Förderverein der Archenhold-Sternwarte
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Dr. Jürgen Rose
The photo meter makes possible the simple comparison of luminosities. The experimental construction consists of two confronted light sources. In the middle, a blotter is located as a projection screen with a fat-stain. The light sources turn up by approaching.
The left comparison light source has got a known brightness, the brightness of the right light source should become measured.
First the brightness of the light source which has to become measured must be pretended with the right setting. Through slow right-turning out of the zero position on the left setting, the brightness of the comparison source will be increased, until the fat-stain on the projection screen vanishes - so to say "optically washed". Initially the stain appears darker than the surroundingly paper since the reflection of light on greasy paper is higher than its permeability. Now, the brightness of the left comparison source can be read at the underlayed scale with help of the diagram, in case of compensation of the brightnesses therefore the unknown brightness of the right light source is equal to the left one.
Beleuchtungsstärke am Projektionsschirm in Abhängigkeit von der Skaleneinstellung
The photo meter makes possible the simple comparison of luminosities. The experimental construction consists of two confronted light sources. In the middle, a blotter is located as a projection screen with a fat-stain.
In the condition of compensation, the relationship-equation is applied to the light intensities:
ISpec/ r12 = I0 / r02
The unknown intensity of the source ISpec thus can be determined through changing of the known intensity of the source I0 or through changing of the distances r0 and r1 to the projection screen.
In the astronomy of the 19th century the brightnesses of stars were estimated after procedures according to the compensation method (Stepping-Estimation Method by F. W. Argelander), before so-called normal lamps were used as comparison light sources (Hefner Candles). White objects appear under candle light behind a distance of 1.8 m as illuminated as under full moonlight virtually. Today the unit for the light intensity is Candela, which considers physiological influences under a wavelenght of 555 nm.