The intensity of traffic lights decreases over time as a result of pollution and ageing. The Dutch Traffic Research Centre of the Ministry of Transport, Public Works and Water Management has asked TNO to develop an apparatus to measure this decrease in intensity out on the road. The purpose is to 1) judge whether a particular traffic light needs to be replaced or cleaned and 2) what replacement and cleaning frequencies are required in general. In the first place, optical requirements were specified, based on photometric measurements and calculation. The apparatus should fit on 200mm as well as 300mm lights.The designed apparatus, abbreviated as "Traffic Light Intensity Control or VLIC", should be robust, usable during daylight conditions, and easy to transport in a car. VLIC should therefore measure only the light emitted by the traffic light and be insensitive to the surrounding daylight. To test this a number of traffic lights have been evaluated with two prototype VLIC's. Several ways are described in which such a device may be constructed. Following the European standard, measurements have been performed on different traffic lights. The (dis)advantages of the various designs are discussed. The final choice is an intensity meter that measures the light emitted perpendicular to the light source. This method gives identical results as the method described in the European standard. To obtain the right value the intensity measured with the proposed method should be multiplied by some factor. It is established that this factor is constant for the light sources measured. (A)
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