Chris Cheal

Lighting for subsidiary streets: investigation of lamps of different SPD.Part 2-Brightness

British Standard BS5489-1: 2003 permits a trade-off between colour rendering and illuminance for lighting in subsidiary streets—if lamps of high colour rendering index are used, such as metal halide instead of low- and high-pressure sodium, a lower illuminance can be used. A series of tests are carried out to validate the trade-off and this paper reports on the new brightness data from these tests. The experimental results support the trade-off but it is suggested that its application may depend on the stage of chromatic adaptation at which the assessment of brightness is made. The experimental results are compared with predictions made by four models of mesopic photometry.

Predicting lamp spectrum effects at mesopic levels. Part 1: Spatial brightness

This paper reports on experimental work carried out to test metrics for predicting spatial brightness at mesopic levels under lamps of different spectral power distribution. The side-by-side matching technique was used following an extensive review of experimental design. Five different types of lamps were presented in all 10 possible pairs, these being selected to compare brightness predictions based on established characteristics of lamp spectrum such as colour rendering index, correlated colour temperature and the scotopic/photopic ratio. The results were also used to test proposed systems for predicting brightness and visual performance at mesopic levels. Of the lamp characteristics examined the scotopic/photopic ratio exhibited the highest correlation with the test results. The new CIE recommended system for visual performance based mesopic photometry was found to give an acceptable prediction of the brightness results.

Obstacle detection: A pilot study investigating the effects of lamp type, illuminance and age

A novel apparatus was used to examine the effect of light source, illuminance and observer’s age on the ability to detect obstacles in peripheral vision, simulating a raised paving slab under mesopic visual conditions. The data collected were used to determine the height of obstacles above the paving surface required for 50% detection. From these detection heights it was determined that: (1) obstacle detection was influenced by illuminance, the 50% detection height being lower at 20 lux than at 0.2 lux, (2) the young observers (<45 years old) showed the smaller 50% detection height at 0.2 lux, but at 20 lux there was no difference in obstacle detection height between the younger and older (>60 years old) age groups, and (3) obstacle detection was affected by lamp type at 0.2 lux, with the 50% detection height decreasing as lamp S/P ratio increased, but at 2.0 and 20 lux there was no significant difference between the three test lamps.

Light source spectrum, brightness perception and visual performance in pedestrian environments: a review

This review considers the impact of light source spectral power distribution (SPD) on brightness perception and visual performance in mesopic conditions, with emphasis on the comparison of metal halide (MH) and high pressure sodium (HPS) lamps. Models of mesopic vision predict that SPD is a significant variable in that at a HPS photopic luminance of 0.100 cd/m2, MH need only produce about 0.070-0.075 cd/m2 to be seen as equally bright. However, attempts to validate the predictions of these models in the field have met with mixed success. As for visual performance, experimentation has shown that there are effects of SPD in mesopic conditions, but the magnitude of these effects depends on the nature of the task. Three alternative approaches are suggested for comparing light sources with different SPDs in mesopic conditions.

Using eye-tracking to identify pedestrians’ critical visual tasks, Part 1. Dual task approach

This paper investigates the critical visual tasks of pedestrians, the first step in a review of design guidance for lighting in residential roads. Eye tracking was used to record pedestrians’ visual fixations when walking outdoors in daytime and after dark with a concurrent dual task to better understand which fixations were critical. Fixations at critical instances, these being shown by slow reactions to the secondary task, were categorised into one of eight groups. Of these, the path and other people were the most frequent items, with people more likely to be fixated at a far distance and the path at a near distance. After dark the path was more likely to be fixated and other people less likely to be fixated compared with daylight.

Using Obstacle Detection to Identify Appropriate Illuminances for Lighting in Residential Roads

This paper uses data regarding detection of pavement obstacles to explore two approaches to establishing an appropriate illuminance for road lighting designed to meet the needs of pedestrians. A previous obstacle detection experiment was repeated using young observers under high pressure sodium (HPS) lighting. One approach was to identify whether there is a plateau-escarpment relationship between obstacle detection ability and illuminance – better detection with increasing light level until further increases bring little improvement: This suggested an appropriate illuminance of 5.7 lux. The second approach was to identify the size of an obstacle that a pedestrian should expect to be able to detect and the associated probability of detection: An obstacle of height 25 mm located 6 m ahead may require 1.8 lux to be detected with 95% probability.

Stimulus range bias explains the outcome of preferred-illuminance adjustments

Review of previous studies suggests that mean preferred illuminances determined using an adjustment task tend to lie in the middle of the range of illuminances available, a stimulus range bias. This hypothesis was validated through the results of an experimental study. It is therefore suggested that the adjustment task is not an appropriate method for determining optimum illuminances.

A comparison of simultaneous and sequential brightness judgements

This paper presents the results of brightness matching and brightness discrimination tests carried out using sequential evaluation (temporal juxtaposition) to compare brightness under lamps of different spectral power distribution at mesopic levels of illumination. These data are compared with the results of previous tests, which used simultaneous evaluations (spatial juxtaposition) to enable comparison of these different modes of evaluation. It is concluded that sequential and simultaneous evaluations yield similar estimates of illuminances required for equal spatial brightness and similar levels of precision in this task.

Lighting for subsidiary streets: investigation of lamps of different SPD.Part 1-Visual Performance

British Standard BS5489-1: 2003 permits a trade-off between colour rendering and illuminance for lighting in subsidiary streets—if lamps of high colour rendering index such as metal halide are used instead of high- or low-pressure sodium lamps, a lower average illuminance can be used. A series of tests are carried out under mesopic conditions to validate the trade-off, and this paper reports on the new visual performance results. Four tests are carried out: acuity of achromatic and chromatic targets, achromatic contrast detection threshold, and colour identification, these being for on-axis targets. It is found that spectral power distribution (SPD) does not affect the performance of achromatic tasks except for an increase in contrast detection threshold under low-pressure sodium lamps. The performance of an acuity task using coloured targets displayed interaction between target colour and SPD. Colour naming accuracy is found to be significantly higher for metal halide lamps than for sodium lamps. For all tasks there is a reduction in visual performance at lower illuminances, and therefore a reduction in design illuminance leads to a reduction in the performance of some visual tasks which may not be offset by lamp SPD. Implications for the performance of real pedestrian tasks are discussed.

Effects of outdoor lighting on judgements of emotion and gaze direction

Road lighting in residential roads should enhance the visual component of interpersonal judgements concerning the apparent intent of other pedestrians – whether friendly, aggressive or indifferent. This paper describes an experiment which collected forced-choice judgements of emotion and gaze direction after 1000 ms exposure under 18 combinations of lamp type, luminance and interpersonal distances. Better performance was found with higher luminance and larger task size, but with diminishing returns according to a plateau-escarpment relationship. The results were used to estimate appropriate light levels for outdoor lighting. Results for judgements of emotion from facial expression suggest a minimum luminance of the face of 0.1–1.0 cd/m2 if facial expressions are to be identified accurately at 4 m, but a luminance above 1.0 cd/m2 for identification at 10 m.