Introduction

A number of photometric and colorimetric instruments are visual optical systems as these devices have a viewing system to observe a scene either for alignment or to make a subjective judgement of brightness (luminance) or colour.

Photometers

Photometers are either subjective or objective instruments and are designed to measure the absolute or comparative brightness of sources or scenes. In the subjective type, the observer has to make a match between the brightness of a standard scene and that of a second scene. In the objective instruments, an observer uses the viewing system only for alignment.

Subjective photometers

The Lummer–Brodhun photometer

The Lummer–Brodhun photometer is designed to compare the luminous intensities of two light sources. Often one is a secondary standard source of known luminous intensity, and in this case, the luminous intensity of the second or unknown source can be found.

The instrument is shown in Figure 24.1. The light from the two sources illuminates both sides of a white diffuse block as shown in the diagram. Both sides of the block are viewed simultaneously through an optical system which contains a prism assembly and an eyepiece. One side of the block (the left side in the diagram) is seen through the central part of the prism and the other side of the white block is seen through the peripheral part. The two beams are brought together but separated by a sharp boundary by the prisms. The eyepiece is focussed on the sharp boundary. The field-of-view seen through the eyepiece is shown in the diagram.