Abstract: Prior art instruments for testing a display unit suffer from the disadvantage that a testing system has to be customized for each type of display unit. The present invention optical measurement system, by utilizing an optical sensor, a three-dimensional transport system and a focus lens, provides for a universal system which is adaptable to test all kinds of display units. To provide for mobility and compactness, the optical sensor, the transport mechanism, the focus lens, in addition to the electronic circuit cards for controlling the motions thereof, are all mounted within a movable cart. To ensure that all kinds of display units may be tested on the same cart, a pivotable mirror assembly is mounted to the cart, with its line of sight axis aligned, closely to the screen of any to be tested display unit, such that the output from the display unit is reflected onto the focus lens and subsequently measured by the optical sensor.