An apertometer directly determines the widest angle of light that a lens can capture. It typically consists of a graduated glass block or scale placed on a microscope stage, allowing the user to read the acceptance angle by observing the limits of the field. While largely replaced by automated beam profilers in laser labs, it remains a historical benchmark tool for high-end microscopy.
Heard on the job
The vintage Abbe apertometer in the cabinet still gives a highly accurate reading for these custom objectives.