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.



XML 地图 | Sitemap 地图