HELIOS A-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 A-Series uniform light sources provide an output that is matched to a target Air Mass (Albedo) spectrum. 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.

Air Mass Sunlight Spectrum

The sun emits optical radiation corresponding to a black body source with a colour temperature of 5800 Kelvin, and the vacuum of space does nothing to modify this. The spectrum of sunlight before transmission through the Earth’s atmosphere is known as Air Mass Zero (AM0). Air mass is a parameter that defines the relative path length of sunlight through the Earth’s atmosphere at a particular zenith angle compared to the path length with the sun directly overhead. With the sun directly overhead (zero degrees zenith angle), the thickness of the Earth’s atmosphere is at its thinnest.

The various gaseous species present in the Earth’s atmosphere modify the AM0 spectrum upon transmission of the radiation to the Earth’s surface, imparting characteristic absorption bands in the infrared. For example, gas-phase H2O and CO2 strongly absorb radiation in the infrared. In the visible range, precipitation, clouds, and sand or dust in the atmosphere all attenuate solar radiation. Of the total AM0 extra-terrestrial radiation, about 75% reaches the surface of the earth. The energy distribution within the solar spectrum as received at the surface of Earth is approximately 5% UV, 43% visible and 52% infra-red.

Radiometric units are used to express the amount of sunlight reaching the Earth’s surface. For the human observer, the illuminance of sunlight from a clear sky is quoted in photometric units, about 100 lux (100 lumens per square meter). For non-human observers, we instead use the radiometric unit of Watts per square meter. Spectroradiometrically, we use the unit of Watts per square meter per nanometer, which is the irradiance per unit wavelength interval.

The modified spectrum of sunlight on the surface of the Earth, with the sun directly overhead, is known as AM1. The greater the distance that the radiation travels through the atmosphere and the higher the zenith angle, the greater the absorption. The air mass for a given zenith angle can be approximated using AM1/cos(z), where z is the zenith angle in degrees. The air mass for most major population centres in the northern hemisphere is located at a yearly average zenith angle of 48.2°, at which the air mass is 1.5 (AM1.5). For assessing the performance of solar cells used in equatorial and tropical regions, the sun’s spectrum at 25° from zenith is often used, which is known as the AM1.1 spectrum.

The AM1.5G (G=global) spectrum was chosen by most international standardising bodies to provide a representative, common frame of reference to allow measurement results to be compared. Also, for standardisation purposes, one “sun” is defined as being equivalent to an irradiance of 1000 Watts per square meter. AM1.5D is the spectrum at 48.2° zenith taking into account only direct, or non-scattered radiation.

Achieving a Solar Spectrum

HELIOS A-Series uniform light sources achieve an AM solar spectrum by combining the output of red-rich tungsten halogen lamps with blue-rich sources in an integrating sphere. Labsphere has employed Xenon sources for bluer spectra for many years, highly stable blue light sources have been very difficult to achieve, to that end Labsphere have developed the new Blue Sun LED light source. In combination with the standard tungsten halogen light source this provides a long term stable solar spectrum.

The sphere diameters available in the HELIOS A-Series are 12 and 20” coated with Spectraflect and 11.5” for the Spectralon option. The sphere radiance is adjustable over a wide dynamic range, from Albedo 0 (AM0) down to night vision light levels.