HELIOS AS-Series Uniform Light Sources

Labsphere’s HELIOS Plus range of integrating sphere uniform light sources serve as stable, calibrated standards of luminance and spectral radiance. The HELIOS Plus AS-Series uniform light sources are bespoke systems for demanding sensor test & calibration applications. The open exit port on an internally illuminated integrating sphere acts as a uniform light source. The sphere output is characterised by both spatially and angularly uniform radiance which can be used to flat-field correct and transfer an absolute luminance or spectral radiance calibration onto cameras, multispectral and hyperspectral imagers, small satellite imagers and earth observation systems.

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.