The Nature of Color. Color Perception. Interpretation of Analysis Results Obtained Using Visual Colorimetric Test Systems

In the course of daily life, humans are in constant contact with the surrounding world. One of the properties of the material world is color, perceived by humans as a conscious visual sensation. Color perception occurs as a result of the eye being exposed to streams of electromagnetic radiation within the wavelength range of 380 to 780 nm. All types of radiation outside this range are not perceived by the human eye.

If a beam of white light is directed through a glass prism, it refracts and splits into several beams, from which bands of violet, blue, cyan, green, yellow, orange, and red can be distinguished. This combination of colored bands is called a spectrum. Each band corresponds to its own electromagnetic wave, characterized by a specific wavelength. The waves themselves have no color. The perception of visible electromagnetic radiation occurs because the retina of the human eye contains receptors capable of detecting this radiation. Scientific research has established that all colors the human eye can perceive can be obtained by mixing three primary colors-red, green, and blue. If three monochromatic light sources (red, green, and blue) are taken and their beams are overlapped partially, new colors will appear at the intersections. Blue and green produce cyan, green and red produce yellow, and red and blue produce magenta. At the center, where all beams intersect, white light is formed. Thus, white is considered the sum of all colors. The opposite of white is black, which is considered the complete absence of light. By varying the brightness of light sources and combining them, a wide variety of color shades can be obtained.

In darkness, the color of all objects and the surrounding space is black. Objects in our world become visible and take on color only in the presence of a light source. The color of an object is determined by the color of the reflected spectral wave, which in turn depends on the molecular structure of the object. If an object absorbs the entire spectrum of light waves falling on it, it appears black. If it reflects the entire spectrum of visible waves, it appears white. The color of an object also depends on the color of the light source. If the light source is a red LED, all objects illuminated by it will appear only in red, gray, or black. Thus, an object’s color is determined by the amount of absorbed and reflected light illuminating it. For example, green grass appears green because, when illuminated by white light, it absorbs red and blue spectral waves while reflecting the green wave.

Each person has an individual color perception, which is associated with varying numbers and sensitivities of eye receptors. Color perception changes with age and depends on visual acuity. However, these differences mainly concern subtle color shades, so with some approximation, it can be said that most people with normal vision perceive primary colors similarly.

One feature of the human eye is its constantly changing sensitivity. The eye adapts to environmental conditions. For example, if a person spends a long time in a room with intense red light, upon exiting into a normally lit room, surrounding objects will temporarily take on a greenish tint. This occurs because the pigment in certain groups of light-sensitive eye receptors breaks down when stimulated. Over time, the pigment regenerates, but this does not happen instantly.

Another feature of color perception is the effect of successive contrast, which arises from a sudden change in visual stimuli and is related to the eye’s inertia. If a bright object is observed and then the gaze is shifted to a uniform color field, a lighter (positive) image will first appear, followed by a darker negative one. If one stares at a red object and then looks at a white field, a phantom green image will appear.

Additionally, there is the effect of visual contrast, where an object appears darker on a light background and lighter on a dark background:

Finally, another type of contrast is edge contrast, also known as the Mach effect. At the boundary of two fields of different brightness, the adjacent part of the dark field appears darker, while the light field appears lighter.

The above features of color perception are normal. However, deviations exist, known as “color blindness.” The retinas of people with normal color vision contain pigments sensitive to red, green, and blue. If one of these pigments is missing, a person can only distinguish two primary colors. If two pigments are missing, the person sees the world in shades of a single color. Color blindness can be congenital (inherited) or acquired (due to aging, cataracts, certain medications, or injuries affecting the retina and optic nerve). To detect color blindness, ophthalmologists use polychromatic tables consisting of colored circles arranged to form numbers. People with color vision deficiencies cannot recognize the numbers indicated in these tables.

Visual colorimetric analysis methods are based on comparing the color of an indicator element with a color scale (a computer-simulated set of reference samples). When interpreting results obtained using visual colorimetric test systems, the physical nature of color must be considered. The comparison of the indicator zone’s color with the color scale should be conducted under normal workplace lighting. The indicator zone should be illuminated by the full spectrum of visible light, which is easiest to achieve under natural (sunlight) conditions. Under artificial lighting, abnormal coloration of indicator zones may occur due to the limited spectrum of light sources. The image below shows the difference in colors observed under natural lighting (top) and fluorescent lighting (bottom).

To obtain reproducible results, it is recommended to conduct a series of preliminary analyses using known control concentrations under conditions as close as possible to the actual work environment to determine whether the workspace meets the requirements for visual colorimetric analysis methods.