Mainly used to determine the correct exposure for bromide prints. Light passes through the negative onto a screen, this is compared to light from the same source but not passing through the negative. The light not passing through the negative is adjusted by a series of wheel stops until it is the same brightness as that passing through the negative. The density number (wheel stop) and type of paper are found in a table which gives the exposure time. The patent talks of adjusting the colour of the light.