Civil Engineering Reference
In-Depth Information
The rods and the cones have different sensitivity to wavelengths. The cones are
sensitive to the entire light spectrum from 380 to 720 nm. Rods on the other hand are not
so sensitive to the upper or red part of the spectrum; their sensitivity is limited to 400 to
about 560 nm (see Figure 4.6). The maximum sensitivity for the cones is at a wavelength
of about 510 nm, corresponding to yellowish green, and for the rods at 510 nm,
corresponding to green (perceived as black/white). Above the cone threshold curve (level
4), colors are visible. Below the rod threshold curve, nothing is visible—it is too dark. In
the area between the cone curve and the rod curve there is black and white vision.
As the illumination drops, there is a shift in color, so that the perceived colors become
more and more greenish. Above the cones sensitivity curve, at level 4 in the figure, only
green is visible. Red is not visible—not even by the rods.
FIGURE 4.6
Relative sensitivity of
rods and cones.
