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This publication gives explanatory comments on the Dutch standard NEN 3322 "Road traffic control light signals. Photometric requirements and test methods", published in December 1972. After a general introduction in chapter 1, chapter 2 deals with the fundamental aspects that were used as a basis when drawing up the requirements for chromaticity coordinates, luminous intensities and luminous intensity distributions, the limitation of the phantom effect, and optical systems presenting a symbolic appearance e.g. green-red arrows and crosses, pedestrians and bicycles. In chapter 3 some aspects are discussed of the locating and directioning of light signals. Some aspects of the construction and maintenance of light signals are discussed in chapter 4. Finally, chapter 5 deals with the test methods of light signals for luminous intensities, limitation of the phantom effect and the colours of the signals.

In a psychophysical experiment the adequate luminous intensities for traffic lights were measured as function of time of day, colour, size and three perception criteria minimum, optimum, and maximum. By day the traffic lights were adjusted at average luminous intensities of 124 cd, 369 cd, and 616 cd for the perception criteria minimum, optimum, and maximum. By night these average luminous intensities are, respectively, a factor 35, 17 and 10 lower. By day small effect of colour and size was found. A performance level 2/1 of standard EN 12368, with a luminous intensity range from 200 cd to 800 cd fits the best on the experimental results. For dark environments a dimming factor of 10 is recommended for yellow and green lights and 4 for red lights.