Figure 1.- -Relationship between candles, lumens, and foot-candles. 



A uniform point source (luminous intensity or candle-power 1 candle) is shown at 

 the center of a sphere of 1 foot radius. It is assumed that the sphere is perfectly trans- 

 parent (i.e., has reflectance). 



The illumination at any point on the sphere is 1 foot -candle (1 lumen per square foot). 



The solid angle subtended by the area, A, B, C, D is 1 steradian. The flux density 

 is therefore 1 lumen per steradian, which corresponds to a luminous intensity of 1 candle, 

 as originally assumed. 



The sphere has a total area of 12.57 (4TT) square feet, and there is a luminous flux 

 of 1 lumen falling on each square foot. Thus the source provides a total of 12.57 lumens. 



Because commercially available light sources of high "brightness" (or "luminous 

 intensity") usually also produce high levels of "luminous flux density" (illumination) in 

 the surrounding space, these two characteristics are difficult to separate in experimental 

 practice. Consequently, many of the studies of insect attraction have involved a com- 

 bination of variations in both "brightness" and "luminous flux density." Such com- 

 parisons clearly show the relative overall attractiveness of different lamps (of similar 

 wavelength output), but do not indicate whether the differences in attractiveness are related 

 to "brightness" or to "illumination." Examples of experiments of this sort are the com- 

 parisons of incandescent and mercury-vapor lamps of various wattages in early corn 

 borer research. A portion of Dr. Pfrimmer's comparisons of various blacklight sources 

 begun in 1955 are shown in table 1. 



Many studies of the effects of differences in "luminous flux density" have been per- 

 formed in 'which differences in "illumination" have beenproduced by using different num- 

 bers of similar lamps. The investigations of the attractiveness of different numbers of 15-watt 

 BL fluorescent lamps, carried out both by Hollingsworth in Texas and by other research 

 workers at Purdue, are typical of this type of study. Table 2 shows results of a study by 

 Deay and Taylor. 



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