Transmittance of human lenses as a function of wavelength and age. Data taken from Charman, (1991). “Optics of the Human Eye” in Vision and Visual Dysfunction 1:Visual Optics and Instrumentation, CRC Press, Boca Raton, FL.
Pre-retinal Media
In the first section, there is a plot demonstrating that human lenses absorb short wavelength light before it reaches the retina. The general comments surrounding that figure are generally true. On average, humans are insensitive to light with wavelengths shorter than around 400 nm. In specific cases, though, that isn’t true. The following plot gives some indication that the “pre-retinal media” of different people can be different. More specifically, the amount of UV that gets through to a person’s photoreceptors is correlated with the person’s age.
Transmittance of human lenses as a function of wavelength and age. Data taken from Charman, (1991). “Optics of the Human Eye” in Vision and Visual Dysfunction 1:Visual Optics and Instrumentation, CRC Press, Boca Raton, FL.
Data such as these aren’t easy to scale properly, which is probably why the short wavelength transmissions here aren’t as low as they are in the prior figure. However, similar data have been acquired by several people, and the results are concordant. Human lenses generally absorb most light with wavelengths shorter than 400 nm. The lenses of older humans generally absorb more light than the lenses of younger humans. Children’s lenses are not only generally more transparent, they are specifically more transparent to light with short wavelengths. A result of these facts is that in normal humans, color vision changes with age!
The data presented in this graph are for absorption by the lens alone. The other two structures labelled in the inset, the cornea and the macular pigment, also preferentially absorb short wavelengths. Their absorption properties also vary from person to person. Additionally, the macular pigment bears special notice because it does not cover the entire retina. It is found only in front of the central part of your eye (the region where the things you are looking straight at get imaged). A consequence of this is that your perception of color depends on exactly what part of your retina you’re using to see a particular object. The effect is subtle, but it shows up reliably in laboratory experiments.
Photopigment polymorphism
In the first section, there is a plot showing the spectral absorptions of the three types of human cone. As with other properties discussed so far, these absorption spectra are normal for humans, but they don’t represent everyone. The spectral absorption properties at the business end of a cone are mainly established by the identity of the photopigment expressed by that cone. In principle, all of our cells contain a complete set of our DNA. In any given cell, not all of the genes are used. Those genes that are used are those that contain instructions to make particular proteins that that cell needs. The protein part of a visual pigment, the opsin, determines its spectral absorption properties. The identity of the opsin expressed by a cone is the primary determinant of its type.
Over the course of the past ten years or so, we’ve come to recognize that within the human population, there are not just three cone opsin genes. For each gene there are variations that shift the spectral sensitivity slightly. Many of these variations are in the form of SNPs (pronounced “snips” ‒ single nucleotide polymorphisms) in the genes. These are locations where different people have one different base pair in the sequence of base pairs that makes up the gene. Also, due to the similarity of the M and L opsin genes, and due to the way they are lined up on the X chromosome, copying errors have produced many hybrid genes. Hybrid genes are sequences that contain parts of two ancestrally different genes, in this case the normal M opsin gene and the normal L opsin gene.
The upshot of all of this is that you can imagine that in the construction of a normal human, there will be three cone pigment genes selected from a pool that contains several different potential genes. Through bad luck, some people (mainly males since the L and M pigments are on the X chromosome) will get fewer than three different genes. Others (all of whom are probably women) may get more than three. To give you some idea of the kind of variation we’re talking about, the following image shows the absorption properties of some of the pigments that could go into a person’s nominal M or L cones.
Spectral sensitivity curves of “normal” and hybrid L/M photopigments. Adapted from Merbs and Nathans, (1992). “Absorption Spectra of the Hybrid Pigments Responsible for Anomalous Color Vision”, Science, 258:464-466.
As in the introductory page, the colors of the curves don’t mean anything. The curves are colored only so that you can tell them apart. Note that this is not a complete set of possible L/M photopigments. Many pigments have absorption spectra that are indistinguishable from some of these at this scale. Similarly, even a person who is lucky enough to have two of these (note that no S cone pigment spectra are shown here) may be so unlucky as to have two pigments that are, for all intents and purposes identical as far as spectral absorption is concerned. We’ll see why that is later.
So what is this all about anyways? If you’ve been reading these pages in the order I wrote them, then you’ve seen a bit about light getting into the eye and a bit about our perception of color. Let’s start filling in some of the middle by looking at how cones respond to light.
This page was last edited on February 3, 2005.
© 2004, 2005, Mickey P. Rowe