1. A method of calibrating a brightness value measured by a camera with an amount of light received by the camera, the method comprising: creating a series of measurements, wherein for each measurement the amount of light received at an image plane in the camera is controlled to be a known ratio of two opposed irradiance values: a high irradiance value and a low irradiance value; correlating each ratio with the brightness value measured by the camera; and obtaining a function describing the correlation.