Grease spot photometer

A grease spot photometer by Robert Wilhelm Bunsen makes it possible to easily compare light intensities .

Experimental setup

Experimental setup

The experimental setup consists of two light sources between which a sheet of paper with a grease stain , typically wax or stearin , is placed in the middle.

Initially, the grease stain appears either lighter or darker than the surrounding paper, since greasy paper reflects light poorly , but lets a lot of light through. You move the sheet until the grease stain seems to disappear. Then applies to the light intensities:

${\displaystyle {\frac {I}{r_{2}^{2}}}={\frac {I_{0}}{r_{1}^{2}}}}$

With this formula you can compare the two light intensities by the distance of the leaf from the light sources. Usually a light source ( ) is selected as the normal light source . This can be a Hefner candle (I = 1 HK = 0.9  cd ) , for example . ${\displaystyle I_{0}}$

It is also possible to avoid squaring the distances with a clever experimental setup. See the general article on the subject of photometers .

However, the Bunsen photometer only works reliably if both light sources are approximately the same color and emit the light evenly (without flickering).

Example of a measurement using the grease spot photometer

The light intensity of a 1500 W headlight is compared to that of a 500 W headlight. Using the above setup, one measures the expected distance ratio of . The 1500 W headlight is now dimmed and the electrical power consumption measured. By moving the grease stain paper you can now see how the light output decreases. If the distance ratio (and thus the light intensity ratio) is 1: 1, then z. B. a consumption of 990 W (± 30 W) measured. That is double the 500 W headlight despite the same light output. ${\displaystyle {\sqrt {3}}:1}$

See also

• Photometry
• Lumens
• Luminous flux

literature

• Dieter Meschede (Ed.): Gerthsen Physik. 22nd edition, Springer Verlag, ISBN 3-540-02622-3 .

Web links

• The gains and blessings of gas light (after approx. 50% of the text there is a very good description of the grease stain photometer: "In a cartridge on the basement [...]")

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