Eyes (Level 7)

This is a portion of the MetaCow test image. The left panel of 9 cows shows the appearance to an average human observer under fixed viewing conditions. Each cow is made up of two distinct spectral power distributions (amounts of the different wavelengths) that happen to be indistinguishable to the human eye. The right panel shows the same 9 cows as they appear when captured by a typical digital still camera. Now the front and back halves of the cows are distinguishable because the differences in wavelengths that were invisible to the human show up clearly to the camera. This is because the camera does not have the same sensitivities to different wavelengths that we do. The front and back halves of each cow are considered metameric matches for humans (stimuli that look the same color but have different spectral power distributions).

Why Can't We See All the Colors (Wavelengths) Within One Color?

It's all because of two properties of the human vision system, trichromacy and univariance. Trichromacy refers to the fact that we have three types of cone photoreceptors, each sensitive to a different range of wavelengths. Vision scientists call them long-, middle-, and short-wavelength sensitive cones (LMS as shorthand), but they are sometimes referred to as red-, green-, and blue-sensitive (RGB). Univariance refers to the fact that there are no mechanisms within a single cone type to distinguish between different wavelengths of light. L cones, for example, respond to a range of wavelengths but the reaction produced by absorbing a photon of light is exactly the same regardless of wavelength. It is just the probability of that absorption that defines the cone's response.

The practical result of univariance is that there are many different ways to produce the same response within a cone. We perceive color by comparing the responses of the three cone types and since there are multiple, essentially infinite, spectral energy distributions that can produce the same response within a single cone type, there are also multiple spectral energy distributions that can produce the same combination of cone signals and therefore the same color perceptions. This is known as metamerism; stimuli with different spectral power distributions can produce the same integrated color responses.

This property of metamerism is what allows us to develop fairly simple methods for measuring and producing colors. For example, it is what allows us to have color televisions with only three primaries, red, green, and blue.

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Updated: Dec. 31, 2010