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Can spectrophotometry replace colorists?

Can spectrophotometry replace colorists?

Each year, new models of cars appear on the market, and with them come new colors. Automotive designers have something in common with fashion designers – they all want to surprise the customer with something new and different. New shapes, a better engine, or maybe a unique color with special effects. And that’s where the trouble begins. Matching colors is often difficult due to the different effects applied, and to differences in shade depending on the angle of light. To make color matching easier, more and more often automotive companies are using spectrophotometers, devices for measuring color.

What exactly is color?

The word “color” appears in daily use, but in fact it can be understood in many ways. For chemists, it is an expression of pigments, dyes and other similar substances. Physicists on the other hand define color as a measurement of its optical properties. For a colorist, the word simply means what he or she sees when waves from the visible electromagnetic radiation spectrum reach the eye. The human eye contains photoreceptors, cells built of light-sensitive proteins called opsins which absorb light waves of various lengths. The brain then receives and processes the information sent by these cells, transforming it into a three-dimensional image. There are three types of photoreceptors in the eye which allow the brain to differentiate three basic colors; red, green, and blue.   

How do we “see” color?

We are able to “see” a given color by means of impulses which stimulate the different types of photoreceptors in different ways. A special situation occurs in the case of black and white. We see white if the light waves which reach the eye stimulate all of the photoreceptors with the same intensity. Black is seen if none of these photoreceptors are stimulated. 

Absorption of light of different wavelengths by photoreceptors

Figure 1. Absorption of light of different wavelengths by photoreceptors      

                 It has been proven that women see colors better than men. This fact results from the positioning of genes responsible for seeing colors. The genes which are responsible for perception of red and green are located very close to one another on the X chromosome. As you probably know, men have only one such chromosome. Any instance of damage to this gene results in the inability to correctly perceive colors. There is no possibility for a second X chromosome to correct this defect, as is the case with women. Moreover, some women have mutations which result in the appearance of four types of photoreceptors in the eye instead of three, giving them an even greater sensitivity to colors. This feature is not solely limited to human beings; most male monkeys can only see dichromatically, or in two colors – they have only one X chromosome which generates only one type of opsin. The females on the other hand have two X chromosomes and can inherit slightly different genes which generate different opsins, allowing them to see trichromatically, or in three colors.

How does a spectrophotometer differentiate between colors?

Increasingly, spectrophotometers are used to assess and differentiate between colors. How is it possible that an electrical device can be made to see colors since it does not have photosensitive cells or chromosomes?

Reflection of light by metallic flakes
Figure 2. Reflection of light by metallic flakes
One essential component of the construction of a spectrophotometer is its light source, usually an LED diode. Light beams shaped by the diode hit a sample at a defined angle and are next deflected onto a series of sensors. A high-precision spectrophotometric analysis takes place along each wavelength segment, and the result is presented as a series of numbers which describe the visible light spectrum. Spectrophotometers with various types of measurement geometry are available on the market. The measurement result depends on lighting conditions, observation angle and the angle at which light falls on the sample. For correct determination of the color, it is necessary to use at least three angles of measurement. Spectrophotometers with 6, and even 12 angles of measurement are available, and are sometimes equipped with cameras.   Nearly 80% of the colors used in automotive paints have special effects, which produce a different appearance depending on the angle at which light falls on them.

Multi-angle measurement with a spectrophotometer
Figure 3. Multi-angle measurement with a spectrophotometer

Light is reflected from a metallic surface at different angles, depending on the orientation of the flakes of metal in the layer of paint. Thus, it is highly recommended to use a multi-angle spectrophotometer on paints with special effects. These devices are equipped with a function which measures grain size, as determined by the diameter of the particles in the paint, as well as the orientation of these grains. It is also possible to measure the so-called sparkle effect. Many spectrophotometers are fitted with onboard memory, allowing measurements to be stored. The measurement data can be displayed in numerical form, in graphs, or in charts in order to give a clearer image of the colors. It is possible to achieve very satisfying results with the proper environmental conditions.  

Can a spectrophotometer replace the human eye?

This question reminds me of the discussion on artificial intelligence. We can safely say, “No, let’s not worry that a machine will be able to see the world the way that people do; let’s worry that people will not be able to see the world the way that machines do.”