Spectra PT PTS Luminance Standard

The Labsphere Spectra PT PLS luminance standard is a portable luminance standard based upon an integrating sphere uniform light source equipped with a quartz tungsten halogen (QTH) lamp. Luminance is manually adjustable from 0 to 5,000 cd/m², at a fixed 2856K colour temperature, emitted from a 3.8cm diameter exit port. The Spectra PT PTS is our most affordable luminance standard and is suitable for testing and calibrating luminance meters, image sensors, photometers, and radiometers and for the field calibration of cameras.

With its compact, integrated design, the Spectra PT PLS delivers laboratory-grade performance in a portable package designed for field applications. The robust design with optional battery operation allows the calibration source to be brought to the device under test. The PT PLS includes an integrated luminance monitor photometer, a touch display that reports lamp voltage, current and output luminance, and high-resolution, manual luminance adjustment.

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.