Metrics for specifying white lighting

This project concerns lighting in subsidiary roads. Anecdotal evidence supported by recent experimental research demonstrates that it is acceptable to install some lamps at a lower illuminance than that specified in BS5489-1:2003, and this illuminance reduction may offer a reduction in the energy consumed by street lighting. The metric currently used to characterise the application of this illuminance reduction is Colour Rendering Index (CRI). Whilst CRI works for some lamps used for street lighting, it is doomed to failure when applied to other lamps. This project will extend recent tests to incorporate a wider range of lamps; use the data set to test tools for predicting whether the illuminance reduction is appropriate; and determine how the findings are best presented to practitioners.

We carried out preliminary trials to verify the side-by-side matching task we used in a previous study. In one set of trials we repeated the side-by-side match but used a range of different field designs, from a neutral uniform surface to an interior space with coloured surfaces: there was little effect on the illuminance ratio at equal brightness. A second study used sequential evaluation (lighting from two different lamps used in temporal succession to illuminate a single space) rather than the simultaneous (side-by-side) evaluation used previously: there is negligible difference between the illuminance ratios at equal brightness derived from the two methods. These findings provide support for continued use of the side-by-side matching task.

A series of trials were set up to identify a lamp quality metric for specifying when it is acceptable to reduce the illuminance of street lighting. Five different lamps were used, these being used in all possible pairs, to enable predictions of brightness based on conventional lamp colour metrics. The tests mainly use the brightness matching procedure and this was validated by a forced choice judgement at equal illuminance; foveal visual performance was tested using a low contrast acuity chart, and preference of skin appearance and a colour array was examined to give an indication of acceptability. 38 test subjects were used, each attending three 2-hour test sessions.

Results

For equal brightness we found that the experimental results (illuminance ratios for equal brightness) were well predicted using the CIE recommended system for mesopic photometry (r2=0.86), even though that system was developed from visual performance data rather than perceptual judgements. While a slightly higher correlation was gained using the Sagawa brightness model (r2=0.89), or the Kokoschka and Bodmann model (r2=0.92), the CIE system is likely to be the more widely used: these data suggest it provides a reasonable prediction of brightness under light sources of different spectral power distribution.

We also found that some lamps which create spaces that appear to be very bright do not give pleasing appearance of skin or of a colour array; these judgements do not correlate well with the S/P ratio (the basis of CIE mesopic photometry) but do correlate well with the colour rendering index. The data suggest that R_a 30 is too low, that R_a>70 is acceptable, but did not provide sufficient data to determine where the threshold lies between R_a 30 and R_a 70; hence until further data are available we are not able to argue against the criteria R_a>60 as is currently used.

These new data, together with our previous work on obstacle detection, suggest that it is possible to reduce illuminance in subsidiary streets whilst maintaining the same level of visual benefits by choice of lamp type. This reduction in illuminance is proportional, as defined by CIE mesopic photometry, but this is applied only when using lamps of Ra>60.

The tests results have been submitted to Lighting Research & Technology for peer review publication, and are the basis of proposals for lighting in subsidiary streets to be published by the ILE and CIE.

Publications

• Fotios S and Goodman T. Proposed UK guidance for lighting in residential roads. Lighting Research & Technology, 2012; 44(1); 69-83. (see also Corrigendum : 2012; 44(3): 377)

• Fotios S. Guidance for lighting in residential streets to account for mesopic vision. Institution of Lighting Professionals (ILP). The Professional Lighting Summit, October 2012, Brighton.

• Fotios S and Goodman T. Research on lighting in residential roads. Keynote presentation. CIE 2012 conference: Lighting Quality & Energy Efficiency. Hangzhou, China. 19-21 September 2012. 21-37.

• Fotios SA and Cheal C, Predicting lamp spectrum effects at mesopic levels. Part 1: spatial brightness, Lighting Research & Technology, 2011; 43(2); 143-157. DOI: 10.1177/1477153510393932.

• Fotios SA and Cheal C, Predicting lamp spectrum effects at mesopic levels. Part 2: preferred appearance and visual acuity, Lighting Research & Technology, 2011; 43(2); 159-172. DOI: 10.1177/147715351039393.

• Fotios S and Cheal C, Road lighting for pedestrians in residential areas: choosing the optimum lamp colour characteristics. SELECTED PAPERS of the CIE Light and Lighting Conference with Special Emphasis on LEDs and Solid State Lighting. 27-29 May 2009, Budapest, Hungary, CIE x034:2010. 29-38.

• Fotios S and Cheal C. Predicting spatial brightness at mesopic levels. Proceedings of CIE 2010 Lighting Quality and Energy Efficiency, 14-17 March 2010, Vienna. 465-474.

• Fotios S and Cheal C, Road lighting for pedestrians in residential areas: choosing the optimum lamp colour characteristics, Svetotekhnika, 2009; 6; 28-36.

• Boyce PR, Fotios S and Richards M, Road lighting and energy saving, Lighting Research & Technology, 2009: 41(3); 245-260.

• Fotios S and Cheal C, Spectrum-balanced illuminance reductions for exterior lighting, Lux Europa, Istanbul, 2009, 1207-1212.

• Fotios S and Cheal C, Road lighting for pedestrians in residential areas: choosing the optimum lamp colour characteristics, CIE Light & Lighting conference, Budapest, 27-29 May 2009.

• Fotios S and Cheal C, The specification of white exterior lighting, Proceedings of the 6th Lux Pacifica, Bangkok, 23-25 April 2009, 93-96.

• Fotios SA, White Lighting in subsidiary roads: can it be done and should it be done? ILE Annual Conference, Bristol, 24-26 September 2008.