UV / visible light from an integrating sphere ( and / or other source such as a black body ) is focused onto a square test target at the focal plane of a collimator ( the mirrors in the diagram ), such that a virtual image of the test target will be seen infinitely far away by the camera under test.
12.
This pitfall ( but not in the setting-sun case ) is avoided by "'incident-light meters "'which measure the amount of light " falling on the subject " using an integrating sphere ( usually, a translucent hemispherical plastic dome is used to approximate this ) placed on top of the light sensor.
