4.8 Trichromatic Theory and Color Perception

Trichromatic Theory Definition and Overview

• Trichromatic theory explains color perception through the activity of three types of cone photoreceptors in the human eye.

• The theory posits that each cone type absorbs a different range of wavelengths of light:

  • Long Wavelength Cone Type:

  • Corresponds primarily with red and orange light.

  • Medium Wavelength Cone Type:

  • Corresponds primarily with green light.

  • Short Wavelength Cone Type:

  • Corresponds primarily with blue and violet light.

Mechanism of Color Perception

• The perception of color arises from the combined activity of these three cone types.

• Graph Representation:

  • On the y-axis, the graph shows the amount of light absorbed by each cone type.

  • On the x-axis, it shows different wavelengths of light.

• For example:

  • Short wavelength cone absorbs the most light at 420 nanometers, indicating a peak sensitivity to blue light.

  • Light absorption decreases as wavelengths deviate from the optimal value for each cone type.

Importance of Multiple Cone Types

• Hypothetical Scenario of One Cone Type:

  • If humans had only the long wavelength cone:

  • Color such as red (e.g., 630 nanometers) would activate that single cone type.

  • Another color, cyan (also causing similar activity), creates ambiguous signals leading to confusion because the brain cannot differentiate between red and cyan.

• Introducing Other Cone Types:

  • When incorporating medium and short wavelength cones, the patterns of absorption become distinct for red and cyan:

  • Red: High activity in long wavelength cone, slightly in medium wavelength cone, none in short wavelength cone.

  • Cyan: High activity in long wavelength cone, high activity in short wavelength cone, slightly in medium wavelength cone.

  • This unique pattern of activity allows the brain to correctly disambiguate between colors.

Unique Signatures of Colors

• Each color has a unique signature determined by the level of activity in each type of cone:

  • Example of Blue:

  • Low in long wavelength cone.

  • Medium in medium wavelength cone.

  • High in short wavelength cone.

• This pattern is distinguishable from other colors such as yellow, green, red, or white.

Phenomenon of Colors Without Wavelengths

• Some colors, like magenta, do not correspond to a specific wavelength of light:

  • Explanation for Magenta Perception:

  • Medium wavelength light is not activated, but light from the short and long wavelength sections is perceived.

  • This creates a unique stimulation pattern as there is no single wavelength to stimulate the medium wavelength cone.

  • The activation occurs when both the long and short wavelength cones are active, but the medium cone is inactive, leading to the perception of magenta.

Conclusion

• Trichromatic theory emphasizes how color perception relies on the complex interactions of three types of cones, leading to the rich variety of colors we perceive, along with colors that exist outside the typical spectra, enhancing our understanding of visual perception.