the human complexion unflattering. A light source which imitates daylight more closely can be obtained by coating the inside of the mercury lamp with a fluorescing powder (commonly an impure calcium halophosphate). The energy emitted by the mercury arc is rich in ultraviolet energy (at 185 and 253 nanometres) and the fluorescence produced in the coating converts the invisible ultraviolet wavelengths to longer wavelengths within the visible range; this extends the lamp's bandwidth into the red, thereby offering a more 'balanced' and useful source of white, interior illumination.
Light emitted artificially from the discharge of static electricity was demonstrated by Otto von Guericke as early as 1683. In 1744 Johann Winkler conducted an experiment in which he produced light by shaking up a vacuum tube filled with mercury. The first modern discharge tube did not appear however until 1856, when Heinrich Geissler experimented with a sealed, low-pressure tube powered by a high-voltage alternating current. Important developments followed from advances made in New York City by Nikola Tesla, competing for an alternative to Edison's new incandescent lamp. The earliest luminescent advertising signs, by McFarland Moore (Newark, New Jersey, 1904) were ultimately developed by Georges Claude in Paris, who manufactured and exhbited his first red neon sign in 1910. (The device was patented in 1915.) The first of the line of modern mercury-vapour lamps was exhibited in Cincinnati, Ohio, in 1935.
Even more relevant info!
In spite of the fact that most designers have color management systems
on their machines (and especially those that are built into the Macintosh
systems), in spite of the fact that graphic software such as Photoshop
can embed color profiles in web graphics, the web visitor's profile is
still an unknown and all bets are off. Furthermore, aside from plug-ins
and file formats that are not fully supported, web browsers have limited
capabilities to deliver the information. There are other complex and costly
solutions. For example, if customers are truly dedicated to an online
store, they might take the time to download the software for color accuracy
at that one site, but that's not realistic for most situations. One of
the best temporary solutions is to design all web graphics on computers
that generate the best colors (as a result of fully corrected gamma and
other standards). You can test your computer's color vision at the following
Still more metamer info!
Still more digressions!
In essence, when you see a color, it's because the three kinds of cones contribute to a sensation that your brain recognizes as a particular color. When you see a frequency of yellow light, say, (as measured by its wavelength), your red, green and blue cones absorb the light in particular proportions. When you see a combination of red and green (as measured by the wavelengths of the light once again), your red, green, and blue cones absorb light in the same proportion, and your visual system once again senses yellow. In fact, there are an infinite number of combinations of light frequencies that will register the same way on your visual system to produce the same sensation -- which is to say, the same color. All of which brings us back to the jacket and pants problem. If the jacket and pants you look at in the store are made from the same bolt of cloth -- which means they are part of the same dye lot -- they will almost certainly both reflect each wavelength of light the same way. Regardless of the light source, then, they will always reflect the same combination of wavelengths, and you'll see them as being the same color regardless of the light source. If they're made from cloth that came from different dye lots, however, odds are that they won't reflect each wavelength of light the same way. The mix of wavelengths they reflect from one light source the light source in the store will produce the same color, because the dyes were mixed to look the same under a light source that matches the light source in the store. But because different dye lots usually reflect and absorb different wavelengths differently, it's highly unlikely that the mix of wavelengths they reflect from a different light source will also look the same. Move to a different light source, in short, and the colors won't match. As you might guess, you can run across problems with metameric pairs in all sorts of situations a set of furniture with fabric from different dye lots, wall paper produced in different runs, paint mixed to match a color from a different brand of paint, and so on. More important, metamerism is something you need to keep very much in mind when dealing with color on a computer system. When you're working hard to match colors of a scanned photo to printed output for example, you need to think in terms of matching the colors in a given light. What matches at home under incandescent light may not match at the office under fluorescent light.
One bothersome complication for anyone trying to match colors is that colors actually change depending on the colors around them. This is an observer effect, or optical illusion. Here's a quick test you can run to prove the point. Find a bright light source, like a bare bulb, and hold your arm out in a thumbs up gesture, with the thumb centered on the light source. You should see little or no detail in the thumb, and may see your thumb only as a silhouette against the light. Then, while still looking at your thumb, slip something opaque between the thumb and light source. Your eye should adjust quickly, and let you see details of your thumb. This change in appearance of a central area in your field of vision because of a change in another area is called simultaneous contrast. It affects not just how much detail you can see, but shades and color as well. Images illustrating this point can be found in the book The Underground Guide to Color Printers, published by Addison-Wesley, and at www.yorku.ca/eye/simcont2.htm. The same color can look very different depending on the surrounding colors, and two very different colors can look the same, given the right interpretation of the colors by your monitor or printer. See www.cs.umb.edu/~ram/courses/color/albers.htm for other examples of this effect.