Filter radiometry is at the heart of optical radiation measurement. It is the tool that provides the traceable link from the cryogenic radiometer to the ‘real world’ of polychromatic sources. Recent developments in high temperature fixed-point research have required us to revisit this established tool to improve the accuracy still further.

In its simplest form, a filter radiometer consists of an optical detector, a wavelength selection device, such as a filter, and a method to define a geometry over which light is collected, such as an aperture. The filter radiometer measures the optical power of a source within a particular spectral band and for a specific geometry. Figure 1 shows NPL’s standard filter radiometer, which includes a water-cooled housing to maintain constant temperature. Of course a spectrometer, which uses a diffraction grating or prism as a spectrally dispersing element that is coupled with a detector or detector array, is also a filter radiometer.

Figure 1: The standard NPL filter radiometer (not to scale)

Filter radiometers have many applications, some requiring absolute calibration – traceable to a primary spectral responsivity scale – and others are simply used as a transfer device.  In some cases the spectral band needs to be tailored to a specific defined function as in a photometer or for some UV meters (UVA, UVB, UVC …).  In addition when using 1- or 2-dimensional diode arrays they can be used for sophisticated imaging applications. For example they are deployed in space or onboard aircraft in Earth Observation applications.  Another demanding imaging application is where we are developing instrumentation to mimic how the eye perceives an object in order to develop and ascribe metrics related to its “appearance”.

Filter Radiometers and Temperature Measurement

Filter radiometers are used to measure the radiance of a blackbody source in one particular wavelength band. This measurement, through Planck’s law, determines the blackbody’s temperature and hence spectral output at all wavelengths. This underpins the spectral emission scales.

As a filtered detector determining the temperature of a blackbody source, a filter radiometer is identical to a radiation thermometer. However a radiation thermometer is calibrated by fixed-point blackbodies and extrapolation based on a ratio of radiance measurements, while a filter radiometer measures absolute radiance directly. This distinction will become less significant following the developments to the kelvin and the International Temperature Scale.

These improvements to the temperature scale are based on the introduction of new high temperature fixed points made from metal-carbon eutectics. The temperature of these fixed-points will be determined using the world’s most accurate filter radiometers. In preparation for the international measurement campaign, which NPL will be leading, we have developed the Absolute Radiation Thermometer and are investigating all aspects of its characterisation.

For more information detailing the services available at NPL, visit the Primary Filter Radiometer Calibration Facility page

For further information, please contact: Emma Woolliams