Infrared radiometry requires large area, linear detectors of spatially uniform response. Currently the choice of high quality detectors of mid-infrared (>8 μm) radiation is far from ideal for radiometric applications. For example, HgCdTe detectors are widely used but exhibit very large (>20%) spatial non-uniformities in their responsivity whereas thermal detectors such as pyroelectric detectors have relatively low D* values. Quantum Well Infrared Photo-detectors (QWIPs) are now well established for use in state-of-the-art cooled thermal imaging systems, driven by military application. For fundamental optical measurement applications (for example, spectral responsivity standards) the expense and complication of imaging arrays is not required. Some QWIPs are made from layers of GaAs/AlxGa1-xAs material which can be mass grown on large substrate wafers with high spatial uniformity. As such, QWIPs offer the potential to be manufactured as a large area single pixel device, with a uniform spatial response, as well as a high D* value. This paper will detail the development of a single pixel QWIP detector and present the results of an initial evaluation of this detector, carried out at NPL.
In its role as the national standards laboratory for the UK, the National Physical Laboratory (NPL) maintains, develops and disseminates, amongst others, the UK's detector spectral responsivity scale and material spectrometric scales (regular, hemispherical and angular reflectance and transmittance). In order to carry this work out detectors, materials, methods and facilities are continually under development at NPL. This paper will present the latest measurement techniques used at NPL that are applicable for the characterisation of infrared detectors and materials. NPL has extensive calibration capabilities, making use of grating and FT spectrometers and tuneable lasers, covering a wide spectral range, catering for single element, array, sub-pixel resolution and photon counting devices. As well spectral responsivity, detector spatial uniformity and linearity measurements are available. The UK spectrometric scales are maintained from 200 nm to 56 μm and include regular, hemispherical and angular reflectance and transmittance scales, and artefacts for the wavenumber and ordinate calibration of mid-infrared spectrometers.
The National Physical Laboratory (NPL) realises and disseminates the UK spectrometric scales from 200 nm to 56 μm wavelength. Its mid-infrared (MIR) regular and hemispherical reflectance and transmittance scales, and transfer standards for the wavenumber and ordinate calibration of MIR spectrometers are realised from 2.5 μm to 56 μm using specially modified grating spectrometers.
This paper will discuss a technique, recently developed at NPL, for the direct absolute realization of the hemispherical reflectance scale. This scale was previously based on a relative method using an absolutely calibrated mirror and published data for BaSO4. The new method has given an improved value for the pure BaSO4 to underpin the relative method. It opens up new applications as it can be applied to types of sample not previously measurable e.g. foil-covered insulation, and this aspect is discussed. The various sources of uncertainties are considered and the existing standards and services are also described briefly to place the technique in context.
Although developed on grating instruments, this new measurement capability can be realized on Fourier transform (FT) spectrometers. Progress is being made at NPL in transferring its other MIR measurement capabilities to FT instrumentation. This will safeguard NPL’s future ability to provide these services to customers.
NPL, in conjunction with many NMIs, has been seeking to improve the accuracy of its primary scales of spectral irradiance and radiance. In common with other laboratories this has been done through the use of an ultra-high temperature blackbody characterized using filter radiometers calibrated against a cryogenic raediometer. While such work is of importance to the Earth Observation community, it is also recognized that of at least equal importance is an improvement in the quality of the scales that are provided to the end user. This paper will describe new transfer standard sources of both spectral radiance and iradiance that have been developed not only to improve accuracy to the end user, but also to provide it in a form that is both robust and convenient to use. For example the radiance source has a spatial non-uniformity of <0.05% over a 50 mm diameter aperture and can maintain its accuracy for more than 100 hrs of operation.