PhD Theses by members of the research group
Investigating the visual tasks of pedestrians and how one of these tasks, obstacle detection, is influenced by lighting
James Uttley. University of Sheffield. 2016. Examined by Tom Foulsham, Essex University.
Current guidelines for pedestrian road lighting are not based on empirical evidence. One approach to providing suitable evidence is to examine the effect of lighting on the visual tasks of pedestrians. This first requires an understanding of what these visual tasks are. An eye-tracking study was carried out in which pedestrians walked a real, outdoor route during the day and after-dark. A novel dual-task method was used to identify the critical visual tasks of the pedestrians. Reaction times to a concurrent audio response task were used to indicate instances when attention may have been diverted towards something significant in the visual environment. Analysis of the eye-tracking videos at these critical times found that the path and other people were the two most significant items looked at.
Observation of the path is important for detection and avoidance of obstacles and trip hazards. Good road lighting should therefore facilitate obstacle detection. An obstacle detection experiment was therefore carried out examining the effect of illuminance and Scotopic/Photopic (S/P) ratio on obstacle detection. The experiment improved the realism and ecological validity of previous research by introducing a dynamic fixation target, realistic apparatus scales and real walking (on a treadmill) whilst carrying out an obstacle detection task. Results showed that obstacle detection only improved with illuminance increases up to 2.0 lux. A higher S/P ratio (2.0) provided better detection performance than a low S/P ratio (1.2), but only at the lowest illuminance used of 0.2 lux.
The data is used to discuss optimal design criteria for pedestrian road lighting based on obstacle detection. However, other purposes of road lighting, such as creating a feeling of reassurance and enabling accurate interpersonal judgements to be carried out, should also be considered when designing pedestrian road lighting.
Lamp Spectrum and Relative Spatial Brightness at Photopic Light Levels
Deniz Atli. University of Sheffield. 2014. Examined by Kevin Mansfield, UCL.
This thesis proposes that the spectral power distribution (SPD) of lighting can be modified to enhance spatial brightness. Energy saving is then possible by using SPD that allows illuminance to be reduced whilst maintaining the same level of spatial brightness. The Akashi and Boyce study demonstrates an energy saving of 33% by using lamps of higher correlated colour temperature but it is widely known that this is not a good metric for predicting spatial brightness.
The aim of this study was to identify a metric for predicting spatial brightness. The first approach followed the method of Cowan and Ware: use the results of past experiments to test potential metrics. 65 studies of spatial brightness and SPD were found. Initially, these lead to different conclusions as to whether SPD affects spatial brightness. The reasons for this are that they used different methodologies and hence review of method was used to screen the credible data from within these 65 studies: only 19 of them were considered to be credible. This thesis focussed on the category rating procedure. The review of methods included an experiment comparing rating scales with different response ranges and a meta-analysis comparing results gained when either brightness or visual clarity were the objective of the experiment. Two potential metrics for spatial brightness are the scotopic to photopic (S/P) luminance ratio and the area of the colour gamut (GA). Results from the credible studies were used to test these models: while both models suggest a reasonable prediction, it was found that they were not independent for this set of data and it was therefore not possible to discriminate between them.
Hence an experiment was carried out to directly test these metrics. The experiment employed full field sequential evaluation of stimulus pairs, with matching and discrimination procedures. Three SPDs were compared, these chosen to isolate the S/P and GA effects. Following Berman et al, one pair had identical chromaticity but different S/P ratios: a second pair had identical S/P ratio but different gamut area; the third pair had different S/P and gamut area. The two procedures led to similar results: null condition trials confirmed that doubt about interval bias in the Berman et al data was unwarranted. It was found that lighting of higher S/P or higher GA enhance spatial brightness: it was also found that their effects appear to be additive.
When the final remodelling was done by adding the data points from the new experiment to the data set, the models of the difference of S/P ratio and the log ratio of GA had the best fits with spatial brightness. Their correlations were equally plausible with mean illuminance ratio of the data set.
This thesis demonstrates that SPD affects spatial brightness, allowing lower illuminances to be used when using lighting of higher S/P ratio and gamut area.
Optimising Lighting to Enhance Interpersonal Judgements for Pedestrians in Residential Roads
Biao Yang. University of Sheffield. 2014. Examined by Peter Raynham, UCL.
Lighting in residential roads is designed to enhance the visual ability to make interpersonal judgements, which is considered to be a critical task for pedestrians. There appears to be little empirical evidence supporting current standards and consistent conclusions cannot be derived from past studies based solely on facial recognition. This work extends investigation of the relationship between lighting and interpersonal judgements beyond the analysis of facial recognition. The results were used to explore how such data might be used to better estimate appropriate light levels for outdoor lighting.
Analysis of gaze behaviour using eye-tracking suggested that the effect of lighting on interpersonal judgements should be examined using the ‘desirable’ distance at 15 m and a duration of 500 ms: in past studies these have been arbitrary. Two pilot studies carried out to inform the experimental design suggested that (i) recognition of facial features is of particular interest, and (ii) standard facial expressions and body postures did not lead to consistent judgements of intent.
The first experiment collected forced choice judgements of emotion (from facial expression and body posture) and gaze direction after 1000ms exposure under 18 combinations of three luminances, two lamp types and three distances. Better performance was found with higher luminance and closer distance, but with diminishing returns according to a plateau-escarpment relationship. Effect of lamp type was not found in judgements of facial expression, but was found in judgements of body posture and gaze direction for some of the conditions lying on an apparent escarpment.
The second experiment provided further examination of facial expressions under 72 combinations of test conditions: six luminances, three lamp types, two distances and two durations. Luminance and distance were found having significant effect on expression recognition. The effect of lamp spectral power distribution (SPD) was not significant and the effect of duration was suggested to be significant only within the escarpment region of the performance versus luminance.
Investigation of Lighting and Disturbing Reflections on Display Screens: A New Model for Judging Acceptability
Tharinee Ramasoot. University of Sheffield. 2010. Examined by Peter Boyce.
This thesis presents an investigation of disturbing reflections on display screens. Preliminary investigations of lighting for the Classrooms of the Future revealed problems with disturbing screen reflections, in particular on the interactive whiteboard, a new type of display screen not covered in current lighting guidance. There is evidence to suggest that the current guidance does not accurately predict user acceptability of visual conditions at display screens and does not accommodate rapid change in display technologies. A new system for predicting the user acceptability of display screen reflections is needed.
An experiment was carried out to evaluate acceptability of screen reflections using two psychophysical test methods: adjustment and category rating. Both methods identified the light source luminances at which screen reflections were just starting to be unacceptable to users. In addition to these subjective test methods, a reading task was carried out to provide an objective assessment of the effect of screen reflections on visual performance. All the tests were done using a range of screen types, chosen to represent those commonly found in ICT classrooms.
Results from the adjustment tests and the category rating tests are consistent with each other. They show that the current guidance for luminance limits to avoid disturbing reflections on display screens, in some cases specifies luminances that are too high leading to unacceptable reflections, and in other cases specifies luminances that are unnecessarily low. Analysis of the results from both subjective tests suggests significant effects of lighting-display parameters on user acceptability of screen reflections including screen type, angle of viewing, and size and luminance of the reflected light source. By contrast, the objective measure of task performance in the reading test was not responsive to reflections on the screens.
The findings of this study suggest that the acceptability of screen reflections can be predicted using knowledge of key lighting and display parameters including the size and luminance of the reflected light source, the screen specular reflectance, the effect from haze reflection, and the screen background luminance. A new model has been developed using these parameters to predict the acceptable luminaire luminances that can be exposed to display screens. And a new approach is proposed: rather than limit the lighting as is currently done, disturbing reflections can be avoided by means of minimum specifications for display screens.
The Impact of Background lighting Complexity on the Visual Saliency of Urban Objects
Navaz Davoudian. University of Sheffield. 2010. Examined by Prof. Stephan Voelker, TU-Berlin.
Spatial design and reassurance for unfamiliar users when wayfinding in buildings
Alison Ching-Lan Chang. University of Sheffield. 2009.
Wayfinding tasks comprise decision points and interconnecting paths leading to a destination. Path choice at decision points is critical to the successful completion of wayfinding tasks. Research has found that signage is not the only influence on path choice and that influences vary depending on familiarity with an environment. People familiar with their surroundings have a cognitive map – a prior understanding of the environment – against which they can compare the environment as they experience it in order to orientate themselves. People unfamiliar with their surroundings, and therefore lacking a cognitive map of them, are found instead to rely upon wayfinding strategies to inform their path choice decisions. This study investigates how aspects of the spatial design of buildings may assist unfamiliar users in finding the destination they are seeking within the building. Observations of people wayfinding in an unfamiliar building suggested that four aspects of spatial design affected route choices made at decision points. Four wayfinding strategies describe the behaviour observed: 1) Maintain a Straight Bearing through the building; 2) Avoid a Change of Level; 3) Walk Towards a Brighter Space; 4) Choose the Wider Corridor. Evidence supporting three of these was found in the literature. For the fourth – Choose the Wider Corridor – only limited evidence was available from the literature and hence further work was carried out to test the predictability of its influence on wayfinding behaviour. An online experiment was conducted to investigate to what degree corridor width influences path choice and the interaction between the Choose the Wider Corridor and Maintain a Straight Bearing wayfinding strategies. A means of categorisation, comprising two wayfinding principles, was devised for information in the environment and means of undertaking wayfinding tasks: Reassurance Principle – wayfinding strategies reassuring the wayfinder that they are taking the correct route - and Tools Principle - signage, maps, landmarks and other sources of information in and representing the environment, available to aid wayfinding decisions. This thesis looks at strategies for wayfinding reassurance. It is proposed that unfamiliar users would find buildings more intuitive to wayfind within if they were designed with routes to likely public destinations that conform to the four wayfinding strategies. An applied test was conducted to confirm whether wayfinding ease could be predicted by analysing the routes within that building against the behaviours described by the wayfinding strategies. It was found that ratings of difficulty given by test participants matched predicted ratings based upon an analysis of the building’s conformance to the wayfinding strategies. It is suggested that if this analysis was conducted at the design stage it could limit potential wayfinding difficulties. Some possible designs as means of achieving this in new buildings and refurbishments are discussed.
Light source spectrum, brightness and visual performance in pedestrian environments
Chris Cheal. University of Sheffield. 2007. Examined by Peter Raynham, UCL.
Street lighting which operates at mesopic light levels has traditionally used low- and high-pressure sodium lamps because of their high luminous efficacies and long reliable lifespans. The characteristic orange light and poor colour rendering properties of these lamps now tend to be accepted less in the lighting of areas with significant levels of pedestrian activity. Here the current trend is towards the use of lamps such as compact fluorescent and metal halide which by comparison with sodium lamps have a white colour appearance and good colour rendering. These effects are a function of lamp spectral power distribution (SPD) and, because SPD is understood to affect visual responses other than colour perception, a clearer understanding of its influence in mesopic conditions is required. Without an established relative spectral sensitivity function for mesopic conditions the matching of pedestrian lighting using standard V(λ)-based photometry does not guarantee equal levels of vision when different lamp SPDs are involved. There is evidence to suggest that the replacement of sodium lamps with white lamps at the same illuminance can provide a brighter environment and increased visual performance. Confirmation of these effects would justify the revision of current lighting specifications for pedestrian areas. This study compared the performance of five different lamp SPDs in a series of psychophysical experiments designed to assess visual performance and brightness in mesopic conditions. Each experiment systematically isolated the effects of lamp SPD from those of the other lighting parameters. The potential for illuminance reductions in pedestrian environments through the use of white lamps over sodium lamps was thus examined. Taken together, the current findings clearly demonstrate the importance of lamp SPD and consistent effects among younger and older pedestrians. As far as the maintenance of brightness levels is concerned, an illuminance reduction of approximately 30% is acceptable for white lamps. The consequences of this for the important pedestrian tasks of facial recognition and orientation are concluded to be small. The result of this study is a better understanding of the relative visual effects of the different lamp types used for outdoor lighting. The overall conclusion is that the potential exists for illuminance reductions with no loss of vision if white lamps replace sodium lamps in pedestrian environments.
The perception of light sources of different colour properties
Steve Fotios. UMIST. 1997. Examined by Prof. Peter Tregenza.
It was confirmed by a literature search that the V(λ) curve, upon which photometry is based, ignores the contribution to brightness from the chromatic (opponent colour) mechanisms of visual processing. Therefore, standard photometry does not adequately represent the visual response to interiors illuminated by light sources of different colour property. Experiments were conducted, comparing the response of human observers to seven discharge and tungsten lamps of different colour property illuminating adjacent booths. When the booths were matched for visual equality, it was found that lamps of good colour quality (full spectrum fluorescent) were set on average to a 20% lower illuminance than a reference source (warm white fluorescent), and lamps of poor colour quality (low pressure sodium) required over 120% more illuminance than the reference lamps. It was also found that at visual equality the lamp of higher colour quality offered equal visual performance, and was preferred by the majority of observers for their workplace, despite being at a lower illuminance than the lamp of poorer colour quality. A number of models of visual response were reviewed and adapted to fit these new results, which include a wider range of lamp spectral quality than has been previously used. Three models are proposed for further evaluation: (1) Cone Surface Area, a colour gamut model that accounts for both the colour appearance and colour rendering properties of a spectrum: (2) SWS-Lumens, a modification of Berman's Brightness Lumens in which the rod response is replaced by a SWS-cone response, and: (3) Chromatic Brightness, a two-channel opponent-colour model which excludes the V(λ) luminance response.