Visión mesopica CIE

CIE AND MESOPIC PHOTOMETRY

by Prof Liisa Halonen & Dr Marjukka Puolakka, Chairman & Secretary of CIE TC1-58
The CIE is the leading international organization in the lighting field and is recognized by ISO as an international standardization body. To be internationally accepted and used, a photometric system has to be adopted and recommended by the CIE.

The CIE TC 1-58 ‘Visual Performance in the Mesopic Range’ has now concluded its work, and the outcome is a recommended system for mesopic photometry based on visual performance. This has just been published as CIE Technical Report 191:2010. The Technical Report will form the basis of a future joint ISO/CIE Standard which is being prepared at the moment.

The development of an effective system for mesopic photometry has been a topic of concern in the international lighting community for several decades. It is found encouraging within the CIE that, after more than 70 years of research the time has now come to publish a practical system of mesopic photometry, as this will be a major breakthrough for the CIE, too.

All lighting technology and practice is based on photometry, the measurement of visible radiation. Photometry provides a method to assess light in terms of human visual spectral sensitivity. Until now, the basis of all photometry has been the CIE photopic spectral luminous efficiency function, V(λ), established in 1924. The CIE scotopic spectral luminous efficiency function, V’(λ), was established in 1951, but it has not been used in practical photometry.

The mesopic luminance region covers a range of luminances between the scotopic and photopic regions. Mesopic lighting applications include road and street lighting, outdoor area lighting and other night-time traffic environments. So far, there has been no internationally accepted system of mesopic photometry. This means that suitable methods to evaluate the visual effectiveness of lighting products and installations in the mesopic region have not been available.

The task of the TC 1-58 was to adopt a visual performance based approach, which means that the underlying spectral sensitivity functions are based on criteria of visual task performance, i.e. on recognition, detection, reaction time tasks, not on brightness matching.

In the mesopic region the spectral sensitivity of the human visual system is not constant, but changes with light level. This is due to the changing contribution of the rods and cones on the retina. Thus, we need not only one mesopic spectral sensitivity function, but instead several functions, together with a defined procedure for using these functions in a photometric measurement system. The new mesopic system describes spectral luminous efficiency, V_mes(λ), in the mesopic region as a linear combination of the photopic spectral luminous efficiency function, V(λ), and the scotopic spectral luminous efficiency function, V’(λ).

For applying the mesopic photometry, the S/P-ratio of the light source, derived from its spectral data, is needed as input value. This is the ratio of the luminous output evaluated according to the scotopic V’(λ), to the luminous output evaluated according to the photopic V(λ). The higher the S/P-ratio the higher the luminous efficacy of the light source in terms of the mesopic design.

The use of mesopic dimensioning changes the luminous output and consequently the luminous efficacy orders of lamps. Many of the ‘white light’ sources currently used for applications such as road lighting have S/P-ratios between about 0,65 (high pressure sodium, for example) and 2,50 (certain metal halide lamps, for example). The S/P-ratios of warm white LEDs are around 1,15 and those of cool white LEDs around 2,15. The use of the new mesopic system to calculate the effective luminance of these white light sources results in significant changes in their apparent efficacy. For example, at a photopic luminance of 1 cd⋅m^-2 the use of the recommended system results in a change between  5 % and +15 % for lamps with S/P-ratios between 0,5 and 2,5; at 0,3 cd⋅m^-2 the change is between about –10 % and +30 %.

Due to their fast development, LEDs are increasingly penetrating the lighting markets. LEDs offer new solutions to various mesopic applications, too, not least because of the possibilities of producing light sources with varying spectral properties. Depending on the LED spectra, their ranking on a luminous efficiency scale may be subject to significant changes if mesopic luminous efficiency functions are used instead of the photopic. A CIE system for mesopic photometry will give manufacturers foundations on which to develop LEDs that are optimised for low light level applications. Consequently, the coming CIE publication on mesopic photometry may also have a major impact on the evolution and adoption of LEDs as the future light sources.

As mesopic dimensioning favours ‘white’ light sources with high S/P-ratio, the extra benefits from using the mesopic design are good colour rendering characteristics of the lighting. This is expected to further pave way for the use of white LEDs in outdoor lighting.

The use of mesopic photometry will promote the development of mesopically optimised lighting products. It will give the manufacturers foundations on which to develop light sources that are optimised for low light level applications. This will result in better energy-efficiency and visual effectiveness in outdoor lighting conditions. The accuracy of photometric instrumentation used in mesopic applications can be increased by taking into account the actual spectral sensitivity at these levels. Industry and users should be strongly motivated to use a photometric method that is valid and functionally relevant.

The development of mesopic photometry is the outcome of a huge amount of research work carried out in several organisations and countries over several decades. It is a major breakthrough for the international scientific community and the CIE. Finally, in 2010 we will have a mesopic photometric system to accompany the photopic V(λ), which has served since 1924.

Actions are now needed to get the new mesopic photometric system into practical use. What is now needed are design guidelines for mesopic lighting dimensioning, i.e. guidelines how to use the mesopic system for example in road, pedestrian way and urban area lighting. There are certain points that require consideration within the various specification organisations, road lighting authorities, designers and the international lighting community. For example, different specification criteria may be necessary in situations where there is a different weighting of on-axis and peripheral visual information to process. In order to reach the full benefits of the new mesopic photometric system, it is hoped that the development of the guidelines are included immediately within the CIE work in the form of a new TC.