Spectra FT Luminance Standard

The Labsphere Spectra FT luminance standard incorporates either 15 or 32-channel LED light engines coupled to a Spectralon integrating sphere to create a spectrally tuneable source of unform luminance or illuminance for calibrating imaging systems and optical sensors with your chosen reference illuminant or spectral power. Spectra FT sources are available for the VIS band (400-700nm + 850nm), the VIS NIR band (380-1000nm) or the SWIR band (900-1600nm).

Please note that compared to the Spectra CT luminance standard which allows you to tune the correlated colour temperature (CCT) of white light, the Spectra FT gives you complete freedom to tune both the colour and spectrum of your luminance or radiance standard.

Create Your Own Spectrum

With the Spectra FT you can create your own, almost infinitely variable spectral power distribution, or select from predefined standard CIE illuminants or black body spectra. White light CCT is adjustable from approximately 1900K to 40000K. The individual LEDs employed in the light engines feature linewidths of approximately 20nm in the visible or 50nm in the NIR. Luminance is adjustable from 10 to 25,000 cd/m² for reference illuminant D65 (actual range varies with model), whilst the luminance uniformity across the exit port is specified at 99% (per the COV method).

The Spectra FT SWIR model serves as a source of uniform radiance in the range from 900-1600nm and is used for testing and calibrating InGaAs sensors.

Real-Time Spectral Monitoring

An integrated internal spectroradiometer offers real-time spectral performance tracking over the dynamic range of the Spectra FT systems (except for the Spectra FT-1000-W). Recalibration and recharacterisation features ensure long life performance and minimal downtime making Spectra FT sources ideal for production applications.

Spectra FT systems with integrated spectroradiometer include a stable quartz tungsten halogen reference source used to recalibrate the absolute spectral responsivity of the spectroradiometer at the discretion of the user. This ensures continuous and accurate spectral monitoring of the performance FT-2300-F system.

The Spectra FT Family

The Labsphere Spectra FT spectrally tuneable luminance standards range takes in 8 models that emit uniform luminance or illuminance in the range from 380 to 1600nm.

• The Spectra FT-1000-W and FT-1100-W operate with 15 channels in the VIS wavelength band, from 400-700nm plus an additional NIR LED that emits at 850nm. The latter incorporates an integrated spectroradiometer for real-time spectral monitoring. Both models are equipped with domes at the sphere exit ports for testing wide angle field-of-view sensors.
• The Spectra FT-1100-A and FT-1100-S also feature the same 15 channel light engine as above, but are equipped with a cosine diffuser at their exit ports (25 and 75mm, respectively).
• The Spectra FT-2200-W and FT-2300-W operate with 32 channels in the VIS/NIR band, from 380-1000nm, with peak luminance outputs of 16,000 and 25,000 cd/m² respectively (for CIE reference illuminant D65).
• The Spectra FT-2300-F also operates with 32 channels in the VIS/NIR band, from 380-1000nm, but instead of a 75mm diameter exit port, the 2300-F output is coupled into a liquid optical fibre bundle. This delivers uniform illuminance/irradiance to the device under test remotely, typically a sensor or sensor array. 
• Last but not least, the Spectra FT-2400-SWIR operates with 10 channels in the near infrared band, from 900-1600nm. The FT-2400-SWIR employs LEDs with spectral linewidths ranging from 35 to 125nm (FWHM) while peak radiance is up to 30W/sr.m² (depending on the spectrum selected).

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.