Colorimetry in perspective of the human eye

Colorimetry is the science used to quantify colour in perspective of the human eye

The science of measuring colour and the appearance of colour is vital: Perception of colour is a subjective process where the brain responds to stimuli produced when incoming light reacts with the several different types of cone cells of the human eye. The way people perceive colour is a subjective psychological phenomenon and is based upon the relation between wavelengths of the light detected by the rod and cone cells and the subsequent processing of those signals by neural pathways. Simply said: the same object illuminated by the same light source can be experienced differently by different people.

Understanding Human Tristimulus Vision
The human eye contains two different types of cells that contribute to human vision. Cone cells operate at high and medium levels of brightness, for example during daytime. Rod cells operate typically under low light conditions, for example at night.

Human tri-stimulus vision is based on three different cone cell types, of which each covers a different wavelength range. Each type of cone cells has a specific spectral sensitivity in short (420 to 440 nm), middle (530 to 540 nm)(M), and long (560 to 580 nm, red) wavelengths. These ranges correspond roughly to blue (S), green (M) and red (L) colours. By combining the stimulation of (mixing) each cell type, we can see different colours.

At night or in the dark, human vision relies on rods as cones are not sensitive enough to provide sight. Rods are in contrast to cones much more sensitive but only come in one type instead of three. This single type has its own typical spectral response with a peak sensitivity around 507 nm. As rods only rely on a specific wavelength range which is not combined with other ranges, human night vision is monochromatic. In other words: the human eye is not able to discriminate colours when it’s dark at night.

The CIE 1931 Colour Space

Although the actual average human eye’s response of S, M and L are known, colorimetry relies on other spectral sensitivity curves. Back in 1931 the International Commission on Illumination (CIE, Commission International de l’Éclairage – CIE in French) developed the still widely adapted CIE 1931 colour space based on a series of experiments. Until then, there was no objective method for describing colour in perspective of the human eye. The CIE model describes colours of any light emitting or reflecting object as a 3-dimensional value of wavelength parts that more or less cover colours we know as red green and blue. These values relate to X, Y and Z respectively and are translated into a two dimensional colour space that covers all colours visible to the human eye.

The CIE 1931 colour space was the first model that linked the distributions of wavelengths in the visible part of the electromagnetic spectrum, and colours perceived by the human eye. One should take into account that this model is, alike any other colour model, a mathematical simplification of human (colour) vision and based on a relatively small population. Still, such colour models allow researchers to define and reproduce colours in most conventional situations and should be considered a tool for measuring colour in applications such as display and lighting industry.

Defining a Standard Human Observer

Considering the fact that human colour perception is fundamentally subjective, it is possible to work with standard human observer to objectively assess an object’s colour. There are a number of standard human observers defined in colorimetry, including the 2° field of vision model as represented by the 1931 model. The 2° standard observer focuses on the high concentration of cones in the fovea whereas the 10° field model uses a broader field of view broadening the visual range. The CIE 1931 model 2° uses chromaticity as a function to describe the two important parameters: hue (colour) and purity (saturation). A complete description of the colour can be provided by measuring the luminance and chromaticity.

Chromaticity itself, also excluding the brightness component, can be plotted in a two-dimensional graph. The CIE 1931 Yxy colour space directly transforms every visible colour the human eye can see into a two dimensional horse shoe type layout. If we consider the colour reproduction capabilities of an RGB emitting source, for example a RGB LED lamp, we can determine the exact area the light source can create. Once we know the colour coordinates of each individual (red, green and blue) LED and plot this into the graph, a triangle can be drawn between these coordinates. This triangle is known as the colour gamut of the source. The exact shape of this gamut entirely depends on the technology of the light source. For example: two different RGB LED light sources may cover a slightly different area as colour points of LEDs of different bins may vary.

 

Spectro-Colorimeter

Example of a colour gamut which is defined by the colour coordinates of three primaries red, green and blue. Any colour within the triangle can be made by mixing the primaries located at the vertices.

Applications of Colorimetry

Colour and colour reproduction has become more and more important with evolving technologies. Technological developments allow even faster and more accurate colour measurement which found its way into many applications. For example brand identity has become more and more important to ensure the right colour reproduction for example on TV. This is directly linked to capabilities of colour reproduction on displays, but also in optimizing differences between different displays. For example the huge technological developments in smart phone displays offers brighter and more vivid colour reproductions, but also requires proper and accurate calibration to ensure the exact same colours amongst millions of phones. For such applications, Admesy has developed in-line production colorimeters such as the Hyperion and 2D imaging colorimeters for full display inspection (Atlas series).