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The human eye does not have the ability to sense light objectively – which is one of its major shortcomings. Also, it cannot quantify absolute luminance levels due to its automatic adaption to different light levels.
The eye encounters the same problem when quantifying color, which is a key characteristic of light.
The Color of Light
Triggering the three cone types in various proportions enable people to perceive millions of different colors. How these colors are actually perceived varies between persons as each person’s eyes show a different physical response. Moreover, the surrounding conditions play an equally important role, for instance, the spectrum of daylight at sunrise, noon, or sunset is totally different.
A green leaf seems to have different colors during a day. Still, people will perceive the color to be the same as their brain automatically compensates for the influence of the light source by ‘knowing’ what color the object should be.
Texture, cultural background, size and the presence of other colors can all have an influence on how people see a color. This reveals that keeping the conditions constant is key for accurate color measurement. Next to the influenced and biased human vision of color perception, communicating perceived colors is limited a person’s vocabulary and often relates only to what is objectively known as hue.
A general way of understandable color communication is based on three parameters: hue, chroma and value. Hue refers to what people commonly talk about as color and explains whether something is yellow, red, blue etc. Chroma explains the saturation or vividness of a color. Value or lightness describes the brightness of an object in comparison with a perfect white reference.
Differences in Color and Reproduction
Keeping the conditions constant during color assessment is key as colors may appear differently under different types of lighting. Even two light sources that seem to have the same color as they trigger the cones in the eye in a similar way may result in objects being perceived differently.
Since white light sources can have different spectral distributions, the mixture of wavelengths reflected by an object may be different, which has an influence on the perceived color. This effect is called metamerism. It is necessary to have light sources and good color rendering capabilities for the accurate reproduction and assessment of colors.
A long-term standard used to describe the rendering capabilities of a light source is called the color rendering index (CRI), Ra, which can have a value of up to 100.
Light sources with a continuous spectrum, encompassing all wavelengths of the visible spectrum, typically have perfect rendering capabilities leading to high Ra value. For instance, the Ra value of daylight is 100. Light sources with discontinuous spectrums that lack parts of the visible spectrum logically have lower Ra values. Still, different types of light sources, both continuous and discontinuous, can appear white to th human eye like the examples below.
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This information has been sourced, reviewed and adapted from materials provided by Admesy.
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