SPD and the perception of interior spaces

Dates: Ongoing
Staff: Steve Fotios, Deniz Atli

This research investigates the impact of light source spectral power distribution (SPD) on the perception of illuminated spaces at photopic levels. Dr Fotios has been working on this since 1993, and is now collaborating with Dr Kevin Houser of the Pennsylvania State University, U.S.A. The objective of this work is to develop models of visual response that can be used by lamp manufacturers to create light sources tailored to human vision, tuning the spectral power distribution of interior lighting to optimise comfort, performance and energy consumption.

Interior electric lighting is a significant energy consumer. Within the EU, lighting in the commercial sector consumes 30% of total electricity consumption and lighting accounts for up to 40% of energy costs in a typical UK office. Lighting recommendations are based almost entirely on ensuring visibility and the data on which these recommendations are based has not taken into account any possible effects of the spectral content of the light source. Visual performance models imply that virtually all tasks done in offices and schools could be done just as well at much lower illuminances than those currently used. However, illuminances have not been reduced because people like an interior to appear bright.

Dim, gloomy lighting can induce a sense of visual discomfort which may change the observer’s mood and motivation to carry out a task, particularly if the work is prolonged. Thus, if a perception of brightness could be maintained at a lower illuminance, energy consumption and carbon emissions could be reduced. There is some evidence that light source SPD affects the perception of brightness and this provides a means for reducing illuminances whilst maintaining the same perception of brightness. To do this requires a tool for predicting the relationship between lamp SPD and illuminance and hence reliable and appropriate evidence of the brightness effects of lamp SPD; that is the long term aim of the applicant's research.

Spaces lit by lamps of different SPD can appear differently bright at the same illuminance because illuminance, defined by The CIE Standard Photopic Observer (Vλ), is derived from a different visual process to that of brightness. The post-receptoral visual system is organised in three channels, one luminance channel where signals from the long- and medium-wavelength sensitive cone types are combined, and two colour channels where the differences between signals from different combinations of cone types are taken. Vλ is based on data collected primarily using flicker photometry and step-by-step brightness matching, techniques that tend to minimize activity in the colour channels; brightness perception is dependent on activity in all three channels.

There have been over 60 studies of SPD and the perception of interior spaces, typically examining brightness or visual clarity. Collating the data from previous studies will generate a larger body of data for initial testing of models proposed to predict the relationship between SPD and brightness. The first stage of this work is thus a review of previous work to identify and collate reliable evidence. This stage demands initially that experimental techniques are reviewed, and potential bias and extrapolation limitations are identified. A problem within the body of previous work is that much of it must be considered unreliable, frequently because of incomplete reporting, for example the published work either reveals an experimental or subjective bias, or does not present sufficient data to check whether an expected bias has been successfully countered, or the results are not presented in sufficient detail.

Variations in experimental protocol include:

  • Method. These tend to fall into three standard categories - matching (an adjustment task), ranking (a discrimination task) and category rating.

  • Evaluation mode: separate, successive, sequential or simultaneous.

  • Perceptual response : e.g. brightness, visual clarity, overall appearance.

  • Stimulus size.

The focus of attention in recent work has been to identify what these differences mean. Two recent studies compared simultaneous and rapid sequential modes of evaluation for brightness matching and discrimination tasks at mesopic and photopic levels. In these studies, the rapid sequential method presented stimuli for 5s each. The results do not suggest a difference between the two modes.

Steve Fotios established a new technical committee within the CIE, TC 1-80: Research Methods For Psychophysical Studies Of Brightness Judgements. The terms of reference are: To report on research methods (both research design and statistical analysis) for psychophysical studies of spatial brightness judgements. The aim is to bring best practices from psychology into the wider awareness of people in the lighting community who wish to use such tools in their own work, to avoid errors that plague the existing literature. Please contact us if you would like to participate.

Update: September 2014

CIE 212:2014, the report on research methods produced by TC1.80, was published in 2014. We used the recommendations for good practise reported in CIE 212:2014 to critically review past studies of SPD and spatial brightness, identifying credible evidence with which to screen potential metrics, somewhat in the manner of Ware & Cowan. However, the exercise was unsuccessful because, for the particular lamps used in these studies, the metrics were not independent.

A pilot study was this carried out by Deniz Atli to evaluate two potential metrics: the S/P ratio proposed by Berman et al and gamut area as found by Boyce to be better than CCT or CRI. S/P is the ratio of scotopic (rod) to photopic (foveal cone) responses: lamps of higher S/P ratios lead to higher spatial brightness.

Gamut area is a measure of the range of surface colours discriminable under a light source, thus being a proxy for the chromatic contribution to brightness: lamps of larger gamut area increase the saturation of colours and thus lead to higher perceived brightness. Two test procedures were used; (1) discrimination judgements of two lamps of equal chromaticity, the procedure used by Berman et al but not previously repeated (and also extended through addition of a null condition and using coloured room surfaces in addition to achromatic surfaces); and (2) the matching procedure we had previously to examine metrics for spatial brightness at the low (mesopic) levels of road lighting.

It was found that:

  1. At equal chromaticity, lighting of higher S/P ratio is brighter;

  2. At equal S/P ratio, lighting of larger gamut area is brighter; and

  3. the mechanisms underlying these metrics may provide an additive contribution to spatial brightness.

The pilot study was not externally funded. The light source was an LED array designed by John Barbur (City University) for a collaborative project at mesopic levels (MERLIN) rather than the higher luminance of photopic adaptation. This led to two limitations: (i) there were sufficient lumens to light only a small booth rather than the room used by Berman et al, although this maintained the requirement of full-field vision; and (ii) the difference in S/P ratios between the two stimuli was smaller than used by Berman et al which meant the potential of S/P ratio could not be fully explored. It also used only younger test participants.

We are currently seeking funding in collaboration with John Barbur to extend this study.


  • Fotios S, Atli D, Cheal C, Hara N. Lamp spectrum and spatial brightness at photopic levels: Investigating prediction using S/P ratio and gamut area. Lighting Research and Technology August 2015 47: 595-612, first published on July 4, 2014. doi:10.1177/1477153514542295

  • Fotios S, Atli D, Cheal C, Houser K, Logadottir A. Lamp spectrum and spatial brightness at photopic levels: a basis for developing a metric. Lighting Research & Technology, 2015; 47(1); 80-102. doi:10.1177/1477153513503170

  • Fotios S, Houser K. Using forced choice discrimination to measure the perceptual response to light of different characteristics. Leukos, 2013; 9(4); 245-259.

  • Logadóttir Á, Fotios SA, Christoffersen J, Hansen SS, Corell DD, Dam Hansen C, Investigating the use of an adjustment task to set preferred colour of ambient illumination, Colour Research & Application, 2013; 38(1); 46-57.

  • Fotios S and Atli D. Comparing Judgements of Visual Clarity and Spatial Brightness Through an Analysis of Studies Using the Category Rating Procedure. Leukos, 2012; 8(4); 261-281.

  • Fotios SA, Logadóttir Á, Cheal C, Christoffersen J. Using adjustment to define preferred illuminances: Do the results have any value? Light & Engineering, 2012; 20(2); 46-55.

  • Logadóttir Á, Christoffersen J and Fotios SA. Investigating the use of an adjustment task to set preferred illuminance in a workplace environment. Lighting Research & Technology, 2011; 43(4); 403-422.

  • Atli D and Fotios S. Rating Spatial brightness: does the number of response categories matter? Ingineria Iluminatului, 2011; 13(1); 15-28.

  • Fotios SA and Cheal C, Brightness matching with visual fields of different types, Lighting Research & Technology, 2011; 43(1); 73-85.

  • Fotios SA and Cheal C, Stimulus range bias explains the outcome of preferred-illuminance adjustments. Lighting Research & Technology, 2010; 42(4): 433-447.

  • Fotios SA and Cheal C, A comparison of simultaneous and sequential brightness judgements. Lighting Research & Technology, 2010; 42(2): 183-197.

  • Fotios S, Houser K, Cheal C and Royer M. A comparison of simultaneous and sequential evaluations of spatial brightness suggests the pupil size mechanism is not responsible for spatial brightness. CIE 2010 Lighting Quality and Energy Efficiency, 14-17 March 2010, Vienna. pp 428-438.

  • Fotios S and Cheal C. Stimulus range bias and estimates of preferred Illuminance. CIE 2010 Lighting Quality and Energy Efficiency, 14-17 March 2010, Vienna. pp 525-532.

  • Houser KW, Fotios SA, Royer MP. A test of the S/P ratio as a correlate for brightness perception using rapid-sequential and side-by-side experimental protocols. Leukos, 2009: 6(2); 119-137.

  • Fotios SA and Houser K, Tuning the spectrum of lighting to enhance spatial brightness: Investigations of research methods. Experiencing Light 2009. Eindhoven, 41-52.

  • Fotios S and Houser K, Tuning lamp spectral power to improve the perception of interior spaces, Lux Europa, Istanbul, 2009, 417-422.

  • Fotios S and Houser K, Tuning the spectrum to match human vision: An opportunity for LED arrays, CIE Light & Lighting conference, Budapest, 27-29 May 2009.

  • Fotios S and Houser K, Visual effects of lamp spectrum: the perception of interior spaces, Proceedings of the 6th Lux Pacifica, Bangkok, 23-25 April 2009, 13-16.

  • Fotios SA and Houser KW, Research methods to avoid bias in categorical ratings of brightness, Leukos, 2009; 5(3); 167-181.

  • Fotios S and Houser K, Measuring lamp SPD affect on the perception of interior spaces: frequently this is misleading. Balkan Light 2008, Ljubljana, Slovenia, 7–9 October 2008, 69-78.

  • Fotios SA, Houser KW and Cheal C, Counterbalancing needed to avoid bias in side-by-side brightness matching tasks, Leukos, 2008: 4(4); 207-223.

  • Fotios SA and Cheal C, The effect of a stimulus frequency bias in side-by-side brightness ranking tests, Lighting Research & Technology, 2008; 40(1); 43-54.

  • Fotios SA and Cheal C, Evidence for response contraction bias in side-by-side matching tasks, Lighting Research & Technology, 2007; 39(2); 159-169.

  • Fotios S and Houser K, Research of lamp SPD effects on the perception of interior spaces: the current state of knowledge, 26th Session of the CIE, Beijing, July 2007.

  • Fotios SA, Effects of SPD on the appearance of spaces and on visual performance, Invited Presentation, Symposium on Lighting and Human Performance, NPL Optical Radiation Measurement Club, 14th December 2006.

  • Fotios SA, Light Colour Specification, presentation to lead discussion, Invited Presentation, Lighting Research & Technology colloquium Light and Human Response with Respect to Lighting Application, UCL, London, 23rd February 2006.

  • Fotios SA, Chromatic adaptation and the relationship between lamp spectrum and brightness, Lighting Research & Technology, 2006; 38(1); 3-17.

  • Fotios S and Gado T, A comparison of visual objectives used in side-by-side matching tests, Lighting Research & Technology, 2005; 37(2); 117-131.

  • Fotios SA and Gado T, Investigating the experimental conditions used to compare the brightness of light sources of different colour characteristics, pp377-386, The First Scottish Conference for Postgraduate Researchers in the Built and Natural Environment (PRoBE), Glasgow Caledonian University, Scotland, 18–19 November 2003.

  • Fotios SA, Experimental conditions to examine the relationship between lamp colour properties and apparent brightness, Lighting Research & Technology, 2002; 34(1); 29-38.

  • Fotios SA, An error in brightness matching associated with the application of dimming, Lighting Research & Technology, 2001; 33(4); 223-231.

  • Fotios SA, Lamp colour properties and apparent brightness: a review, Lighting Research & Technology, 2001; 33(3); 163-181.

  • Fotios SA, An investigation into the relationship between luminance and brightness of strongly chromatic light sources by K van Creveld, Lighting Research & Technology, 2000; 32(4); 234-235.

  • Fotios SA and Levermore GJ, The effect of lamp colour properties upon perception: A summary of research, CIE Symposium ‘99: 75 Years of CIE Photometry, Hungary, September 1999.

  • Fotios SA and Levermore GJ, The perception of daylight, 154-158, Proceedings of the First international CIE symposium on Lighting Quality, 9-10 May 1998, Ottawa, Canada (1998).

  • Fotios SA and Levermore GJ, Chromatic effect on apparent brightness in interior spaces, I: Introduction and colour gamut models, Lighting Research & Technology, 1998; 30(3); 97-102.

  • Fotios SA and Levermore GJ, Chromatic effect on apparent brightness in interior spaces, II: SWS lumens model, Lighting Research & Technology, 1998; 30(3); 103-106.

  • Fotios SA and Levermore GJ, Chromatic effect on apparent brightness in interior spaces, III: Chromatic brightness model, Lighting Research & Technology, 1998; 30(3); 107-110.

  • Fotios SA and Levermore GJ, Perception of electric light sources of different colour properties, Lighting Research & Technology, 1997; 29(3); 161-171.

  • Fotios SA, The perception of light sources of different colour properties, PhD thesis, UMIST (1997).

  • Fotios SA and Levermore GJ, Visual perception under tungsten lamps with enhanced blue spectrum, Lighting Research & Technology, 1995; 27(4); 173-179.

  • Fotios SA and Levermore GJ, The perception of an artificial daylight source, CIBSE National Lighting Conference, Bath, U.K. (1996).

  • Fotios SA and Levermore GJ, The perception of enhanced blue light sources, CIE Symposium on Advances in Photometry, 1-3 December 1994, Central Bureau of CIE; Vienna, (1995).