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Imaging photometer tests car headlight performance

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Ring Automotive, based in Leeds, UK, supplies performance bulb upgrades as replacements for the original equipment (OE) lamps fitted to car headlights. The company needed a way of comparing the luminance, chromaticity and colour temperature of their aftermarket bulbs and competitor products against OE lamps in order to establish their performance benefits.

Traditionally, the illumination performance of a headlamp has been determined using a specialist device called a goniophotometer. This type of instrument places the headlamp on a motorised stage that rotates and tilts the headlamp in relation to a photometer that views the headlamp along a fixed direction of view.

Pro-Lite Technology, based in Cranfield, UK, proposed an imaging photometer to make the measurements, which is a powerful, CCD-based light and colour measurement instrument that provides for vastly increased productivity compared with traditional goniometric and 'spot' measurements of lamps. Whereas a goniophotometer can only measure the luminance and colour from the lamp one direction at a time, a CCD-based imaging photometer can measure millions of angles simultaneously. Moreover, because the imaging photometer views the whole illumination pattern at once, localised luminance and colour differences can be easily detected – artefacts that goniometric measurements performed at defined angles might easily miss. In addition, the capital cost of an imaging photometer is much less than that required for even the most basic motorised goniophotometer.

Ring selected a ProMetric model PM-1423F-1 CCD imaging photometer which is manufactured by Seattle-based Radiant Imaging and sold and supported by Pro-Lite in the UK. The model is a 1536 x 1024 pixel camera with 16,000:1 photometric sensitivity and spectral response accurately matched to the CIE tristimulus observer functions. This provided the necessary level of luminance sensitivity, spatial resolution and spectral match to the human visual response.

Of importance to Ring was the ability of the chosen measurement system to analyse the luminance pattern from a headlight in accordance with the multi-point test pattern specified in ECE regulation R20. The information necessary to perform this analysis manually was already contained within the standard ProMetric software. However, to further standardise the testing of Ring's lamps and enhance accuracy, consistency and productivity, Radiant Imaging agreed to develop an additional software application for the ProMetric camera that would automatically determine the datum position (beam elbow) in the illumination pattern, align the camera image to this position, measure the luminance at the ECE R20 specified test points for left or right hand drive, and provide for simplified pass/fail type reporting of the device under test. This application is called ProMetric Headlamp Test (PM HL-Test for short) and is now up and running in the photometric laboratory at Ring Automotive in Leeds.

Douglas Kreysar, Radiant Imaging's CEO commented: 'While the ProMetric system has been used in the automotive lighting field for several years, I am extremely pleased that we have developed a product that provides specific solutions for headlamp measurement and analysis. These tools provide further productivity gains for the use of the ProMetric system for automotive lighting.'

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