(Young-Helmholtz theory /
Dichromatic color blindness / Spectral
sensitivity curves)
In this section, Feynman discusses Young-Helmholtz theory of
color vision, dichromatic color blindness, and spectral
sensitivity curves of a
normal trichromat’s receptors. In a sense, the title of
the section “the mechanism of
color
vision” may imply the interaction of light, photoreceptor cells in the retina,
three types of cone cells, and complex processing in the eye-brain system. However,
the trichromatic theory and
the opponent-process theory (instead of Young-Helmholtz theory) help explain how the eye-brain system perceives and interprets a wide spectrum of colors and color
blindness.
Alternatively, the section could be titled as “Three types of cone visual
pigments” that are closely related to dichromatic color blindness and spectral
sensitivity curves.
1. Young-Helmholtz theory:
“The simplest theory, proposed by Young and Helmholtz, supposes that in
the eye there are three different pigments which receive the light and that
these have different absorption spectra, so that one pigment absorbs strongly,
say, in the red, another absorbs strongly in the blue, another absorbs in the
green (Feynman et
al., 1963, p. 35–7).”
Historically, Young and Helmholtz did not propose that the three
different cone-pigments are primarily sensitive to red, green, and blue. In 1802, Young initially thought the eye required
receptors that were sensitive to three principal colors (red, yellow,
and blue). In “Chromatics” (an entry in Encyclopaedia
Britannica), Young (1817) proposed that the three primary colors are red,
green, and violet. Building on Young’s theory, Helmholtz classified the cone
photoreceptors as short (violet), middle (green), and long (red). In Handbuch
der Physiologischen Optik, Helmholtz (1866) writes, “In the eye there are
three types of nerve fibers. Stimulation of the first one excites the sensation
of red, stimulation of the second the sensation of green, stimulation of the
third the sensation of violet (Valberg, 2007, p.
278).”
“Now if we adjust the
brightness or the intensity of one color against the other, there comes an
intensity where the flicker at 16 cycles disappears… It is possible to match
two colors for “equal brightness” by this flicker technique. The results are
almost, but not exactly, the same as those obtained by measuring the threshold
sensitivity of the eye for seeing weak light by the cones. Most workers use the
flicker system as a definition of the brightness curve (Feynman et al., 1963, p. 35–8).”
We may use the term, flicker
fusion, which refers to the phenomenon where the eye perceives a continuous image
(or still image) when presented with a rapid succession of discrete images (or
flickering image), typically above a certain frequency threshold. Feynman suggests that we
can adjust the brightness of one color against the other such that the flicker
disappears at 16 Hz, but the eye may perceive visual
flicker artifacts at rates over 500 Hz when the image includes high frequency
spatial edges (Davis, Hsieh, & Lee, 2015).
However, some opin that the frame rate of computer displays should be
72 Hz to avoid flicker completely (Barten, 1999). (Standard-definition television may
operate at 25 or 30 frames per second, or sometimes at 50 or 60 half-frames per
second.) In short, flicker fusion could be related to the
Talbot-Plateau law, which describes the conditions under which the perceived
brightness of a flickering image will appear to be equal to the brightness of a
still image.
2. Dichromatic color blindness:
“By
measuring all these types we can determine the three curves! It turns out that
there are three types of dichromatic color blindness;
there are two common types and a third very rare type, and from these three it
has been possible to deduce the pigment absorption spectra (Feynman et al., 1963, p. 35–8).”
Feynman explains that there are three types of
dichromatic color blindness. However, color
blindness can be categorized as monochromatism,
dichromatism, and anomalous trichromatism. Firstly, monochromatism (total color
blindness) refers to the condition characterized by the total inability to
perceive color. Secondly, dichromacy includes
protanopia, deuteranopia, and tritanopia, where one type of cone is
non-functional, leading to difficulties in perceiving or distinguishing certain
colors. Lastly, anomalous trichromatism refers
to conditions where there is an abnormality in two
types of cones, often leading to a variation in color perception but not
complete color blindness. In addition, there are variations within the sub-categories,
such as protanomaly, deuteranomaly, or tritanomaly, which refer to a reduced
sensitivity of the cone cells instead of a complete absence (see figure below).
Source: What Is Color Blindness? Condition and Types Explained (verywellhealth.com)
“Figure 35–6 shows the color mixing of a particular
type of color-blind person called a deuteranope. For him, the loci of constant
colors are not points, but certain lines, along each of which the color appears
to him to be the same. If the theory that he is missing one of the three pieces
of information is right, all these lines should intersect at a point (Feynman et al., 1963, p. 35–8).”
Feynman says that the loci of constant colors for a color-blind
person are not points, but certain lines along each of which the color appears
to him to be the same and all these lines should intersect at a point. Specifically,
these lines of confusion* cannot be distinguished (or confused) by the
protanope or deuteranope, are also known as pseudo-isochromatic lines. One may
clarify that these apply not only to the colors on the confusion lines, but all
the colors between any two closest lines, especially under certain lighting
conditions. Furthermore, we may adopt the term copunctal
point, which refers to the convergence point of these
confusion lines outside the chromaticity diagram. This is a theoretical reference
point where all the confusion lines meet or intersect.
 |
| Source: Color blindness - Wikipedia |
In the Audio Recordings* [46 min: 00 sec] of this lecture,
Feynman says: “lines of confusion” instead of “loci of constant
colors.”
*The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html
“If we carefully measure on this graph, they do intersect
perfectly. Obviously, therefore, this has been made by a mathematician and does
not represent real data! (Feynman et
al., 1963, p. 35–8).”
The
co-punctal-point of the CIE diagram could be attributed to James Clerk Maxwell.
In a letter dated Jan. 4, 1855 to G. Wilson, J. C. Maxwell writes, “If we find two
combinations of colors which appear identical to a color-blind person, and mark
their positions on the triangle of colors, then the straight line passing
through these points will pass through all points corresponding to other colors,
which, to such a person, appear identical with the first two. We may in the
same way find other lines passing through the series of colors which appear
alike to the color-blind. All these lines either pass through one point or are
parallel, according to the standard colors which we have assumed, and the other
arbitrary assumptions we may have made. Knowing this law of color-blind vision,
we may predict any number of equations which will be true for eyes having this
defect.” Maxwell was a Scottish physicist, but he was also known as a
mathematician.
3. Spectral sensitivity curves:
“Yustova gets approximately the same position in this case. Using the
three different kinds of color blindness, the three pigment response curves
have finally been determined, and are shown in Fig. 35–8 (Feynman et al., 1963, p. 35–9).”
It could be confusing to some that Feynman mentions pigment
response curves, but the caption of Fig. 35–8 is “The spectral sensitivity curves of a
normal trichromat’s receptors.” However, the spectral sensitivity curves are
essentially a representation of how the human visual system responds to light
across the spectrum, and they are due to the responses of color-sensitive
pigments in the cones of the retina. Interestingly, Feynman explains that the
spectral sensitivity curves were obtained using an ophthalmoscope in the
next section. This experiment relied on the subjective judgment
of the observer to interpret the reflection of light from the retina and
determine the spectral sensitivity. Thus, Feynman adds that “[e]ven today it can be said
that the color pigments of the cones have never been obtained in a test tube (Feynman
et al., 1963, Section 35–6).”
Spectral
sensitivity curves represent the response of the eye's different types of cones
to varying wavelengths of light. There are at least three aspects that make
them somewhat arbitrary: (1) Individual differences: Spectral sensitivity
curves are based on averages derived from studying groups of individuals with normal
color vision. (2) Experimental limitations: Color matching experiments or
testing with different monochromatic lights have their constraints, and the
accuracy of the measurements might be influenced by the experimental setup
used. (3) Environmental factors: Factors such as lighting conditions,
adaptation to different light levels, and background colors can influence cone
responses. In essence, the spectral sensitivity curves are not entirely
arbitrary but are based on measurements obtained through the experiments.
No comments:
Post a Comment