A microdensitometer is an optical instrument used to measure optical densities in the microscopic domain.^[1][2][3] A well-known microdensitometer, used in the photographic industry, is a granularity instrument or granularity machine.^[2] The granularity measurement^[2] involves the use of an optical aperture, 10-50 micrometers in diameter, and in the recording of thousands of optical density readings. The standard deviation of this series of measurements is known as the granularity^[2][3] of the measured transmission surface, optical film, or photographic film, in particular .

An alternative version to the traditional point-by-point microdensitometer is the beam expanded laser microdensitometer.^[3][4] This instrument can illuminate simultaneously an area a few centimeters wide with an ultra thin height, in the micrometer regime.^[4] Advantages include increased depth of focus, significant increases in data collection speed, and superior signal to noise ratios.^[3][4] In microscopy applications, this type of ultra thin beam-expanded illumination can also be known as light sheet illumination or selective plane illumination.

This measurement technique, using ultra-thin expanded laser beams, is particularly useful to detect microscopic imperfections in optical coatings or transmission optical surfaces.^[5]