Trichromatic Color Representation
 
 
 
Explain
Glossary

Chapter 1. Trichromatic Color Representation

Demonstration 1.1
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You must read each slide, and complete any questions on the slide, in sequence.
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Trichromatic Color Representation

Control the wavelength of a light to see how S-, M-, and/or L-cones respond to the light.

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This image of a human retina reveals the three types of cones: S-cones (blue),
M-cones (green), and L-cones (red).
Photo: Republished with permission of the Society for Neuroscience, from Hofer et al., 2005.
Permission conveyed through Copyright Clearance Center, Inc.

What Is Trichromatic Color Representation?

There are three types of cones; each type contains a different photopigment; and each photopigment has a particular spectral sensitivity. The three types of cones are referred to as L-cones, M-cones, and S-cones, according to whether their peak sensitivity is to a long, medium, or short wavelength, respectively. The spectral sensitivity function of the photopigment in each type of cone is the probability that a photon of light with any given wavelength will be absorbed by that cone's photopigment. You can see from the graph the photopigment in each type of cone is most responsive to a particular range of wavelengths, but all three respond to a wide range of wavelengths, and their spectral sensitivity functions overlap considerably.

Although each type of cone varies in its sensitivity to different wavelengths of light, it's impossible to work backward from the response of a single cone type to determine the wavelength of the light that caused the response. The reason for this is expressed by the principle of univariance as it applies to cones: the absorption of a photon of light causes a fixed response by a cone, regardless of the photon's wavelength—in other words, the strength of the response generated by a cone when it transduces light depends only on the amount of light transduced, not on the wavelength of the light. For example, L-cones have a relative sensitivity of 50% to light with a wavelength of 513 nm and to light with a wavelength of 623 nm. That is, L-cones respond identically to equal amounts of those two wavelengths of light, making it impossible to distinguish those wavelengths based just on the responses of L-cones. This means that color vision depends crucially on the patterns of relative responses of multiple cone types. Since people normally have three cone types, normal color vision is termed trichromatic.
activity_5_3_slide_2

Select the cone types whose responses you want to show, and then use the slider to specify the wavelength of light stimulating the cones. Compare the patterns of responses to various wavelengths. See if you can produce the same pattern in response to two different wavelengths. To save response patterns for comparison, click SAVE.

Relative sensitivity (%)
Wavelength (nm)
100
50
0
380
400
450
500
550
600
650
700
Show
S-cones
M-cones
L-cones
Saved response patterns
Wavelength
Wavelength
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50
0
S-cones
M-cones
L-cones
Relative sensitivity (%)
380 nm
380 nm
535 nm
50%
50%
50%
535 nm
50%
50%
50%
535 nm
50%
50%
50%
535 nm
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50%
50%
535 nm
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50%
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535 nm
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535 nm
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50%
explain_text

What Is Trichromatic Color Representation?

There are three types of cones; each type contains a different photopigment; and each photopigment has a particular spectral sensitivity. The three types of cones are referred to as L-cones, M-cones, and S-cones, according to whether their peak sensitivity is to a long, medium, or short wavelength, respectively. The spectral sensitivity function of the photopigment in each type of cone is the probability that a photon of light with any given wavelength will be absorbed by that cone's photopigment. You can see from the graph the photopigment in each type of cone is most responsive to a particular range of wavelengths, but all three respond to a wide range of wavelengths, and their spectral sensitivity functions overlap considerably.

Although each type of cone varies in its sensitivity to different wavelengths of light, it's impossible to work backward from the response of a single cone type to determine the wavelength of the light that caused the response. The reason for this is expressed by the principle of univariance as it applies to cones: the absorption of a photon of light causes a fixed response by a cone, regardless of the photon's wavelength—in other words, the strength of the response generated by a cone when it transduces light depends only on the amount of light transduced, not on the wavelength of the light. For example, L-cones have a relative sensitivity of 50% to light with a wavelength of 513 nm and to light with a wavelength of 623 nm. That is, L-cones respond identically to equal amounts of those two wavelengths of light, making it impossible to distinguish those wavelengths based just on the responses of L-cones. This means that color vision depends crucially on the patterns of relative responses of multiple cone types. Since people normally have three cone types, normal color vision is termed trichromatic.

You must select an answer and then click SUBMIT before you can click proceed to the next screen.
test_single_choice

Select your answer to the question below. Then click SUBMIT.

What is shown by the spectral sensitivity functions in the graph?

A.
S-cones don't play an important role in color vision.
B.
M-cones and L-cones are sensitive to a wider range of wavelengths than are S-cones.
C.
L-cones have about the same sensitivity to 550-nm light and 650-nm light.
D.
All three cone types respond to all wavelengths in the range of 500–600 nm.
Correct! Click EXPLAIN if you want to review this topic.
Incorrect. The correct answer is B. Click EXPLAIN if you want to review this topic.
You must complete the question and then click SUBMIT before you can click proceed to the next screen.
test_single_choice

Select your answer to the question below. Then click SUBMIT.

All of the following statements are true. Which one indicates why it's impossible to consistently distinguish different wavelengths of light based on the responses of a single cone type?

A.
S-cones don't respond to wavelengths greater than 550 nm.
B.
All three cone types respond to light with a wavelength of 500 nm.
C.
The spectral sensitivity functions of M-cones and L-cones are quite similar.
D.
M-cones have about the same sensitivity to 500-nm light and 600-nm light.
Correct! Click EXPLAIN if you want to review this topic.
Incorrect. The correct answer is D. Click EXPLAIN if you want to review this topic.
You must complete the question and then click SUBMIT before you can click proceed to the next screen.
test_single_choice

Select your answer to the question below. Then click SUBMIT.

M-cones have about the same sensitivity to 500-nm light and 600-nm light. Which of the following statements indicates why people with normal color vision can distinguish lights with these wavelengths?

A.
S-cones respond differently to these wavelengths, and so do L-cones.
B.
M-cones play only a minor role in color vision.
C.
S-cones don't respond to 600-nm light.
D.
600-nm light is usually brighter than 500-nm light.
Correct! Click EXPLAIN if you want to review this topic.
Incorrect. The correct answer is A. Click EXPLAIN if you want to review this topic.
You must complete the question and then click SUBMIT before you can click proceed to the next screen.
test_dnd

Slide 7/Test Yourself 4 Matching Activity

Based on the graph below, drag each label into the blank box with a matching description. Then click SUBMIT.

Peak sensitivity of S-cones

Cone type that doesn't respond to 600-nm light

Wavelength that elicits a greater response from L-cones than from M-cones

Cone type with the least sensitivity to 450-nm light

Cone type that responds about the same to 450-nm light and 640-nm light

Wavelength that only S-cones respond to

Wavelength that elicits equal responses from M-cones and S-cones

S-cones
M-cones
L-cones
390 nm
443 nm
483 nm
650 nm
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Trichromatic Color Representation.
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