RELATION OF LIGHT INTENSITY TO INSECT RESPONSE 



J. G. Hartsock 1 



To discuss the effects of the various characteristics of light, one is almost inevitably- 

 faced with the necessity for defining terms to reduce the possibility of confusion. It seems 

 especially appropriate to begin this discussion in that manner. Dr. Hienton has already 

 defined light in terms of the wavelengths of radiant energy. It can also be defined on the 

 basis of other characteristics: Illuminating engineers define light as "visually evaluated 

 radiant energy." This visual evaluation depends upon the wavelength of the energy, its 

 distribution in space, and its distribution in time. 



Standard measurements have been adopted for the evaluation of these space and time 

 distributions of light which are of interest in understanding the reactions of insects. 

 Studies concerning the effects of light "intensity" on insect attractance have involved 

 at least two of these different concepts. These standard measurements include: 2 



1. Quantity of light- -the time -rate of light energy flow x time, a total energy meas- 

 urement expressed in lumen-hours. Somewhat analogous to kw.-hr. of electrical 

 energy. 



2. Luminous flux--the time-rate of light energy flow, measured in lumens , One 

 lumen is defined as the light emitted by a standard-candle source into a unit solid 

 angle of space. Essentially a power measurement, analogous to watts. 



3. Luminous intensity, also termed candlepower- -a measurement applied only to 

 point sources, also evaluating time -rate of light energy flow (luminous flux) 

 through a unit solid angle, but in one given direction. The measurement is ex- 

 pressed in candles. 



4. Photometric brightness or "brightness"--a more practical measurement of 

 luminous intensity applied to sources of appreciable size. The luminous flux per 

 unit of surface area of the source, expressed in candles per unit area, foot- 

 lamberts (1 lumen/sq. ft.) or lamberts (1 lumen/sq. cm.). 



5. Luminous flux density at a surface- -luminous flux per unit of surface area, but 

 applied as follows: 



a. Illumination- -applies to surfaces receiving light energy, measured in foot- 

 candles (1 lumen/sq. ft.). 



b. Luminous emittance- -applies to a sizeable surface emitting light, measured 

 in lumens/sq. ft. 



The relations between candles, lumens, and foot-candles are shown in diagrammatic 

 form in figure 1. 



TYPES OF "INTENSITY INVESTIGATIONS" 



On the basis of these definitions, investigations of the insect response to light 

 "intensity" have involved both "luminous intensity" (or "photometric brightness") of the 

 light sources used and the "luminous flux density" (or "illumination")produced in the 

 surrounding space. From this confusion of concepts it is also easy to understand why 

 workers concerned with illumination problems avoid using the term "intensity" and are 

 careful to attach appropriate modifiers to clarify its meaning when used. 



1 Agricultural Engineer, Farm Electrification Research Branch, Agricultural Engineering Research Division, U.S.D.A., 

 Lafayette, Ind. 



2 Illuminating Engineering Society Lighting Handbook, 3rd. Edition, 1959. 



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