Development of Visual Performance Based Mesopic Photometry

This work started by investigating the applicability of the photopic V( ) function to predict visual task performance at mesopic light levels. Visual acuity and pedestrian visibility experiments were carried out in varied lighting and viewing conditions. The results indicated the inadequacy of photopic photometry to characterise the response of peripheral vision at low mesopic light levels. They also indicated that mesopic spectral sensitivity is visual task dependent. Based on these findings, and on an extensive review of mesopic research work, it was evident that there was a demand for visual performance based mesopic photometry. In order to establish a basis for mesopic photometry, a European research consortium with multi-disciplinary expertise was formed. Following this, a framework for the development of performance based mesopic photometry was developed. With this framework the international lighting community and EC Fifth Framework Programme were convinced to identify the urgent need for mesopic photometry. The work continues by introducing a multi-technique approach developed and adopted in a European research work MOVE. In the work of MOVE, a large data-base of mesopic visual task performance was generated by investigating the visual performance of night-time driving using three visual sub-tasks. The data was used in modelling mesopic spectral sensitivity. The MOVE work resulted in two distinct mesopic models; a practical (i.e. linear) model and a more complex chromatic model. The practical model is applicable for the visual task of night-time driving in situations where the background and target both have fairly broad spectral power distributions. The chromatic model gives a better prediction of performance for tasks which colourfulness (chromatic saturation) is high. The MOVE practical model was applied in road lighting dimensioning via road luminance measurements. Road lighting installations using HPS and MH lamps were measured with a CCD luminance photometer. Analysis of the applicability of the MOVE model is conducted on the basis of luminance measurements, calculations and on comparison to a recently proposed X-model by Rea et al. The work concludes by describing how the new performance-based mesopic models of MOVE are integrated into the CIE (Commission Internationale de l’Eclairage) work in order to contribute to the establishment of a standard for mesopic photometry. Finally, the impacts of standardisation of mesopic photometry on lighting dimensioning and products are discussed.