An example of a lower frequency is infrared radiation. When shooting with black-and-white infrared film, a stock that has a higher sensitivity to the infrared spectrum, the lens focus must be set differently than you would for visible light because the glass does not bend the longer wavelengths as much. Doing so enables the film to record a sharp image that takes advantage of what infrared reveals about the subjects by how the subjects and their variations in reflectance modulate the IR waves.
When light strikes an object, certain wavelengths are absorbed and others are reflected. Those that are reflected make up the color of the object orange, for example. Orange has many variations that can be described as light, deep, rusty, bright, light, etc., and each is its own individual color. To explain this in more scientific terms than “deep” or “rusty,” orange is distinguished by several characteristics: hue, lightness, and colorfulness. Hue is the property that identifies a color with a part of the spectrum for example, orange is between red and yellow. The lightness description simply locates the color somewhere between light and dark colors. The colorfulness label indicates whether a color is either pale or pure (vivid).
Turn on a fluorescent Kino Flo and a tungsten 10K and then look at the output. What do you see besides spots? You will perceive white light emanating from each, yet the two fixtures have vastly different spectrums. Though the two throws of light may be of different spectrums, an object illuminated by each will look the same a phenomenon known as metamerism. The reason we see white light from two different sources is due to the fact that the eye reduces the full spectral distribution of light into three bands, and there are an infinite number of different spectral distributions that can result in the same three signals to the eye. The spectral distributions that appear visually identical are called metamers.
Of the colors that make up the visible spectrum, red, green and blue are called primaries. Newton argued that each color of the visible spectrum was a primary, but he considered the definition of primary to mean “simple,” as in simply a lump-sum color called red or yellow, etc., without getting into characteristics such as lightness or colorfulness or a specific wavelength of red within the red range. The other four colors, known as secondary colors, are made by combinations of the three pure colors (see diagram). Orange light is created by roughly two parts red to one part green. In addition to creating the secondary hues, adding these three primary colors together in various combinations builds every other color up from black, and this is known as the additive color process. The three bands in our color vision cells correspond directly to the three primary spectral colors, and because of this, it is possible to match a wide range of colors via a mixture of just three primary colors.
Yes, we learned in art class that red, blue and yellow are primary colors (and that we aren’t supposed to eat paint). But those primaries apply to dyes and pigments, and they actually are called subtractive primaries (see diagram). (In light, the subtractive primaries are cyan, magenta and yellow.) Visible light, whose primaries consist of red, green and blue (RGB), is what allows those primary red, blue and yellow pigments (or cyan, magenta and yellow) to have color through varying degrees of wavelength reflection and absorption by the pigments and dyes themselves. The colored dye of a solid object will absorb (or subtract) unwanted frequencies from white light and reflect the desired frequency say, blue. As another example, a piece of glass dyed yellow subtracts the unwanted frequencies from white light and allows the yellow frequency to pass through.
Plato believed that our eyes contained fire placed there by the gods. The fire produces rays that extend from our eyes and interact with the particles that emanate from all the objects in the sight line to create visual perception. Obviously, Plato’s theory has since been proven false. Even Aristotle, Plato’s pupil, refuted this theory by arguing that if it were true, vision at night would not be any different than vision during the day. But, he noted, it is different.
Back to the more logical explanation. Three kinds of lightsensitive pigments in our eyes are contained in three types of cones: blue (short wavelengths), green (medium) and red (long). These color vision cells (or photoreceptors) truly are shaped like cones. Because there are three cones, color spaces are mapped in three dimensions. In reality, each is sensitive to a range of colors, but they peak at the correlating primary-color wavelengths. How the brain sees and interprets color signals sent from the cones is still not fully understood, but it is known that processing in the brain plays a big role in determining the perceived color. Though the blue cone’s peak sensitivity is to blue light, when only those cones fire, our brains for some reason perceive violet rather than what we would assume to be a typical blue. Call it one of nature’s quirks. Other quirks are color vision abnormalities, where some of a person’s cones may be absent or deficient in a particular pigment, which causes the interpretation of some colors to be affected. This is known as color-blindness, and it predominantly affects about 8 to 12 percent of males. Only about half of 1 percent of females has a form of color-blindness. Red-green color blindness is the most common form.
Rods, too, are not immune to abnormalities. Rods are retinal cells that are indeed shaped like rods and function more at low light levels. They do not distinguish color, only picture detail and intensity, or white and black. Rods are saturated above about one foot-lambert and are not considered to play a significant role in normal color vision. However, the shadows in a typical motion-picture presentation fall into a light level where both cones and rods are active, known as mesopic vision, which falls between photopic (under normal light conditions) and scotopic (human vision in the dark). Few people have total color-blindness with no cone activity, in which they see only in black-and-white or shades of gray. (For the Internet-savvy, the Web site http://colorfilter.wickline.org allows you to type in a Web address and view that page as though you are color-blind.)