LMS color space

LMS is a color space represented by the response of the three types of cones of the human eye, named for their responsivity (sensitivity) peaks at long, medium, and short wavelengths.

It is common to use the LMS color space when performing chromatic adaptation (estimating the appearance of a sample under a different illuminant). It’s also useful in the study of color blindness, when one or more cone types are defective.

theory
All colors can be represented (for a human observer) by the first Grassmann’s law by three primary colors. Therefore, each color shade can be assigned a color location in a three-dimensional vector space. This approach is the abstracted symbolism that was necessary for coloring methods, the colorimetry and technical treatment of colors, such as the color reproduction of this screen. Color spaces are adapted to different tasks and are in use as CIE standard color space, RGB color space, CMYK color space or LAB color space.

Radiation in the visible range directly from a light source or indirectly from a surface exerts a color stimulus. This causes in the three cones of the human organ of vision a color valence, a color value. In the subsequent process in the body, this is perceived as a hue. The term “tristimulus” is used for the “stimulated” reaction of the color centers, although this term is used for the modified standard valences.

For illustration, the “spectral valences” of the pins are shown in the diagram. The values were measured directly on human L, M and S cones, as well as human rods with a microscope spectrometer . In addition, the readings are registered to rhesus monkeys, which were performed by Bowmaker.

The color receptors of each eye have an individual spectral sensitivity. In the process of perception, this is shaped into a specific sensory impression in the nervous system. This applies to every eye, whether animal or human and the subsequent nerve apparatus. Every normal-color person has three types of “color-sensitive” cones. These are referred to as the location of the maximum of their sensitivity as L, M and S cones.

In German-language literature is sometimes set for S-pin K-pin. The L-cones perceive primarily the color stimulus of the radiation from the long-wave red range, the M-cones the middle green area and the S / K-cones the short-wave blue range of the spectrum. The reception system of the sense of sight also includes the rods, English: rods.

Despite individual differences in the spectral absorption properties of these cones, caused for example by genetic variations, and the specific influence of lens or vitreous in the eye, which is determined by personal staining or in age by turbidity, the absorption curves are in good agreement for all normal-sighted people ,

The totality of perceivable color stimuli, ie the colors, is ultimately mapped to these three quantities L, M, S. In the “objective world”, it is spectral distributions that are each with an intensity of 0% to 100% at each (even continuously graded) wavelength between about 380 nm and 780 nm color stimuli.

Occasionally, these three causative color values after the sensation maximum are also denoted by R (ot), G (green), B (lau). Since this can lead to confusion with the coordinates of the RGB color space, P, D, T is also common, whereby the failed receptor is used in color deficient ones, ie P [rotanopia], D [uteropanopia] and T [ritanopie]. Another system uses the Greek letters ρ, γ, β. Rho stands for L- or R-, gamma for M- or G- and beta for S-cones or the blue-sensitive ones.

It can form a three-dimensional vector space, which is spanned by the three axes L, M, S.

A spectral color is a sufficiently narrow section of the spectrum in the colorimetry with the bandwidth Δλ almost 0 nm, in practice at best this width can be 1 nm.

History
The measurement of the individual absorption spectra L (λ), M (λ) and S (λ) is a complex measuring task. The foundations for the CIE systems were laid by the measurements and work of Maxwell, König, Dieterici and Abney, which were summarized in 1922 by the OSA (Optical Society of America) and published in edited form. Since at that time the possibilities and accuracy of the measurements were inadequate, David Wright (1928) and John Guild (1931) independently performed new and more accurate color matches and photometric comparisons, and created a new base of basic data. The respective data agree very well with each other and also confirm the old measurements within the scope of accuracy. In 1931, Wrights and Guild’s data were recommended by CIE International as a database. Stiles, Burch and Speranskaya later provided further data that expanded the system and also confirmed the measurements of Wright and Guild. Bowmaker then used a microscope spectrometer to measure the absorption properties of the cones directly on the object. The direct measurements showed that the LMS sensitivity values, which could only be calculated indirectly up to that point, corresponded very well with the measurement results, ie the actual values.

Since the original LMS color space for technical purposes contains some disadvantages, the pin valences L M S were replaced by the virtual norm valences X Y Z and based on the CIE standard 1931. The number of individuals was limited to a total of 17 selected individuals for these metrological reasons of the 1930s. Guild himself had only performed measurements on 7 people. This is still considered a further disadvantage and potential source of error. Nevertheless, Stiles found in subsequent measurements in 1955 that the data from these 17 individuals represented and ensured adequate representation of the 2 ° standard observer. However, since the CIE standard values have prevailed today, it is mainly corrected with transformations such as the DIN99 color space using computer technology.

To accommodate all normally sighted observers who deviate from the standard observer, there are supplementary data sets (standard deviate observers, standard deviation observers) to the CIE data that apply to both the 2 ° and the 10 ° standard observers.

XYZ to LMS
Typically, colors to be adapted chromatically will be specified in a color space other than LMS. The chromatic adaptation matrix in the von Kries transform method, however, expects the LMS color space. The relationship between the XYZ and LMS color spaces is linear, so the transition is representable by a transformation matrix.

Since the LMS color space is supposed to model the complex human color perception, no single, “objective” transformation matrix between XYZ and LMS exists[dubious – discuss]. Instead, various Color Appearance Models (CAMs) offer various Chromatic Adaptation Transform (CAT) matrices M as part of their modeling of human color perception.

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