How do we see colored light

2. Light and color perception

Our primary organ of perception for processing light stimuli is the eye.

Fig. 2: Schematic representation of the visual process

2.1. Visual process

The light waves emitted or reflected directly by an object are bundled by the lens of the eye and projected onto the retina through the vitreous humor. There they stimulate specialized nerve cells that are specialized in detecting such light waves. As a result of the irritation, an electrical signal is generated in the nerve cells, which is conducted into the visual center of the brain via a conduction path, the optic nerve. The actual visual impression is created there by mechanisms that are still very little understood.

Our eye has the task of creating a picture of our environment that is as bright and detailed as possible. On the one hand we have the light sensitivity (bright picture) and on the other hand the resolution (detailed picture). Both cannot be maximized at the same time. The highest resolution is only achieved if the light sensitivity is not limited - i.e. on the brightest day. However, when it comes down to capturing every available light quantum - during twilight or under water - the resolution and thus the wealth of spatial details in the image must recede.

2.2. Color perception, rods and suppositories

The process of light perception is directly linked to color perception. Whenever we see light, we see colors. The perception of different color nuances helps us to interpret our surroundings. Shapes and materials can only be differentiated through colors. So we perceive our environment in color. This is based on the fact that light is electromagnetic radiation and has different wave properties. Our eyes can recognize these different wavelengths as color information.

So that our eyes can perceive the light and color information, there are several light receptors on the retina: so-called rods and cones. That only light-sensitive rods are distributed over the entire retina, while the color-sensitive suppositories lying mostly in the middle of the retina. Objects at the outermost edge of the field of vision are therefore only seen in gray. When the brightness is low, the uvula recede and the rods protrude. This is why colored objects often appear gray at dusk. The impressions are mainly transmitted by the uvula and you can see in color only when the brightness is sufficient. During the transition from daytime vision to twilight vision, the sense of color fails. Our twilight vision is therefore not capable of color either. Since we only approx. 7 million suppositories (Color receptors), while having color perception approx. 100 million chopsticks (Brightness receptors) ensure that we perceive light, we are much more sensitive to light than we are color-sensitive.

The characteristic color sensation that is caused by a light stimulus of a certain wavelength is called color tone. How is it possible that man can distinguish so many shades of color? Color differentiation is possible because there are three types of cones, each of which reacts to different wavelengths of light. One type of cone reacts particularly strongly to light in the short-wave range, the second to light in the medium-wave range and the third to light in the long-wave range. If light of a certain wavelength hits the cones, the cones form an energy potential. This energy potential is then passed on to the brain as a color stimulus via the nerve tracts. The brain then responds with a color sensation. If the short-wave light rays predominate, the color sensation is blue, if the medium-wave light rays predominate, the color sensation green, and if the long-wave light rays predominate, the color sensation red have suspected; namely on the objects in our environment.

Rather, color arises as a kind of sensory perception in our brain and is the answer to a color stimulus that was received by the eyes.

To understand this, we have to look more closely at the process of action between light, matter and color perception.

The rays of light from the sun sooner or later fall on matter (see 1 in Fig. 3) of some form. Matter has the property of absorbing / swallowing certain parts of sunlight (see 2 in Fig. 2). The parts of the sunlight that are not absorbed are reflected by the matter and then reach the beholder's eye (see 3 in Fig. 3).

Fig. 3: Schematic representation of color perception

An object in our environment reflects only part of the light waves contained in sunlight. If an object reflects light of a certain wavelength (e.g. light in the medium-wave range at around 500 nm), it does not just appear lighter or darker than its surroundings, but is characterized by its color (in the case of light waves in the 500 nm range, we feel that Color than green). In order to produce this perception, our visual system uses the aforementioned photoreceptors (cone types), whose maximum sensitivities are in the short, medium and long wave range. The actual color stimulus is determined from the non-absorbed - the reflected - part of the general lighting.

The spectral composition of sunlight is not always the same. Depending on the season, weather conditions and angle of incidence, there are fluctuations in the composition of the light and the spectral intensities change. For example, when we see the sun as an orange disk at sunset, the long-wave rays of light predominate and we are dealing with one warm light to do. However, if we look at the midday sun in midsummer, with a bright blue sky, the short-wave light rays predominate and we have it with us cold light to do. There are also differences between what we perceive as daylight and the light from lightbulbs and spot lamps. The artificial light of a halogen lamp with 3200 Kelvin appears yellow in comparison with the average daylight of 5600 Kelvin. However, the human eye has the ability to adapt to the changed spectral composition of the light to a certain extent. These differences in light are therefore not always visible to our eyes. We are also not equally sensitive to all light waves. Our eyes react most strongly to medium-wave light (green), less strongly to long-wave light (red) and least to short-wave light (blue). Therefore, in an 8-bit representation in the computer, 3 bits each are used for red and green and only 2 bits for blue.

2.3. Color sensation problem

A major problem of our color perception is its pronounced subjectivity. For us, color is a general sensory impression, and we are therefore not in a position to make a statement about the spectral composition of light. Therefore, it is fundamentally not possible for us to feel how our fellow human beings perceive colors in comparison to us, and even for a single person the perception of color is subject to daily fluctuations. Thus, color definitions such as `` wine red '' or `` sky blue '' are extremely imprecise and are assigned quite different colors by different people on a color scale. If we order a `` sky blue '' car by phone, we may be very surprised at the color of the vehicle that will later be on our doorstep.

In technology, and therefore especially in image processing, such a subjectivity is highly undesirable. Only if objective measuring systems are available that allow a clear definition of color can it be achieved that monitors or television sets from different manufacturers display a color image in approximately the same way, or that a color film reproduces the original colors as faithfully as possible.

It is precisely for this purpose that various methods for the mathematically exact description of colors have been developed, each of which is particularly suitable for certain areas of application, so-called Color models.