National Physical Laboratory

Properties of optical detectors

Detector Characterisation

Detectors may vary in response across their surface, and also in their response to different power levels. The uniformity and linearity facility can fully measure detectors for both these characteristics.

Detector Uniformity Scan
Detector uniformity scan

NPL's uniformity and linearity of response facility is able to characterise working standard detectors for both spatial uniformity, across the detector's surface, and the linearity of response to different incident power levels. Uncertainties achieveable depend on the detector under test, but typically results can be expected with a confidence of 0.1%. The detectors can be measured over the wavelength range 400 nm to 14 mm, and at a bandwidth selected by customer defined band-pass filters.

The uniformity of response across the sensitive area of a detector can be determined using equipment that scans a small circular patch of radiation of a selected diameter across the surface. The patch of radiation will determine the resolution of uniformity measured.

The linearity of optical radiation detectors can be determined by comparing the output of a detector when two equal fluxes are incident separately onto it, with the response when they are incident in combination.

The ratio of the combined flux to the sum of the fluxes measured separately is called the linearity factor. For a  perfectly linear detector, this factor would be unity, but will be less if the detector is saturating.

The linearity factor can be measured from a power level of approximately 0.1 mW (bandwidth dependent) down to the noise limit of the detector or a few picowatts, over six orders of magnitude.


The uniformity and linearity of response facility is a dual-purpose system. All modes are fully automated and the system is supplied with the appropriate software and support electronics.

Operating Details
Spatial uniformity Typically ± 0.1% for 1 µm to 14 µm
Linearity of response Typically ± 0.2% for 400 nm to 990 nm & ± 0.1% for 1 µm to 14 µm
Sources Tungsten lamp, for UV and visibleCeramic globar for infrared
Optics Two large off-axis parabolic mirrors for focusing the image onto the detector
Pinholes Set of 11; 25µm, 50µm, 100µm, 200µm, 400µm, 600µm, 800µm, 1mm, 2mm, 3mm, 5mm
Band pass filters Set of 14; 488 nm, 514.5 nm, 530 nm, 632.8 nm, 850 nm, 1.06 µm, 1.3 µm, 1.55 µm, 2.4 µm, 3.7µm, 4.8 µm, 7.8 µm, 10.3 µm, 12.8 µm
Neutral density filters 50%, 25%, 12%, 6%, 3%, 1.5% and 0.8% transmission levels, visible and infrared individual sets, on associated filter wheels
Detector stage x-y translation stage for test detector, ± 0.5 µm
Calibrated detectors as transfer standards
Transfer Standard Detectors
Transfer standard detectors
Further detector characterisation can be achieved by using the NPL spectral responsivity facility

NPL's support includes the installation of the facility and training in its use.

Contact us for further information

Call the Customer Service team on: 020 8943 6151

Last Updated: 28 Apr 2017
Created: 19 Jun 2007


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