It is now well established that the spectral, spatial, temporal and absolute sensitivities of the human circadian system are very different from those of the human visual system. Although qualitative comparisons between the human circadian and visual systems can be made, there still remains some uncertainty in quantitatively predicting exactly how the circadian system will respond to different light exposures reaching the retina. This paper discusses attempts to model the spectral sensitivity of the circadian system. Each of the models discussed here varies in terms of its complexity and its consideration of retinal neuroanatomy and neurophysiology. Future testing to validate or improve any of these computational models will require a targeted hypothesis, as well as a suitably high level of experimental control before one model can be rejected in favour of another. Until specific hypotheses are formulated and tested, it would be premature to recommend international acceptance of any model or system of circadian photometry.

Consideration is being given to the adoption of a new metric characterizing light as it affects the human circadian system. Much has been learned over the past couple of decades about light as a stimulus for circadian system regulation, so it is appropriate that these discussions take place. The present paper develops an argument for adopting circadian stimulus as a metric for quantifying light in architectural spaces. The circadian stimulus metric (a) was developed from several lines of biophysical research, including those from basic retinal neurophysiology; (b) has been validated in several controlled experiments; and (c) has been used successfully in a number of real-world applications. Any discussions of new metrics should take each of these foundational points into consideration.

A uni® ed system of photometry is proposed that is based on human vision and allows the speci® cation of visual stimuli at all light levels. To be useful, however, photometry can never be entirely synonymous with vision. Additivity is an essential characteristic of photometry, yet many visual responses, such as brightness matching, incorporate visual channels that are inherently non-additive in responding to light. Following a review of models of mesopic photometry based on a number of experimental techniques, a system of photometry based on reaction times is proposed to bridge conventional photopic and scotopic luminous ef® ciency functions through the mesopic region. The basis for the uni® ed system is a parameter, X, which describes the proportion of photopic luminous ef® cacy at any luminance, and luminance can be calculated from a simple closed-form equation. The utility of the system is described, including instrumentation for the measurement of uni® ed luminance.

Light quantities based on the photopic luminous efficiency function do not predict brightness perceptions of lighted outdoor scenes such as streets, parking lots and plazas. This paper summarises a series of experiments conducted using scalemodel outdoor scenes illuminated by different light sources to assess judgements of brightness. From the results and from previously published literature on the relative increase in short-wavelength spectral sensitivity for brightness, a tentative model for brightness perception of outdoor scenes is proposed. The model can serve as a starting point for efficiently testing future hypotheses regarding brightness perception in lighted outdoor scenes.

Metal halide (MH) lighting systems are gaining in acceptance relative to highpressure sodium (HPS) lighting systems for many night time applications. The present paper describes a series of studies carried out to address some of the 'quality' issues associated with MH and HPS lighting systems. HPS and MH installations were compared in terms of perceptions of brightness and safety, acceptability for social interaction, facial recognition and eyewitness identification. At equal photopic light levels, a street scene illuminated at night by an MH lighting system was reliably seen as brighter and safer than the same scene illuminated by an HPS system. In terms of acceptability for social interaction, facial recognition and many aspects of eyewitness identification, the measured differences between lighting systems were not as clear.