Human Cone Action Spectra
In most vertebrates that have what humans term “color vision”, there appear to be three classes of cones, based on their absorption spectra. Although many fundamental aspects of color vision remain to be worked out, the chemistry of the cones’ visual pigments is pretty much the same as in the rods. A photon is absorbed by a cis-11-retinal, converting it to the trans- configuration and initiating the phototransduction process. One of the most important differences between cones and rods is in the amino acid sequence of the opsin component of the rhodopsin molecule. The cones in the three classes of vertebrate cone differ in their absorption spectra because the amino acid sequence of their opsin molecules (opsin is the protein component of rhodopsin, the visual pigment that actually absorbs photons) differs.
In humans, the three classes of cone show maximal absorption efficiency at wavelengths (lmax) of 420 nm (blue cones), 530 nm (green cones) and 560 nm (red cones), as illustrated by the following graph (I’ve included the absorption spectrum for rods as well, but the graph of the rod absorption spectrum is not to scale; it’s included solely for comparative purposes. Rods are far more sensitive, and show a many-fold stronger response, to light than cones. You should simply note that the absorption spectrum for rods is very similar in shape to the spectra for cones, and that the lmax is approximately 500 nm.):
You can see that in spite of their names (“Red”, “Green”, and “Blue”), each class of cone shows an absorption, or action, spectrum that extends over a range of wavelengths, with the result being considerable overlap in the three action spectra. In addition, the names we assign to the three classes of cones are a bit of an unfortunate misnomer: with the possible exception of green cones, the names do not correspond well with the color we perceive when viewing monochromatic light with a wavelength corresponding to the lmax for the cone in question. For example, our perception of what we call “Blue” light is not a response to light that preferentially activates only so-called blue cones. Instead, light with a wavelength of light to which we assign the color “Blue” (from 450 to 490 nm, give or take) actually stimulates each of the three cone types with roughly comparable effectiveness. Likewise we perceive yellow, orange, or red, depending on the relative activation of green and red cones.
The overlapping action spectra, coupled with the complexity of the neural pathways involved in processing the information provided by the cones (see the links below for more on this fascinating topic), allow us to perceive (= discriminate) a huge number of different colors, depending on the wavelength(s) of the photons impinging on the cones. This is because the color we perceive when viewing light of a given wavelength depends on how much the cones of each class are stimulated by that wavelength, which in turn is determined by how efficiently the cones of each color class absorb the photons with that wavelength. The perceived colors for various wavelength ranges are indicated across the top of the graph.
For More Information
Excellent, and (considerably) more detailed, discussions of color vision, may be found at:
1. http://webvision.med.utah.edu/Color.html -- a primo site, but it's a large file, so be patient when opening it. Also, this URL’s home page, http://webvision.med.utah.edu is a treasure trove of information about vision……..highly recommended!
2. http://dinsdale.psych.ndsu.nodak.edu/HomePage/Psy460/Color%20Vision/Color%20Vision.html -- this URL contains a link to some software that let’s you extend your investigations of color vision.
3. http://www.colormaxtech.com/colorblindscience/intro.htm -- Colormax® Technologies, Inc. manufactures lenses designed to ‘correct’ various color deficiencies, so you have to put up with a bit of self-promotion & hype at this site, but it still contains interesting medically-related information.
4. http://www.ucc.uconn.edu/~wwwphil/ccompan.html -- less flashy (no pictures), but chock full of information; perhaps a bit technical for some tastes.