Light Filters of Coloured ‘Perspex’

Abstract

LIGHT filters of coloured glass and gelatin are in common use for visual and photoelectric photometers. They are satisfactory, on the whole; but the glass filters are easily broken, and those of gelatin require to be mounted between glass to protect them from water. The transparent polymethyl methacrylate ‘Perspex’ is now obtainable in several colours and in sheets. This material is very tough and difficult to break, but is easily worked, and can be cut with a saw. The spectral characteristics of the different kinds of coloured ‘Perspex’ are not such as to make them directly usable as light filters in photoelectric photometers, because of considerable transmission in the red and infra-red by those which are violet, blue and green (Fig. 1); but by combining sheets of the coloured ‘Perspex’ with the Ilford copper red-absorbing gelatin filter 803, a set of nine serviceable light filters, covering the visible spectrum, has been prepared (Fig. 2), and these have been found to be sturdy and almost unbreakable, and well suited to laboratory use.