Revolutionary changes for lighting discussed at Society of Light and Lighting event
Posted: 04 August 2014
Industry leading lighting professionals delivered presentations and engaged in debates on the subject of changes to lighting metrics at University College London (UCL) on 15th July. Hosted by the Society of Light and Lighting (SLL) and its Journal, Lighting Research & Technology (LRT), the Better Metrics for Better Lighting Symposium highlighted new metrics that could be introduced to ensure the more effective use of light.
Peter Raynham, University College London, introduced the Symposium by discussing why metrics are required in standards and how standards are developed in Europe. Today, the most common metrics used in lighting practice are the illuminance on a horizontal working plane, daylight factor, colour rendering index (CRI) and various power limits, but these all have defects.
Lou Bedocs of Thorn Lighting evaluated nine different metrics used to limit lighting energy consumption. He advocated the Lighting Energy Numeric Indicator (LENI) as the best because this provides a true estimate of the energy consumption of a lighting installation. None of the others, including those used in the current Building Regulations, do this.
John Mardaljevic of Loughborough University discussed the defects of daylight factor as a metric. His alternative was Climate-Based Daylight Modelling (CBDM) which is now a requirement in any funding application for the Priority Schools Building Programme. Daylighting design based on CBDM can help reduce the energy needed for lighting whilst reducing overheating in summer.
Kevin Smet of KU Leuven assessed the many different metrics available for characterizing the colour properties of light sources. He split them into two broad categories identified as the fidelity with which a light source renders colours, which is what CRI measures, and the effect on how colour is perceived, which is what many people are concerned about. As yet there is no decision about future colour metrics but it is likely that, eventually, a two-metric system will be recommended by the CIE.
A revolutionary change to how lit environments should be designed was discussed by author and educator, Kit Cuttle. He called for a change in general lighting practice away from lighting the horizontal working plane to lighting the room. Mean Room Surface Exitance (MRSE) is a measure of ambient illumination in a room but good lighting requires a hierarchy to give emphasis to interesting features of the space. This is achieved through a metric called the Target / Ambient Illumination Ratio (TAIR). A spreadsheet system that enables both these metrics to be used to guide design was demonstrated.
Mark Rea of the Lighting Research Centre, Troy, New York, made an even more revolutionary proposal – a redefinition of light and the separation of light from lighting. At present, light is part of the SI system and is necessary for international trade. It is defined by the V (λ) function, a function that represents the response of only two of the five photoreceptors in the human retina. Instead, he suggested the universal lumen, an envelope that contains the spectral sensitivities of all the presently known photoreceptor systems. Then, lighting could be separated from light in the sense that depending on what aspect of vision was required, an appropriate spectral sensitivity could be selected from within the universal lumen to match the light spectrum to the application.
Mark Rea called for a redefinition of light and the separation of light from lighting
Throughout the Symposium it was evident that the purpose of these proposed new metrics was to more accurately reflect how lighting is perceived so that light can be used more efficiently and effectively. This is important because by reducing electricity consumption lighting could make an important contribution to limiting climate change.
For more information visit www.cibse.org/Society-of-Light-and-Lighting