This section is from the book "The Fundamentals of Photography", by C. E. K. Mees,. Also available from Amazon: The Fundamentals Of Photography.
IF we take a piece of blue cloth and put an orange on it and then photograph the combination we shall find that instead of the orange being lighter than the cloth, as it looks to the eye, the photograph (Fig. 107) shows it as being darker. This difficulty in photographing colored objects so that they appear in the print in their correct tone values, as they are seen by the eye, has been well known to photographers from the earliest days of the art.
In order to understand the cause of it we must consider the nature of color itself. When we speak of a colored object we mean one which produces a distinct sensation, which we call the sensation of color. This, of course, is due to a change in the nature of the light which enters the eye and causes the sensation of sight, and this change is produced in the light by the colored object so that the light after reflection from the colored object is different in composition from the beam of light before reflection.
Fig. 107. Picture of an Orange on Blue Cloth.
In Chapter II (Light And Vision) we have seen that light consists of waves, and that these waves are of various lengths, the color of the light depending upon the wavelength.
Fig. 108. Divisions of Spectrum.
In white light there are waves of all lengths and if white light is passed through a spectroscope it is spread out into a band of various colors which is called a spectrum. The various colors of the spectrum correspond to definite lengths of light waves and if we measure their length in the very small units which are used for measuring waves of light we shall find that the red waves are 700 millionths of a millimeter, the yellow ones are 600, the green 550, the blue-green 500, the blue 450, and the violet waves, the shortest which we can see, are 400 millionths of a millimeter long (Fig. 108). Thus, we can scale the spectrum by the length of the light waves of which it is composed (Fig. 109).
Fig. 109. Pink filter passing violet, blue, yellow, orange and red rays but absorbing green.
If we take a piece of colored glass or gelatine, say pink gelatine, and hold it in front of the spectrum, we shall find that the pink gelatine will not let some of the waves of light through; it will stop them completely, while it will let the other waves through without any difficulty. The pink gelatine, in fact, cuts out or absorbs the green light (Fig. 109). This is because of its pinkness; that is, it has the property of absorbing green light from the white light and of letting through the other light which is not green, that is to say, to a less degree this pink film sorts out the light just as the spectroscope does, but instead of separating the waves of different lengths it stops some of them and lets the others go on, and the eye, missing those which are stopped, records the absence as a sensation of colour.
If, instead of having a transparent substance like film, we have an opaque colored object, like a sheet of orange paper, and let the spectrum fall on it, we shall find that the orange paper will reflect the red and yellow and green light but will refuse to reflect the blue light; it absorbs it, and its orangeness is due to the fact that it absorbs the blue light and refuses to reflect it. All objects which are colored are colored because they have some selective absorption for some of the waves of light; they do not treat them all alike but reflect some and absorb others, and the modified light which reaches the eye we call "color." Any object which treats all the waves of light alike, which absorbs them all or absorbs them equally or reflects them all in equal proportion, is not colored. If it absorbs them all it will be dead black since it will reflect no light. If it absorbs them to a small extent, but equally, it will be gray; if it reflects them all it will be white, but if it absorbs some of the wave lengths and not others, it will be colored.
Fig. 110. Purple filter passing violet and red, but absorbing the blue, green, yellow and orange.
Limit of Visibility
If we try a series of experiments in our spectrum we shall find that things which absorb red light are colored blue, and those which absorb green light are colored pink or magenta, or if they absorb a great deal of the light, purple (Fig. 110). Those that absorb blue-green light are orange, and those that absorb blue-violet light are yellow. We see, then, that to each color there corresponds a region of the spectrum which is absorbed.