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The fluorescent lamp is a very popular light source for machine vision applications. However, many fluorescent lamp installations perform below expectation due to a misunderstanding of their nature. Of all light sources, fluorescent lamps are the most sensitive to their environment and special care is needed to extract the maximum performance. Some of the more critical aspects are examined below.
Heat and the Fluorescent Lamp
The fluorescent lamp is particularly affected by even small changes in ambient temperature, but this characteristic is routinely overlooked by machine vision integrators. One reason is that there is very little technical information available about the idiosyncrasies of fluorescent lamps in confined spaces. Even the most tedious and scrupulous design engineer can overlook heating problems in an application that may cause problems in the field.
A little background is helpful. The granddaddy of fluorescent lamps, the F40-T12, is the standard against which all other fluorescent lamps are judged. It is a 48 inches long, 1.5-inch diameter lamp that operates at 40 watts of input power. (By the way, the T12 nomenclature means 1.5 inches in diameter because the letter "T" in the lamp business means 1/8-inch of lamp diameter; so T12 means 12 times 1/8-inch which equals 1.5 inches.)
The F40-T12 lamp was designed to operate in an open-air fixture in the ceiling of an ordinary office building. The ambient air temperature was assumed to be about 80°F. Under these circumstances, the normal heating of the lamp would cause the surface of the lamp to rise about 25°F and result in a lamp body temperature of 105°F. This temperature is optimum for lamp efficacy and brightness. If the surface area of the lamp is calculated, the F40-T12 has about 5.6 square inches for each watt of input power.
For comparison, the F48-T12-VHO lamp is practically the same size, but it operates at 115 watts of input power. It has the same surface area, but three times the wattage. This lamp has about 1.9 square inches of surface area for each watt of input power. To operate with a body temperature of 105°F, this lamp would need either a much cooler environment, or forced air cooling.