PhotoGroupy

by Nanette Salvaggio

The wavelengths of energy referred to as light are located near the middle of the electromagnetic spectrum. It is important to note that the location of this region is solely dictated by the response characteristics of the human eye. In fact, the international standard definition states: "Light is the aspect of radiant energy of which a human observer is aware through the visual sensations that arrive from the stimulation of the retina of the eye." Simply stated, light is the energy that permits us to see. By definition, all light is visible, and for this reason the word visible is an unnecessary (and perhaps confusing) adjective in the common expression visible light. This definition also may be interpreted to mean that energy that is not visible cannot and should not be called light. Thus it is proper to speak of ultraviolet radiation and infrared radiation, not ultraviolet light and infrared light. The popular use of such phrases as black light and invisible light to describe such radiation makes it impossible to determine what type of energy is being described and should be avoided.

The understand more about light, it is necessary to become familiar with the way the human eye responds to it. Figure 1-3 represents the photopic luminosity function of the human eye as defined by the International Commission on Illumination (CIE).

The plot illustrates the sensitivity of the eye to different wavelengths (or colors) of light. These data indicate that the sensitivity of the eye drops to near zero at both 400 and 700nm, thus specifying the limits within which radiant energy may be referred to as light. It also shows that the response of the eye is not uniform throughout the visible spectrum. Human vision is most sensitive to green light. If equal physical amounts of different colors of light are presented to an observer, the green portion of the spectrum would appear the brightest, and the blue and red parts would appear very dim. This is the reason that a green safelight is used when processing panchromatic film. Since the eye is most sensitive to green light, it takes less of it to provide adequate visibility in the darkroom than any other color of light.

The plot shown in Figure 1-3 has been accepted as an International Standard response function for the measurement of light. Therefore, any meter intended for the measurement of light must possess a sensitivity function identical to it. Most photoelectric meters used in photography have response functions significantly different from the standard ad are not properly called light meters, although the international standard curve can be approximated by the use of appropriate filters with some meters. (It is worth nothing that the determination of the proper f-number and shutter speed for a given photographic situation does not require a meter with this response function. It is more important for the meter to match the sensitivity of the film or digital sensor being used than that of the human eye.)

When all of the wavelengths between 400 and 700nm are presented to the eye in nearly equal amounts, the light is perceived as white. There is no absolute standard for white light because the human visual system easily adapts to changing conditions in order to obtain the perception of whiteness. For example, the amounts of red, green, and blue light in daylight are significantly different from those of tungsten light; however, both can be perceived as white due to physiological adaption and the psychological phenomenon known as color constancy. (See Figure 1-4.)

Thus our eyes easily adapt to any reasonably uniform amount of red, green and blue light in the prevailing illumination. This means that our eyes are not reliable for judging the color quality of the prevailing illumination for the purposes of color photography.

If a beam of white light is allowed to pass through a glass prism as illustrated in Figure 1-5, the light dispersed into a series of colors we call the visible spectrum. This separation of colors occurs because lights of varying wavelengths are bent by different amounts. The shorter-wavelength blue light is bent to a greater extent than the longer-wavelength green and red light. The result is a rainbow of colors that range from a deep violet to a deep red. Experiments indicate that human observers can distinguish nearly one hundred different spectrum colors. However, the visible spectrum is often arbitrarily divided into the seven colors listed in Figure 1-5.

To describe the properties of color photographic systems in simple terms, the spectrum is divided into just three regions: red, green and blue. The color of the light may also be specified at a given wavelength, thereby defining a spectral color. Such colors are the purest possible because they are unaffected by mixture with light of other wavelengths. It is also possible to specify a certain region or color of the spectrum by the bandwidth of the wavelengths. For example, the red portion of the spectrum could be specified as the region from 600 to 700nm.