Analysis of information sources in references of the Wikipedia article "Light meter" in English language version.
In [the Heyde] instrument, blue-glass prisms are used to cut out the light reflected from the object. One looks through the eyepiece and turns the thicker portion of the prisms (one or both, according to the luminosity of the object) into position until the shadow details are suppressed. By reference to Tables, the necessary exposure may readily be found.
