The Road Less Travelled

THE ROAD LESS TRAVELLED

From lighting at night for cyclists and pedestrians to the general V( λ) mismatch index, Iain Carlile selects three of the most recently published papers in Lighting Research and Technology

Uttley et al have investigated the effect of light levels on promoting cycling at night. They comment that while there are many benefits to cycling, people may be put off from doing it at night because of reduced visibility, road safety concerns and personal security.

Taking results from previous studies and accounting for factors such as time of day, seasonal weather variations and holiday periods, the authors extended this earlier work by analysing cyclist counts in Norway, a country at a higher latitude than those previously studied.

They found that darkness affected cycling rates, but that this impact was lower than previous work had shown, suggesting other factors may be important.

Road lighting can encourage more cycling after dark by increasing average illuminance on routes used by cyclists. The authors note, however, that there may be an optimal illuminance for encouraging cycling at night, and anything beyond this illuminance value would actually have no further beneficial effect.

In addition to optimal illuminance, other lighting criteria such as uniformity and spectrum are likely to have an effect and further evidence is being collected through the Saturn (supporting active travel using roadlighting at night) project.

Also focusing on lighting at night, Abboushi et al’s paper looks at pedestrians and their potential to experience discomfort glare caused by outdoor lighting. While several models measuring discomfort have been proposed, they point out that no consensus exists as to which is the most appropriate model to use.

Seven models were tested using four independent datasets. From the range of different experimental conditions used, the authors conclude that direct illuminance at the eye is the most suitable model as it tended to offer similar or better predictions than other models. They note that a 2008 study by Bullough et al exhibited best performance, but this method requires additional measurements that may not be easy either to predict at design stage or to take field measurements.

Krüger et al’s paper examines the general V(λ) mismatch index f’ between a photometer’s relative spectral responsivity, and the spectral luminous efficiency function for photopic vision, V(λ). They present a review of the historical development, explaining the reasons for the current definition and potential future adjustments.

The authors postulate that in future photometers will be calibrated with a white LED light source as the reference, resulting in the need for a more appropriate definition of the general V(λ) mismatch index, either by using a different normalisation in f’ for the photometer’s relative spectral responsivity, or alternatively introducing a different function to assess the mismatch.

They also note that the measurement of coloured LEDs is increasing in importance and therefore suggest that a general mismatch index for white and coloured light sources may be more suitable.

Iain Carlile, FSLL, is a past president of the SLL and a senior associate at dpa lighting consultants

Lighting Research and Technology: OnlineFirst In advance of being published in the print version of Lighting Research and Technology (LR&T), all papers accepted for publishing are available online. SLL members can gain access to these papers via the SLL website (www.sll.org.uk)

The effect of changes in light level on the numbers of cyclists J Uttley, S Fotios, CJ Robbins and C Moscoso

Predicting discomfort from glare with pedestrian-scale lighting: A comparison of candidate models using four independent datasets B Abboushi, S Fotios, and NJ Miller

General V(λ) mismatch index: History, current state and new ideas U Krüger, A Ferrero, A Thorseth, V Mantela and A Sperling