Section 35–2 Color depends on intensity

 (Dark-adapted vision / Purkinje effect / Peripheral vision)

 

In this section, Feynman discusses the dark-adapted vision, Purkinje effect (related to mesopic vision), and peripheral vision that depend on the intensity of light.

 

1. Dark-adapted vision:

“If the intensity of the light is very low, the things that we see have no color. It is known that this dark-adapted vision is almost entirely due to the rods, while the vision in bright light is due to the cones. As a result, there are a number of phenomena that we can easily appreciate because of this transfer of function from the cones and rods together, to just the rods (Feynman et al., 1963, p. 35–2).”

 

It is potentially misleading to say “if the intensity of the light is very low, the things that we see have no color.” In very low-light conditions, we rely on our rod cells, which are highly sensitive to low levels of light, to see in shades of gray (or grey). While rod cells do not provide detailed color information, they allow us to distinguish different levels of brightness and perceive objects in a grayscale view. On the other hand, gray is a color because it can be achieved by adding and adjusting the intensity of red, green, and blue light (see below). However, rod cells may take about 20-30 minutes for the human eye to become fully dark-adapted, that is, allowing the rod cells to reach their maximum sensitivity in the dark night.

 

Source: Butler, 2005

Dark-adapted vision primarily refers to scotopic vision, which is the vision that occurs in very low-light conditions. In his autobiography, Feynman (1997) mentions that he could see colors during an atomic explosion in the dark: “… a big ball of orange, the center that was so bright, becomes a ball of orange that starts to rise and billow a little bit and get a little black around the edges, and then you see it's a big ball of smoke with flashes on the inside, with the heat of the fire going outwards. All this took about one minute. It was a series from bright to dark, and I had seen it. I am about the only guy who actually looked at the damn thing--the first Trinity test (p. 134).” In a sense, this is another reason why it is incorrect to say that the things that we see have no color in the dark. (Interestingly, some claim to achieve beatific vision during the dark night.)

 

2. Purkinje effect:

“It turns out that the rods see better toward the blue than the cones do, and the cones can see, for example, deep red light, while the rods find that absolutely impossible to see. So red light is black so far as the rods are concerned. Thus two pieces of colored paper, say blue and red, in which the red might be even brighter than the blue in good light, will, in the dark, appear completely reversed. It is a very striking effect. If we are in the dark and can find a magazine or something that has colors and, before we know for sure what the colors are, we judge the lighter and darker areas, and if we then carry the magazine into the light, we may see this very remarkable shift between which was the brightest color and which was not. The phenomenon is called the Purkinje effect (Feynman et al., 1963, p. 35–2).”

 

Historically, Jan Evangelista Purkinje observed that his favorite flower appeared bright red in the afternoon, but it became bluish-red in the evening. We can explain Purkinje effect as follows: (1) Lighting conditions: It pertains to the shift in perceived colors under different lighting conditions, specifically during transitions from bright light (photopic vision) to low-light conditions (mesopic vision). (2) Color perception: In bright light, shorter wavelengths dominate our perception, making blue and green hues more vibrant, whereas in low-light conditions, longer wavelengths become more prominent. (3) cone cells: The effect occurs due to the different sensitivity of S-cones*, M-cones, and L-cones under different lighting conditions. In short, Purkinje effect may refer to red objects that appear darker under dim lighting conditions compared to green objects due to the reduced sensitivity of the cone cells to red wavelengths (See below).

*Humans have three types of cone cells in the retina, each sensitive to different wavelengths of light: short-wavelength cones (S-cones) are sensitive to blue light, medium-wavelength cones (M-cones) are sensitive to green light, and long-wavelength cones (L-cones) are sensitive to red light.

 

In Fig. 35–3, the dashed curve represents the sensitivity of the eye in the dark, i.e., using the rods, while the solid curve represents it in the light. We see that the peak sensitivity of the rods is in the green region and that of the cones is more in the yellow region. If there is a red-colored page (red is about 650 μm) we can see it if it is brightly lighted, but in the dark it is almost invisible (Feynman et al., 1963, p. 35–2).”

Feynman initially explains the phenomenon using two pieces of colored paper, blue and red, but elaborates the spectral sensitivity of the eye involving the cones that is more in the yellow region. However, it could be explained in terms of photopic vision and scotopic vision: (1) Photopic vision: When our eyes are exposed to bright light conditions, such as well-lit indoor environments, they are in a state of photopic vision. In photopic vision, our eyes (cones) are most sensitive to the yellow-green part of the spectrum, about 555 nanometers (nm) wavelength. (2) Scotopic vision: When our eyes are exposed to very low-light conditions, they gradually adapt to scotopic vision, which is highly sensitive to dim light. In scotopic vision, our eyes (rods) are most sensitive to about 505 nm wavelength, which corresponds to the color green on the visible spectrum.

 

3. Peripheral vision:

“Another interesting phenomenon is that the periphery of the retina is very sensitive to motion. Although we cannot see very well from the corner of our eye, if a little bug moves and we do not expect anything to be moving over there, we are immediately sensitive to it. We are all “wired up” to look for something jiggling to the side of the field (Feynman et al., 1963, p. 35–3).”

 

The phenomenon described by Feynman is related to the distribution of rod cells in the human retina and how it affects our peripheral vision. We may define peripheral vision (or side vision) as follows: 1. Field of vision: It covers a wider field of view compared to central vision. 2. Sensitivity to motion: It is highly sensitive to motion and is crucial for detecting movement in the surrounding environment due to a higher density of rod cells. 3. Lower acuity and color perception: It has lower visual acuity and reduced color perception compared to central vision due to a lower density of cone cells. Thus, the periphery of the retina is better suited for detecting motion and objects in dim lighting conditions.

 

“Another interesting effect of the fact that the number of cones decreases as we go farther to the side of the field of view is that even in a bright light color disappears as the object goes far to one side. The way to test that is to look in some particular fixed direction, let a friend walk in from one side with colored cards, and try to decide what color they are before they are right in front of you. One finds that he can see that the cards are there long before he can determine the color. When doing this, it is advisable to come in from the side opposite the blind spot, because it is otherwise rather confusing to almost see the color, then not see anything, then to see the color again (Feynman et al., 1963, p. 35–3).”

 

It is unclear if one can almost see the color of moving cards, then not see anything, and see the color again while we are looking in a fixed direction. Firstly, the phenomenon related to the blind spot typically occurs when observing a stationary object, not a moving one. When the eyes are fixated on a specific point, the brain fills in the missing information from the blind spot, creating the illusion that the object is still visible, even though it is not being detected by the photoreceptor cells in the blind spot area. This occurs seamlessly and almost instantaneously, so one does not notice the gap in the visual field due to the blind spot. On the other hand, the decrease in the number of cones and the dominance of rod cells in the peripheral vision result in reduced color perception and decreased visual acuity in the outer edges of the visual field. This effect can make colors less noticeable as the object moves toward the side of our visual field.

 

Feynman initially mentions that another interesting effect is: even in a bright light, color disappears (instead of almost disappear) as the object goes far to one side. This is related to the near absence of cones and abundance of rods in the outer part of retina that is the reason for our not getting color or detailed information from our peripheral vision. However, a more interesting phenomenon is Troxler’s fading, which occurs when the eyes fixate on a particular point. As a result, details of objects located in the periphery of the fixated point can fade away or become less visible. One common example of Troxler's fading is staring at a small, high-contrast object (like a dot) in the center of a large, uniform background as shown below.

Source: Pegoraro, 2016

 

Review Questions:

1. How would you explain dark-adapted vision and whether one may see colors under very low light conditions?

2. How would you explain the Purkinje effect?

3. How would you define peripheral vision under normal light conditions?

 

The moral of the lesson: human vision depends on the intensity of light because it is based on rods (sensitive to dim light) and three types of cone cells: S-cones (sensitive to blue light), M-cones (sensitive to green light), and L-cones (sensitive to red light).

 

References:

1. Butler, Y. (Ed.). (2005). The Advanced Digital Photographer's Workbook: Professionals Creating and Outputting World-class Images. Taylor & Francis.

2. Feynman, R. P. (1997). Surely You’re Joking, Mr. Feynman! : Adventures of a Curious Character. New York: Norton.

3. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

4. Pegoraro, V. (2016). Handbook of Digital Image Synthesis: Scientific Foundations of Rendering. CRC Press.