Spectra CT Luminance Standard

The Spectra CT-1000 uniform light source serves as a standard of luminance with which you can adjust both the luminance and also the correlated colour temperature (CCT), the shade of white light. By combining separate warm white and cool white LEDs, the CCT can be continuously adjusted between 2800 and 7500 Kelvin. This allows for the flat field correction and the calibration of camera systems, reducing colorimetric errors due to the matching of the calibration CCT to the target illuminant that the camera will be used to measure.

The CT-1000 is available in three sphere sizes of 20, 30 and 50cm sphere diameters and with corresponding exit ports of 5, 10 and 20cm. The luminance of the CT-1000-S (20cm sphere) can be adjusted from 1-50,000 cd/m², while that for the CT-1000-M (30cm sphere) spans the range 0.5-25,000 cd/m². The luminance range for the CT-1000-L (50cm sphere) is 0.1-5,000 cd/m². Luminance adjustment is accomplished by means of programmable LED drive currents.

An integrated luminance photometer reports the real-time output brightness of the sphere. The CT-1000 is controlled via a USB interface from a user-provided PC running Labsphere’s LSS-PT software. The operator can set both a target luminance and CCT using the software.

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.

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.