LRT Digest 1 Maintaining brightness while saving energy in residential roads
Authors: Fotios Steve 
Published: February 2013

Executive summary

This digest presents a summary of recent research on the relationship between lamp spectrum and brightness in the context of residential roads. Lighting design for such roads considers primarily the needs of pedestrians rather than drivers. For pedestrians, brightness is important because brighter lighting tends to produce higher levels of perceived safety in a particular location. Studies of brightness perception for lamps with different spectral power distributions at illuminances representative of those used for residential road lighting have been made using controlled laboratory conditions and field surveys. The findings from these two approaches are in good agreement. It has been found that in the mesopic region (the lighting levels found on roads at night time) lamps with a higher scotopic / photopic (S/P) ratio appear brighter at the same illuminance than lamps with lower S/P ratios. This means that lamps with a high S/P ratio can be used either at the same illuminance to create a higher brightness or at a lower illuminance but the same brightness, the latter leading to a reduction in energy consumption. The recently established CIE system of mesopic photometry, which involves S/P ratio, can be used to predict the illuminance reduction that will ensure the same level of brightness for a range of lamps with different spectra. However, brightness is not the only factor that matters for the lighting of residential roads. The ability to detect obstacles is also important, as is the appearance of the environment and the confidence that the intent and identity of other people can be recognised. Other studies have shown that the ability to detect obstacles is influenced by lamp spectrum in a similar manner to brightness, that is, a higher S/P ratio improves obstacle detection. As for the acceptability of the appearance of the environment, this is more consistently related to the colour rendering of the light source than the S/P ratio. How light spectrum affects the ability to recognise the intent of other people is still uncertain. Given these findings it is possible to modify the illuminances used in residential roads when using different light sources. These illuminances should be chosen based on two characteristic of the light source, S/P ratio and colour rendering index. The Institution of Lighting Professionals (ILP) have proposed two rules: 1 Reducing illuminance from the levels recommended in the S-series of lighting classes can be considered when using lamps which have a CIE general colour rendering index greater than or equal to 60. Where lamps with a colour rendering index greater than or equal to 60 are to be used, the illuminance reduction allowed can be calculated using the CIE system of mesopic photometry, assuming the low pressure sodium lamp is the reference. The table below gives the examples of the illuminances predicted by this process for a number of alternative lamps for the S-classes given in BS 5489-1:2003 and BS EN 13201-2:2003. This demonstrates that MH lighting, for example, may be used at a lower average illuminance than HPS lighting, and this may lead to a saving in energy consumption. [Table: see text]