The three curves in the figure above shows the normalized response
of an average human eye to various amounts of ambient light. The
shift in sensitivity occurs because two types of photoreceptors called
cones and rods are responsible for the eye's response to light.
The curve on the right shows the eye's response under normal lighting
conditions and this is called the photopic response. The cones
respond to light under these conditions.
As mentioned previously, cones are composed of three different
photo pigments that enable color perception. This curve peaks
at 555 nanometers, which means that under normal lighting conditions,
the eye is most sensitive to a yellowish-green color. When the
light levels drop to near total darkness, the response of the
eye changes significantly as shown by the scotopic response curve
on the left. At this level of light, the rods are most active
and the human eye is more sensitive to the light present, and less sensitive to the range of color. Rods
are highly sensitive to light but are comprised of a single photo
pigment, which accounts for the loss in ability to discriminate
color. At this very low light level, sensitivity to blue, violet,
and ultraviolet is increased, but sensitivity to yellow and red
is reduced. The heavier curve in the middle represents the eye's
response at the ambient light level found in a typical inspection
booth. This curve peaks at 550 nanometers, which means the eye
is most sensitive to yellowish-green color at this light level.
Fluorescent penetrant inspection materials are designed to fluoresce
at around 550 nanometers to produce optimal sensitivity under
dim lighting conditions.
Robinson, S. J. and Schmidt, J. T., Fluorescent Penetrant Sensitivity
and Removability - What the Eye Can See, a Fluorometer Can Measure,
Materials Evaluation, Vol. 42, No. 8, July 1984, pp. 1029-1034