Two papers presented at PLDC today bring us close to the “Holy Grail “ of a measurable value for lighting quality.

As Lighting designers we are all aware that the standards and measurement of light that we have all been working with have little or no relationship with how we see space and how we feel about its visual appearance. We know that measuring the the light output of fittings and that largely in a horizontal plane notionally connected with tasks relevant to the space does not in any way describe the visual appearance of that space. We are aware of this and we see many attempts to devise ways of working with this measuring system to try and avoid the worst visual results that it tends to create.

Kit Cuttle’s paper “Percieved Adequacy of Illumination. a new basis for lighting practice” abundantly demonstrated that the existing lighting standards based on the measurement of illuminance have strayed a long way from ensuring the delivery of adequate and appropriate task based lighting through a wholly erroneous attempt to produce the appearance of well lit space. As the plane of measurement is almost universally horizontal at a relatively low level in the space and our normal visual field encompasses more of the vertical and ceiling planes, the amount light measured in the horizontal required to ensure any degree of luminance on the vertical and ceiling becomes quite unreasonable! In fact the combined results of this approach and the ever increasing search for “efficient” lighting with reduced energy use are delivering lighting solutions that reduce the possibility of creating what are perceived as well lit spaces.

If we accept in very general terms that a space perceived to be well lit meets at least some of the requirements for lighting quality we are now on our way to establishing something we can model and measure. Kit has defined an approach to this problem in the form of the concept of Mean Room Surface Excitance (MRSE). Instead of measuring or calculating the light that comes directly from the fittings, this approach quantifies all the light that is reflected from all the surfaces in the room, the actual light that we see. In terms of design this will have a significant impact on the working practices of not only the Lighting Designer, but also the Architect and Interior Designer as, quite obviously, the reflectance values of all the surfaces in the space need to be decided in order to determine MRSE. While this may sound onerous and challenging, attempts to achieve good quality workplace lighting using the current approach has already resulted in surface reflectances being included in the 2011 revision of the European Workplace Lighting Standards, for now this might have escaped the attention of architects and interior designers however it won’t be long before the lighting designer is forced to bring this to their attention in order to meet increasingly limiting power densities required in LEED, BREEAM and building regulations.

How we will approach calculating and measuring MRSE and any other luminance based quantification of lighting formed the content of Dr Craig Bernecker’s paper “ A Practical Method for Commissioning Lighting using High Dynamic Range Photography.” HDR is a technique that has sprung from image making photography and Computer Graphic Imagery. The initial desire was to create images that relate more closely to what we see with the eye than the restricted ranges of luminance that can be captured by cameras. Digital photography and computer image processing allow the capture of a number of images at different exposures and processing them to create a single image containing a true record of all the luminances within the image. To create a picture another set of processing is required to re-compress all this information into a range that can be output on a screen or printer. For our purposes the intermediate data is immensely useful. In software it is possible to identify a numeric value for each and every pixel in the image that proportionately represents the luminance in that area of the camera’s field of view. Dr. Bernecker’s paper describes how it is possible to calibrate these values to luminance values using easily available tools such as a photographic grey card and an illuminance meter. He also pointed us towards “Photoshere” by Anywhere Software  This program provides a function to create HDR images and calibrate the camera from a single measured point, typically the grey card that should be located in the image and a luminance or illuminance measurement taken at at hat point to provide the calibration value. Currently this software is available for MAC OS however it is being ported to Windows and eventually Linux.

Elsewhere at PLDC other speakers have talked around the subject of designing to Luminance values and considering the values of darkness in visual compositions. notably Edward Bartholomew’s paper “Applied Darkness - A model for Luminance based design”.  Unfortunately I had to leave on Friday afternoon however the message of this convention is clear, Luminance design must replace Illuminance design and we do now have the tools to do this. The next challenge is to change the basis of standards and legislation to a Luminance based mode.

Kit invited us all to join the conversation on luminance design issues on his blog here see you there (and here if you want to comment!)


Kevan Shaw 21 October, 2011