What is Spectral Responsivity?

The spectral responsivity of an optical detector is a measure of its response to radiation at a specified wavelength. The precise definition depends on the way that the detector is used:

• When the whole optical beam falls within the detector aperture the responsivity is equal to the ratio of detector response to beam radiant power.
• When the detector is placed in a radiation field which over-fills its active area (the aperture) the responsivity is equal to the ratio of detector response to the irradiance of the field.

How is it Measured?

Since spectral responsivity is defined at a specific wavelength monochromatic (i.e. single wavelength) optical radiation must be used to make the measurement. Traditionally, such radiation has been generated by means of a prism or diffraction grating monochromator, which selects a narrow wavelength band from a broad band lamp source such as a tungsten lamp. Such techniques are used at NPL for generation of radiation between 200 nm and 20,000 nm. The wavelength can readily be tuned continuously allowing measurement of spectral responsivity across the wavelength range for which the detector is used. The limitations of this method are relatively low power levels and low wavelength resolution, both of which restrict the accuracy of calibration.

Alternatively, lasers can be used as calibration sources. These have much narrower spectral bandwidth and generally much higher output power, making them the source of choice for high accuracy work. NPL has a suite of lasers for this purpose covering much of the wavelength range between 210 and 1700 nm, as well as important spectral regions at longer wavelengths up to 11,000 nm (see NLRF web page).

In all cases the detector under test is compared with a standard detector of known spectral responsivity traceable to the NPL primary cryogenic radiometer (see cryogenic radiometer web page). Such detectors include silicon and InGaAs photodiodes. and pyroelectric detectors.

Detector Characterisation

No detector has a constant spectral responsivity under all conditions of use. For instance, the responsivity of photodiode detectors falls off above a certain incident power level due to saturation effects. Full evaluation of a detector requires characterisation of such effects.

This information allows the identification of the detector with the best performance for a particular application. Also, a detector to be used for calibration purposes should be fully characterised in order to evaluate the uncertainty of such calibrations. NPL has assembled dedicated facilities which permit the characterisation/measurement of the following detector parameters:

• Linearity range: This is the range of incident radiant power levels over which the detector output varies linearly with incident power. Incident power levels in the range 10^-10 – 10^-2 W can be generated at different wavelengths using either lamp-based or laser-based methods. In this way the saturation threshold of a detector can be determined.
• Spatial non-uniformity of response: the variation of responsivity across the active area of a detector is determined by scanning a small patch of radiation at a selected wavelength over the surface.
• Noise Equivalent Power (NEP): the incident radiant power at a specific wavelength and modulation frequency necessary to produce a signal-to-noise ratio of 1 at the detector output.
• Temperature coefficient of response: the variation of responsivity with temperature
• Detector shunt resistance: the slope of the current-voltage curve of a photovoltaic detector at zero-volt bias.

Filter Radiometry

A particularly important class of detectors are “filter radiometers”. They are used for the calibration of spectral radiance and irradiance of sources and also for “spectrally weighted measurements”, eg erythema dose or photometry. In their simplest form, filter radiometers consist of a combination of a precision aperture, a band-pass filter and a detector which detects the radiant power transmitted by the aperture and the band-pass filter. NPL has developed laser-based methods for the calibration of the spectral irradiance response of filter radiometers.

Calibration of Cameras and Detector Arrays/Matrices.

NPL are assembling facilities for the calibration of the spectral response of imaging systems (cameras) in terms of spectral radiance and detector arrays and matrices in terms of spectral irradiance.