In the standard approach for calibrating a remote sensing imaging device, a uniform light source is used to determine the sensor’s spectral radiance and radiance responsivity over a large dynamic range.
To do this, the field-of-view (FOV) of the instrument under test is focused at infinity through the integrating sphere’s radiance port. The sphere itself is designed with a radiance port large enough to overfill the instrument’s FOV and is equipped with one or more lamps positioned outside this FOV so the radiance received by the instrument is uniformly integrated. This region is irradiated by multiple diffuse reflections so that a uniform field-of -view is presented to the instrument.
For applications requiring photometric calibration as well as spatial uniformity, the luminance of the uniform source system is calibrated with a luminance monitor attached to the sphere. The luminance monitor provides a linear correlation of the luminance through the exit port of the integrating sphere. The uniform source system is a source of known uniform spatial luminance, which can be used to calibrate the instrument’s photometric scale.
Integrating sphere uniform sources significantly outperform alternatives such as reflective diffuser targets and are readily adaptable to provide variable levels of radiance (and even variable correlated color temperature) without affecting the uniformity of the scene presented to the system under test.
Labsphere’s line of Uniform Source Spheres and Systems are used to calibrate and characterize large, multi-sensor imaging systems, such as those used in earth-observing satellite instruments.