In order to drive safely into a tunnel, the so-called visual critical details in the tunnel must be visible for the road user. Therefore the tunnel entrance must be sufficiently illuminated, especially during daytime. The illumination in tunnels is set to such level that a reference object in the tunnel is just visible for a road user at a certain distance outside the tunnel. However, this method is only based on luminance contrast and the possible contribution of the colour is ignored. The contribution of the colour can be taken into account by multiplying the contrast of the object with the so-called colour factor. This is the ratio of the colour difference and the lightness difference of the object and the background, according to the colour difference formula in the CIELUV colour space. Weighting factors must be used for objects with a a visual angle smaller than 1 degree. For a representative colour set the colour factor was calculated for various conditions. This has produced a number of conclusions, some of which are: The visibility of neutral (colourless) objects is the best with coloured tunnel lighting; The colour factor is largest for large, coloured, objects under a daylight lamp shining into the tunnel; Objects illuminated by high-pressure sodium lamps have a colour factor larger than 1; and tunnel lighting with a daytime colour is better for coloured objects if the light is shining into the tunnel. For symmetrical lighting daylight illuminant is advantageous only for large coloured objects. For lighting oriented towards the tunnel entrance, daylight colour does not aid object delectability.
Abstract