Introduction

Responsivity standards and measurement techniques have changed significantly in the past 20 years. In addition to relatively simple single element detectors, more complex radiometers with multiple-reflection input geometries (trap, wedge, and sphere), filters, windows, diffusers, and apertures are now widely used. Also, small field-of-view and/or narrow band-pass elements are frequently used in modern radiometers. In addition to radiant power, many applications require irradiance and/or radiance responsivity measurements. This document describes how to perform spectral radiant power, irradiance, and radiance responsivity measurements. Measurement methods are described for detectors, radiometers and photometers to determine the relative spatial, angular and spectral variations of their (absolute) responsivities. The document describes measurement geometry, measurement setups, typical types and properties of different detectors, radiometers and photometers, and measurement methods. The measurement methods include procedures to obtain traceability to National Metrological Institutes (NMIs) and guidance on selecting standards. Evaluation of measurement uncertainty and state of the art measurement uncertainty values are discussed. Common problems in all of the above three radiometric quantity measurements, such as spectral issues, DC and AC measuring instruments are also discussed. Details of the primary standards and the procedures for the realization of units (the tasks of NMIs) are not discussed in this report. Also, multi-element devices, such as two dimensional array detectors, spectrographs, and imaging devices, including hyper spectral instruments, are not discussed here. Multi-element devices are discussed in other documents such as the TC 2-51 Technical Report¹.

A test device can be calibrated against the detector standard if it can be substituted for the detector standard. The responsivity of the test device will be the ratio of its electrical output signal (current or voltage) to the magnitude of the measured radiometric quantity determined by the detector standard. Responsivity measurements can be made broadband or spectrally (versus wavelength). This document is mainly focused on spectral responsivity measurements. Even in the case of broadband responsivity measurements it is often necessary to measure the spectral responsivity of the detector (radiometer) as well e.g., a broad-band UV source can be measured with a broad-band UV-A detector only if the spectral responsivity of the detector and the spectral power distribution of the source are known, so that any spectral mismatch corrections can be determined. The relative standard measurement uncertainty of the test device will be always larger than that of the standard device because of the increased number of the calibration steps (during the measurement transfer).

Responsivity related calibrations are:

- relative spectral responsivity (in power, irradiance, and radiance modes),

- absolute spectral responsivity (in power, irradiance, and radiance modes),

- uniformity:

- relative variation of the spatial responsivity (typically varies with wavelength),

- relative variation of the angular responsivity (typically varies with wavelength),

- aperture effective area measurement,

- linearity (typically varies with wavelength),

- temporal characteristics,

- temperature coefficient of detector responsivity (typically varies with wavelength).

Responsivity measurements can be utilized in the field of:

- optical radiometry,

- photometry,

- colorimetry,

- temperature measurements of blackbody radiators,

- optical pyrometry,

- solar photovoltaic quantum efficiency measurements.

¹ In preparation by CIE TC 2-51 The Calibration of Multi-Channel Spectrometers.