Page 39 - Fundamentals of Light Microscopy and Electronic Imaging

22 LIGHT AND COLOR

The brightness of a light wave is described physically and optically in terms of the
amplitude (A) of its E vector, as depicted in a graph of its sine function. Indeed, the
amplitudes of sine waves are shown in many figures in the text. However, the nervous
activity of photoreceptor cells in the retina is proportional to the light intensity (I),
where intensity is defined as the rate of flow of light energy per unit area and per unit
time across a detector surface. Amplitude (energy) and intensity (energy flux) are
related such that the intensity of a wave is proportional to the square of its amplitude,
where

2
I A .
For an object to be perceived, the light intensity corresponding to the object must be dif-
ferent from nearby flanking intensities and thereby exhibit contrast, where contrast (C)
is defined as the ratio of intensities,

C
I/I ,
b

I is the difference in intensity between an object and its background, and I is the inten-
b
sity of the background. If I obj I , as it is for many transparent microscope specimens,
b
C 0, and the object is invisible. More specifically, visibility requires that the object
exceed a certain contrast threshold. In bright light, the contrast threshold required for
visual detection may be as little as 2–5%, but should be many times that value for
objects to be seen clearly. In dim lighting, the contrast threshold may be 200–300%,
depending on the size of the object. The term contrast always refers to the ratio of two
intensities and is a term commonly used throughout the text.

PHYSICAL BASIS FOR VISUAL PERCEPTION AND COLOR

As we will emphasize later, the eye sees differences in light intensity (contrast) and per-
ceives different wavelengths as colors, but cannot discern differences in phase displace-
ments between waves or detect differences in the state of polarization. The range of
wavelengths perceived as color extends from 400 nm (violet) to 750 nm (red), while
peak sensitivity in bright light occurs at 555 nm (yellow-green). The curves in Figure
2-6 show the response of the eye to light of different wavelengths for both dim light
(night or rod vision) and bright light (day or cone vision) conditions. The eye itself is
actually a logarithmic detector that allows us to see both bright and dim objects simulta-
neously in the same visual scene. Thus, the apparent difference in intensity between two
objects I and I is perceived as the logarithm of the ratio of the intensities, that is, as
1
2
log (I /I ). It is interesting that this relationship is inherent to the scale used by Hip-
10 1 2
parchus (160–127 B.C.) to describe the magnitudes of stars in 6 steps with 5 equal inter-
vals of brightness. Still using the scale today, we say that an intensity difference of 100 is
covered by 5 steps of Hipparchus’ stellar magnitude such that 2.512 log 100 5. Thus,
10
5
each step of the scale is 2.512 times as much as the preceding step, and 2.512 100,
demonstrating that what we perceive as equal steps in intensity is really the log of the
ratio of intensities. The sensitivity of the eye in bright light conditions covers about 3
orders of magnitude within a field of view; however, if we allow time for physiological
adaptation and consider both dim and bright lighting conditions, the sensitivity range of
the eye is found to cover an incredible 10 orders of magnitude overall.