Spectra PT Luminance Standard

The Labsphere Spectra PT luminance standard is a high dynamic range luminance standard based upon a 13.5cm Spectralon integrating sphere uniform light source equipped with a quartz tungsten halogen (QTH) lamp. Luminance is adjustable from 0 to 50,000 cd/m², by means of a motorised variable attenuator, with a fixed 2856K colour temperature, emitted from a 5cm exit port. An optional dome at the exit port allows cameras with a 180° field of view to be tested. The Spectra PT’s high luminance and wide dynamic range is suitable for testing and calibrating a wide variety of products that includes CCD and CMOS cameras, small area remote sensing devices, electronic imaging devices, medical endoscopes, ambient light sensors and security cameras.

With its integrated design, the Spectra PT-1000 is an excellent “all-rounder” if you need a no-frills, wide dynamic range, high brightness uniform light source. Luminance adjustment is from zero to 50,000 cd/m² at a fixed CCT of 2856K and is achieved by means of a precision, motorised variable attenuator. The PT-1000 is available in two versions to suit the field-of-view of the cameras that you wish to test. The PT-1000-S features a simple 5cm exit port, whereas the PT-1000-W is equipped with a dome placed at the exit port which provides high luminance uniformity over 180° subtended from the plane of the exit port. The latter version is ideal for testing cameras with wide fields-of-view. An integrated luminance photometer reports the real-time output brightness of the sphere.

The PT-1000 is controlled via a USB3.0 interface from a user-provided PC running Labsphere’s LSS-PT 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.