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2: CONTROL LIGHT LEVEL AND COLOR Libraries need lighting that is bright enough and makes colors look good. Library lighting systems need to provide enough light for reading and to read the labels on the spines of shelved books. The level of illumination on a surface is measured in footcandles. One source of recommendations for appropriate lighting levels is the IES, the Illuminating Engineering Society of North America. For library purposes, 50 to 75 footcandles at tabletop is plenty bright, as long as the light bounces around everywhere rather than shines straight down. We usually suggest 60 footcandles. Inadequate lighting leads to real misery. Lighting can also be too bright. We know one library with a central service area illuminated at about 60 footcandles and an adjacent children's department with about 150. The children's department is much too bright for comfort, especially if one moves back and forth frequently between the two areas. How do we avoid problems with areas that are too bright or too dim? All engineers should have software that calculates illuminating brightness at table top throughout all areas of a building. These programs adjust for ceiling configuration, height, color, typical dust on lamps, aging lamps, etc. And they show lighting levels for relatively small areas. Insist that these calculations be carried out for your library during the design development phase, and demand that any excessively low or high figures be corrected. Remember, however, that this kind of computer output tells you only brightness at table top. It won't tell you if your lights provide unpleasant glare, or whether the lateral light on the spines of books meets your needs. But if you are using reflected uplighting, and the printouts show a uniform illumination of about 60 footcandles, you should be in pretty good shape. Color rendition One major problem with some kinds of lighting (both cheap fluorescent lamps and many HID lamps) is their failure to reproduce colors accurately. This is because some wave lengths are actually missing entirely. A standard measure of the ability of lighting to reproduce colors adequately is CRI, which stands for "color rendering index." A CRI of 100 is great, and a CRI of 0 is wretched. Daylight and incandescent light both have a CRI of 100. With modern fluorescent lighting, you should expect a minimum CRI of at least 85. Good quality fluorescent lamps will produce this, but cheap ones (such as the old warm white and cool white, both of which are still around but technically obsolete) won't. And a CRI of this level may be nearly impossible to achieve with metal halide lighting. The light output of lamps is portrayed in charts with distribution curves indicating how much of each wavelength is present. While daylight and incandescent light have smooth curves, with all wavelengths present in relatively similar amounts, the curves for fluorescent and HID light frequently consist primarily of spikes of a handful of wavelengths. Color temperature measures how warm or cool light is. Unlike CRI, color temperature is not concerned with the presence or absence of wavelengths, but just the balance between warm and cool. Color temperature is measured in degrees Kelvin. (You don't have to know much about this except to remember that the higher the number the cooler the light.) If you like the appearance of incandescent lights, so your library looks warm and homey rather than cold and industrial inside when people drive by at night, a good first try at color temperature is 3500 degrees Kelvin. In daily life, our eyes adapt fairly well to changes in color temperature that are not extreme. Typically, we notice them most when we are outdoors at twilight, and we see the warmth of incandescently lighted rooms contrasted with the last of the bluer daylight.
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