Visibility is the real safety topical issue of good functional road lighting installations and a means to improve our knowledge of vision in other outdoor lighting applications. Until now the methods of measurement on site have been usually limited to illuminances or, in the better cases, to luminances on grid points of reference surfaces. There is therefore still a gap between calculations that can be relatively sophisticated thanks to existing visibility models and the practical measurements.The only way to prove that visibility levels of a lit realisation are sufficient for the observers is to check whether they can benefit or not of satisfactory lighting distributions to ensure at least minimal visibility. And this is manifestly something which is difficult to translate even for drivers in road lighting by the conventional requirements of luminance level, uniformity and limitation of glare.Nowadays however, there is an opportunity to link calculations to real measurements. The new digital luminancemeters that certain manufacturers provide on the market allow accurate evaluation of point and area luminances with a large dynamic. Consequently they give the means to calculate from measured values of luminance the visibility of physical obstacles, delimited on the road or on other vertical background surfaces, including possible glaring luminances due to the luminaires and other light sources present in the field of view of the observer.The present paper shows, from measurements on a test road and its background, how visibility can be evaluated on site confronted to calculations.
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