GIS : Classes of photosensors

Posted on May 5, 2007 By : Mizake Laziaf
Where do the properties of the eye get involved?

It's know that the eye does not see all wavelengths equally. The eye has two general classes of photosensors, cones and rods." 

Cones:
The cones are responsible for light-adapted vision; they respond to color and have high resolution in the central foveal region. The light-adapted relative spectral response of the eye is called the spectral luminous efficiency function for photopic vision, V(l) or V(wavelength). This empirical curve, first adopted by the International Commission on Illumination (CIE) in 1924, has a peak of unity at 555 nm, and decreases to levels below 10–5 at about 370 and 785 nm. The 50% points are near 510 nm and 610 nm, indicating that the curve is slightly skewed. The V(l) curve looks very much like a Gaussian function; in fact a Gaussian curve can easily be fit and is a good representation under some circumstances. I used a non-linear regression technique to obtain the following equation:

Vlambda.gif (557 bytes)
More recent measurements have shown that the 1924 curve may not best represent typical human vision. It appears to underestimate the response at wavelengths shorter than 460 nm. Judd (1951), Vos (1978) and Stockman and Sharpe (1999) have made incremental advances in our knowledge of the photopic response.

Rods:
The rods are responsible for dark-adapted vision, with no color information and poor resolution when compared to the foveal cones. The dark-adapted relative spectral response of the eye is called the spectral luminous efficiency function for scotopic vision, V’(l). This is another empirical curve, adopted by the CIE in 1951. It is defined between 380 nm and 780 nm. The V’(l) curve has a peak of unity at 507 nm, and decreases to levels below 10–3 at about 380 and 645 nm. The 50% points are near 455 nm and 550 nm. This scotopic curve can also be fit with a Gaussian, although the fit is not quite as good as the photopic curve. My best fit is

Vlambda'.gif (577 bytes)
Photopic (light adapted cone) vision is active for luminances greater than 3 cd/m2. Scotopic (dark-adapted rod) vision is active for luminances lower than 0.01 cd/m2. In between, both rods and cones contribute in varying amounts, and in this range the vision is called mesopic. There are currently efforts under way to characterize the composite spectral response in the mesopic range for vision research at intermediate luminance levels.

The Color Vision Lab at UCSD has an impressive collection of the data files, including V(l), V’(l), and some of the newer ones that you need to do this kind of work.

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