Digital cameras and other array-based electronic imagers must be normalized in much the same way as bare focal plane arrays (FPA). However, one more element is introduced that must be accounted for — the optical imaging system.
Imaging systems, whether they are refractive, reflective, or both (catadioptric) suffer from irradiance that varies with field angle. The most common variation is the cosine-fourth law. A procedure similar to that described for simple FPAs will correct for cosine-fourth irradiance falloff and other sources of irradiance variation in the image.
To calibrate a digital camera with an integrating sphere system, typically the camera is positioned at the integrating sphere’s radiance port and is focused at infinity or at the plane of the sphere’s exit port. The sphere’s exit port should be large enough to fill the camera’s field-of-view, ensuring that this area is completely featureless and devoid of shadows.
The integrating sphere is equipped with one or more lamps positioned outside the camera’s field-of-view. Multiple diffuse reflections within the sphere integrate the light creating uniform sphere wall radiance which is collected through the camera’s optics to achieve uniform irradiance on the array. The uniform distribution of light on the array is then used to identify and correct for non-uniformities in the signal conversion of the array (see the Focal Plane Array section for additional information).
For camera applications requiring absolute photometric calibration as well as spatial uniformity, the luminance of the uniform source system may be calibrated so that a monitor detector attached to the sphere provides a reading of the sphere luminance output during the camera test procedure. The system then becomes a source of known uniform luminance, which can be used to calibrate the camera’s photometric scale.
An integrating sphere provides the best means of characterizing and calibrating the response of an imaging system, significantly out-performing alternatives such as reflective diffuser targets.
Labsphere’s line of Uniform Source Spheres and Systems are specifically designed for camera calibration applications and are readily adaptable to provide variable levels of radiance and spectrum without affecting the uniformity of the scene presented to the camera being tested.