SphereOptics provides an image sensor and camera calibration service. Using a calibrated, large area, uniform luminance source, the absolute radiometric or photometric response of machine vision, multispectral, hyperspectral cameras, imaging photometers & colorimeters and focal plane arrays is provided.
The open exit port on an internally illuminated integrating sphere acts as a uniform light source. The sphere output is characterised by a spatially and angularly uniform radiance which can be used to flat-field correct and transfer an absolute luminance or spectral radiance calibration onto imaging systems. The sphere also provides a field of uniform irradiance that can be used to perform pixel gain normalisation of 2D array detectors.
SphereOptics provides an image sensor and camera calibration service. Using a calibrated, large area, uniform luminance source, the absolute radiometric or photometric response of machine vision, multispectral, hyperspectral cameras, imaging photometers & colorimeters and focal plane arrays is provided.
The open exit port on an internally illuminated integrating sphere acts as a uniform light source. The sphere output is characterised by a spatially and angularly uniform radiance which can be used to flat-field correct and transfer an absolute luminance or spectral radiance calibration onto imaging systems. The sphere also provides a field of uniform irradiance that can be used to perform pixel gain normalisation of 2D array detectors.
Uniform Light Source Theory
An internally illuminated integrating sphere simplifies what would otherwise be complex procedures in the calibration and distortion correction of cameras and image sensors. Generating a field of uniform irradiance or radiance is not easy – unless you use an internally illuminated integrating sphere. Any light entering a sphere reflects with equal radiance in all directions from the diffusely reflecting (Lambertian) sphere wall coating. The high reflectance coating (typically 96-99%) ensures a high number of reflections, resulting in a near-perfectly uniform radiance at all points on the sphere wall. The open exit port on an internally illuminated integrating sphere is the “uniform source”. The sphere can be illuminated by lamps or LEDs placed inside the sphere or held outside at a sphere port.
A uniform light source integrating sphere functions both as a source of uniform radiance and irradiance. A camera whose lens is focussed onto the plane of the exit port of the sphere collects defocussed light from the sphere wall opposite. The irradiance is uniform at all points on the sphere wall, and the wall reflects light with constant radiance at all angles. Therefore, the camera sees a field of uniform radiance or luminance.
Without imaging optics, the integrating sphere would deliver a field of uniform irradiance directly onto an image sensor. An image sensor placed directly in the plane of the exit port of the sphere receives uniform (but diffuse) irradiance. Note that the irradiance uniformity decreases in the near-field, but recovers in the far-field.
Key Capabilities of the Camera Calibration Service
The Need for Uniform Radiance/Irradiance with Image Sensors & Cameras
Focal plane array (FPA) image sensors (PDA, CMOS, CCD etc) suffer from pixel-to-pixel differences in responsivity (photo response non-uniformity, PRNU) as well as photon (shot) noise, dark (thermal) noise, read noise and non-linearity. By placing the sensor at the exit port of a uniform light source, we uniformly illuminate each pixel on the array and can perform pixel gain and offset normalisation.
An imaging system (reflective or refractive) introduces additional distortions with angle: vignetting and cos4 intensity drop-off. An integrating sphere uniform light source provides spatially and angularly uniform radiance and allows the camera (sensor with lens) to be flat field corrected.
Common Applications
Datasheets
Technical Notes
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