SPECTRO - PHOTOMETER (Pr., Spectro photorizetre ; Ger., Spektrophotometer) An instrument by means of which the lumi nosity of various regions of two spectra can be directly compared. Various forms have been suggested.
There is, first, the double - slit photometer of Vierorot, in which the lights to be compared fall on the slit, the two halves of which can be opened or closed, and thence passed through a prism, to appear at the eyepiece as adjacent spectra ; they are compared by opening and closing the one slit which transmits the light to be measured, whilst that transmitting the standard light is kept constant. The eyepiece of the telescope is provided with sliding shutters which enable one to limit the field of view to the particular region under examination. The disadvantage of this method is that the two spectra are not of equal purity, and therefore not of the same colour, this being an inherent defect in the use of two slits of different widths.
bummer and Brodhun introduced a photo meter with two collimators at right angles to one another, the lights being brought into juxta position by means of a Lummer-Brodhun rhomb and thence dispersed. The reduction of one of the lights is produced by rotating sectors.
Polarising photometers have been devised by Glan, Crova, Hiifner, Glazebrook, etc. Their principle is that the lights are received through two contiguous slits, then pass through the dispersing medium, a Wollaston prism, and are brought into juxtaposition by a biprism, thence reaching the eyepiece, which is fitted with an analyser. This method also has its faults, and is really only strictly correct when mono chromatic spectrum rays are used.