Optimization of lighting installations should be a priority in order to reduce energy consumption and obtrusive light while providing optimal visibility conditions for road users. For the design of lighting installations, it is assumed that the road has homogeneous photometric characteristics and only one viewing angle is used. There are often significant differences between the design of lighting installations and their actual performance. In order to examine whether these differences are due to the photometry of the road, this study proposes metrics to assess the influence of road heterogeneity and observation angle. These metrics have been used on many measurements conducted on site and in the laboratory for different pavements. A calculation engine has been developed to realize road lighting design with several r-tables in the same calculation or for different observation angles. Thus, this study shows that a root mean squared deviation (RMSD) calculation, including average luminance and uniformities associated with different r-tables, is directly correlated to a normalized root mean squared deviation (NRMSD) calculation between these r-tables. With these proposed metrics it is possible to optimize lighting installation while taking into account different types of urban surfaces and the diversity of users.

This paper focuses on the visibility of road users in urban environment. Firstly, a virtual reality experiment was carried out to determine where road users look on the road surface when travelling in city. The data collected were used to deduce observation angles for each type of road user. Measurements on an experimental site and simulations were carried out with these new observation angles. These were used to assess the impact of changing the observation geometry on the quality criteria of a lighting installation and on the visibility criterion. The results show that increasing the observation angle leads to a reduction in the average luminance but that, despite this, visibility is not affected. This led us to propose adaptations to the current recommendations: using a mobile observer for an observation angle greater than 1° and downgrading by one class when dimensioning a lighting installation in the city in the standard way, while taking into account the visual needs in an urban environment.

This paper addresses both geometrical definitions and a methodology of computation of the average luminance factor. The road lighting community deals with a dedicated coordinate system adapted to the different lighting situations. The road surface characterisation is used both for the road lighting and road marking community and is also useful for visual appearance models that use Bidirectional Reflectance Distribution Function (BRDF). In this paper, conversion relations are given between BRDF and road/marking lighting conventions. The second issue deals with the calculation of the average luminance coefficient Q0 defined in CIE 144:2001 for road surface characterisation. The calculation has been defined in the seventies and its explanation cannot be found as it has not been published. The paper proposes a new definition of the solid angle for the integration that provides the best compromise between mathematical rigour and previous customary approach.

In lighting design, knowing the pavement reflection properties is the best way of optimising road luminance, but using the CIE standard r-tables remains the common practice. A lesser evil is to scale these r-tables using Qd or Q0 coefficients to consider the lightness of the pavement. Using various r-tables databases and numerous lighting calculations, we show in this study that the scaling with Qd is a good option for most surfaces. This result is interesting because there are devices on the market capable of measuring Qd directly. However, they are dedicated to the visibility of road markings and operate with an observation angle of 2.29° instead of the conventional 1° in road lighting. Our investigations show that this geometrical difference can be considered by applying a correction factor to the measured Qd. It could then be used to scale standard r-tables, which would represent a significant operational advance.

In road lighting, calculations are made within the rectangular CIE grid while measurements conducted with Imaging Luminance Measurement Devices (ILMD) have a trapezoidal shape. We propose to build a grid that takes the observer’s perspective into account by projecting ellipses of adjustable dimensions than can match human vision or ILMD angular aperture and allow direct comparison with luminance images. Lighting standard criteria definitions remain unchanged and values of average luminance and longitudinal uniformity are equivalent. However, as the grid in perspective describes more finely the luminance distribution, the overall uniformity is lower than in the classical grid but more representative of observer’s perception. We present in detail our approach, give the corresponding code, expose calculations performed with this new grid and show an example of direct comparison between calculations and measurements.