The term "Scheiner scale" refers to a type of measurement scale used in optics. A Scheiner scale is a circular disk with black and white segments that is used to measure the resolution of an optical instrument. The scale is placed in front of the instrument and the observer looks through the instrument to see the disk. The observer adjusts the focus until the black and white segments are just visible. The resolution of the instrument is then calculated based on the size of the segments and the distance between them. Synonyms for Scheiner scale include Scheiner disk, Scheiner plate, and Scheiner test. These terms all refer to the same type of measurement scale used in optical testing.