The majority of techniques developed for filter radiometer characterisation are based on comparative scale transfer. The response of the filter radiometer to light at a particular wavelength is directly compared to the response of a calibrated detector exposed to the same radiation. This comparison is done at each wavelength interval across the entire response range of the filter radiometer. In this way the spectral responsivity, that is responsivity (amps per watt) versus wavelength (metres), of the filter radiometer can determined.

The NPL Primary Filter Radiometer Calibration Facility is illustrated in Figure 1. This facility uses high power, highly monochromatic, tuneable lasers as the source of optical radiation. At NPL the National Laser Radiation Facility (NLRF) can provide wavelength coverage from  200 nm in the UV, through the visible spectrum, out to beyond 10 μm in the infra-red (IR). A number of different laser sources are often required to generate sufficient wavelength coverage to calibrate broadband filter radiometers.

Figure 1: Schematic of the NPL laser-based Primary Filter Radiometer Calibration Facility

The comparative calibration technique requires the radiance of the light source to be stable between the measurement of the reference detector (for example a trap detector) and the FR being calibrated. This is achieved by actively stabilising the optical radiation. The laser stabiliser unit monitors the signal from a photodiode mounted in the integrating sphere and continuously adjusts the voltage applied to a Pockel cell in order to maintain a constant light output.  The laser beam is passed through a (multi-mode) fibre optic that is agitated in an ultrasonic bath to remove coherence. The output of the fibre optic is connected to an integrating sphere, which generates a near ideal (Lambertian) source with the added advantage of scrambling the laser beam polarisation. The resultant light is finally imaged onto an orthogonal translation stage, which systematically positions the trap and filter radiometer detectors in front of the light beam.

The reference detectors used, to which the FR response is compared, are typically trap detectors for the UV to near IR spectral regions. At NPL, the trap detector acts as the transfer standard from the cryogenic radiometer, from which spectral responsivity scales are ultimately realised.  In other spectral regions transfer standard detectors such as cavity pyroelectrics may be used in place of the trap detectors.

A number of additional spectral responsivity characterisation facilities exist at NPL and more information regarding these measurement services can be found here

For further information, please contact: Martin Dury