Tristimulus colour values are numerical representations of colour perception, describing the responses of three types of cone cells (L, M, and S) in the human eye to different wavelengths of light.
These values serve as the foundation for various colour spaces, such as LMS and XYZ, and are essential for accurately measuring, representing, and matching colours in the field of colour science, colour matching, and colour management.
Tristimulus colour values
- Tristimulus values are the backbone of colour measurement whether in terms of the physiological response of the human eye to light or within the world of colour matching or colour management.
LMS tristimulus colour values
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- LMS tristimulus colour values form the foundation for measuring and representing colour perceptions within the LMS colour space. The system is based on the premise that any colour can be described physiologically by measuring the response of L, M, and S cone cells in the human eye’s retina to different wavelengths of light.
- LMS tristimulus colour values have a genuine association with the range of colours that fall within the observable visible spectrum of a typical human observer.
- LMS tristimulus colour values have three components corresponding to the response of the L, M, and S cone types. Each response is measured on a scale with values between 0 and 1.
XYZ tristimulus colour values
- XYZ tristimulus colour values are equivalent to LMS colour values. The CIE (1931) XYZ colour space utilizes XYZ tristimulus colour values as the basis of the CIE colour system, which has become the global standard for conveying accurate colour information worldwide.
- XYZ tristimulus colour values have a virtual correspondence with observable colours, meaning that some colours are hypothetical and require adjustments to account for variations in brightness. For instance, fully saturated yellow, green, or cyan may appear much lighter than red or blue.
- XYZ tristimulus colour values correspond with the response of the L, M and S cone types.
- Tristimulus colour values are colour-matching functions insofar as they allow you to predict the corresponding colour experience when you know a tristimulus value.
LMS tristimulus colour values & the human eye
- The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in:
- Short wavelengths: S = 420 nm – 440 nm.
- Middle wavelengths: M = 530 nm – 540 nm.
- Long wavelengths: L = 560 nm – 580 nm.
- Every human colour sensation can be explained in terms of the stimulus each cone type receives.
- The LMS cone cells underlie human colour perception in conditions of medium and high brightness.
- However, in very dim light, colour vision diminishes, and the low-brightness, monochromatic “night vision” receptors, known as “rod cells,” become effective.
- The three parameters denoted as “S”, “M”, and “L” are represented in a 3-dimensional space known as the “LMS colour space,” which is one of many colour spaces designed to quantify human colour vision.
- The LMS colour space was the subject of intense scientific study during the 1920s because it established a direct link between the subjective human experience of colour and wavelengths of the visible spectrum.
- There were technical problems interpreting the LMS colour space, which led to the development of the CIE 1931 colour space. In the CIE 1931 colour space, LMS tristimulus values are denoted by X, Y, and Z tristimulus values.
- One of the most important innovations associated with the CIE 1931 colour space is the CIE xy chromaticity diagram.