by Bud Wood, ANCAL, INC

As is usually known, photopic vision is the response of the eye to radiant energy (light). More specifically, this is the response of the cones in the eye to light. Similarly, scotopic vision is the response of the rods in the eye to light. Both these responses follow relative photopic efficiencies and relative scotopic efficiencies, respectively. However, what is generally not realized is that these responses differ.

Not only do these responses differ, as shown in the photopic and scotopic efficiencies chart (below), but they have different functions. The contraction of the pupil to light intensities corresponds to the scotopic efficiency curve. That is, scotopic vision is the reception of light that regulates the opening of the pupil of the eye. When one thinks about this, it makes sense because the rods are more sensitive to light and they receive more of the "environmental" or surrounding light. This arrangement works very well when we are in sunlight or even in an incandescent lighted area because these types of illumination are continuous "full spectrum" illumination. However, when we are in distinct lighting environments, such as that created by gas discharge lights or by fluorescent lights, things don’t work out as well. That is because our eyes are built to correctly respond to full spectrum sunlight, not to partial spectrum lighting.

As can be seen in the chart, photopic sensitivities are at a maximum at about 555 nm whereas scotopic sensitivities peak about 50 nm lower. Practically, there can be problems in environments which are lighted by partial spectrum lighting. For example, high pressure sodium lamps are not human eye "friendly"; these are the high intensity "yellow-orange" lamps which increasingly are used for highway and outdoor area lighting. Such lamps have very little output below 550 nm. Hence, the scotopic response is at a minimum because it responds poorly in the wavelength ranges of typical sodium vapor lamps. Fluorescent lights, although not as bad, do engender similar problems, depending, of course, on the specific fluorescent light. This is the main reason that special ("full spectrum") fluorescent lights are used for aquariums and for other animal enclosures. There is a lot of investment in fish and zoo animals, so it pays to have appropriate illumination. Although standard fluorescent lamps do not provide illumination which is compatible with human eye response, people are usually not restricted to such environments.

However, for people who work under artificial illumination, there may be risks. If one’s desk or workbench is illuminated only by standard fluorescent lights, one would be well advised to have an incandescent or tungsten-halogen light as a supplement. Alternately, replacing a standard fluorescent light with an aquarium or other "full spectrum" fluorescent light can be beneficial. Also, if one is in the proximity of a window in which natural sunlight comes, the illumination situation can be even better. Being aware of the potential problems of inappropriate lighting and taking whatever steps are relevant can minimize consequences that can range from having a simple headache to that of compromising one’s vision.

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