Maker's Name: 
Max Kohl, Chemnitz
Where made: 
circa 1920
50 × 20 × 50 cm

This item is a Weber Photometer. Made by Max Kohl around 1912, the Photometer is made up of large brass tubes that are connected with a rotating joint in a T-configuration. The vertical tube on the left has an eyepiece on one end and a lens opening on the other. The small black case at the end of the horizontal tube on the right can hold a lamp. The lamp can be moved in and out as well as up and down with the rack-and-pinion adjustment. Inserted directly above the eyepiece is a slide with three color filters on it, to allow the user to choose which color light is observed through the eyepiece. When the photometer is in use, both tubes should be horizontal, however one tube has been turned to vertical to allow it to fit in the cabinet.

This Weber Photometer was used to measure the intensity of a given light source by comparing it with a known, standard source. The user would insert a standard candle into the lamp housing of the photometer. They would then aim the lens at the end of the observation tube at the light source to be measured and look through the eyepiece. The standard candle would illuminate an unpolished disk in the horizontal tube and the light source to be measured would illuminate a similar disk in the vertical tube. The user can then compare the the light intensity of the two sources using the inbuilt Lummer-Brodhun cube.

A standard candle is a light source that gives off one candela of luminous intensity. Originally the candela was based on the brightness of the flame of a candle with known composition. However different countries used different candles and so there was not a consistent unit of measurement. In 1948 the International Committee for Weights and Measures and the International Commision on Illumination standardized the unit. Now, if a source has a luminous intensity of one candela in a given direction it is emitting monochromatic radiation of frequency 540×1012 Hertz and has a radiant intensity in that direction of 1⁄683 watt per steradian. When considering the units of light intensity it is important to distinguish between the color or frequency of the light, the amount of power that the light is carrying (the radiant intensity) and its luminance, which is measured in candela per square meter.

This instrument was developed in 1883 by Leonhard Weber, who called it a ‘milk glass photometer’.

It was able to measure not only the brightness of sources but also the illumination of surfaces and sprang from a collaboration with Hermann Cohn who needed to measure light levels in schoolrooms.

This item is part of the UQ Physics Museum ‘Before Photonics’ Tour
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  • from Max Kohl catalogue 1912 
  • standard candle holder