Metrics for specifying white lighting

Dates: June 2008–June 2010
Staff: Steve Fotios, Chris Cheal
Funding: EPSRC grant EP/F035624/1

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.


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.

Test results (mean illuminance ratios at equal brightness) plotted against brightness predictions (ratios of brightness model outputs)

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 Ra 30 is too low, that Ra>70 is acceptable, but did not provide sufficient data to determine where the threshold lies between Ra 30 and Ra 70; hence until further data are available we are not able to argue against the criteria Ra>60 as is currently used.

Preference scores for five lamps (mean of judgements for appearance of hands, colours and space at equal illuminance and equal brightness) plotted against CRI (R2=0.91).

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.


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