Explain the measurement of illumination using Macbeth illuminometer?

A Macbeth illuminometer is a type of portable visual photometer in which the light to be measured is balanced by a Lummer-Brodhun sight box* against a comparison lamp, whose apparent brightness can be varied by moving it along a tube; a control box supplies a calibrated current to the comparison lamp, and calibrated optical filters can be placed in the light paths to correct for color differences in the comparison and measured sources and to extend the range of the instrument.

* A Lummer-Brodhun sight box is a device, having a series of prisms, for viewing simultaneously the two sides of a white diffuse plaster screen illuminated by light sources whose luminous intensities are being compared