Yukio Akashi

Several Views of Metal Halide and High-Pressure Sodium Lighting for Outdoor Applications

Metal halide (MH) lighting systems are gaining in acceptance relative to high-pressure 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.

Driver decision making in response to peripheral moving targets under mesopic light levels

A field study was conducted to extend fundamental findings about visual performance at mesopic light levels to a driving context. Subjects participating in this field study drove a vehicle along a lighted street while performing a high-order decision-making task. Subjects identified the direction of an off-axis target, toward or away from the street, and braked or accelerated, accordingly. Two sets of light sources were compared: a set of ceramic metal halide light sources and a set of high-pressure sodium light sources. The same study was also performed during the day-time. The results demonstrated that both braking and acceleration response times decreased monotonically as unified luminance increased, suggesting that unified luminance is a suitable rectifying variable for characterizing light levels for different light sources with respect to a complex visual task.

The impact of spectral power distribution on the performance of an achromatic visual task

This experiment tests the hypothesis that light sources that produce smaller pupil sizes ensure better achromatic visual task performance at the same photopic illuminance. Two groups of subjects, one in the age range 18-28 years and the other in the range 61-78 years, performed a Landolt ring task for eight different gap sizes, two different illuminances, and two lamp scotopic/photopic ratios. For both age groups, pupil size was determined by both illuminance and lamp spectrum. The speed and accuracy of performance of the task was determined by the gap size, and to a much lesser extent, by the illuminance. Lamp spectrum had no effect on the performance of the task.

Headlamp Parameters and Glare

This paper discusses a field study which examines headlamp glare. It investigates the role of spectrum, size and illuminance. It ascertains the relative magnitude of each of these variables and its impact on glare from oncoming vehicles. Results of the study indicate that illuminance at eye level is the main factor that influences glare produced by the headlamp of an oncoming vehicle.

Brain potentials associated with eye fixations during visual tasks under different lighting systems

The variations of eye fixation related potentials (EFRPs) were examined in two tasks under three lighting conditions for assessment of lighting environments. Sixteen subjects participated in two tasks; a difficult and an easy reading task under three lighting conditions: Spot light (S), General light (G) and Mixed light (M). EEG (Oz) and EOG were recorded. EEG epochs time-locked to onset of eye fixations were collected at random and averaged separately in two arrays to obtain a pair of EFRPs. Two wave forms under the S were similar, although those under the G showed the disparity, the largest disparity being in the easy task under the G. Under the S, wave forms of EFRPs were stable in the difficult task. The amplitude changed with the task load. The results suggested that EFRPs might be an index of the work load under lighting conditions.

Strategies for Optimizing Headlamp Illumination and Visibility Along Curves

This document reports on a field study undertaken to measure peripheral visual performance under various headlamp conditions. The study include conditions that are typical of halogen and high intensity discharge (HID) headlamps and explored functions that could be incorporated in advanced forward-lighting systems (AFS). Targets of varying size were located at different locations along the edges of both left- and right-hand curves, and different headlamp illumination conditions were used. Reaction times and missed targets were measured. The authors found a benefit of increased peripheral illumination commonly found in HID headlamps and with AFS systems on peripheral target detection. These results are consistent with previously published studies. The authors describe how these data could be used to make predictions of peripheral visibility under arbitrary beam patterns as a function of target characteristics and headlamp intensity, and could help identify locations within beam patterns where increased illumination is likely to be beneficial when entering curves, and where it is not likely to benefit. Compared to configurations most closely matching the illuminance profile of halogen headlamps, the configuration with illuminance conditions most closely matching that of HID headlamps improved detection for targets located along a curve. The authors also conclude that once an asymptotic level of visual performance is achieved, there are diminishing returns from the increased illumination that could be gained by including AFS functionality.

Headlight Glare Exposure and Recovery

There is concern that the greater light output and increased beam pattern widths of some headlamp systems may be resulting in higher glare exposures to drivers for longer times. A set of experiments is described that examines how headlamp glare exposure affects recovery time and ratings of discomfort. Theoretical glare exposures were examined to study different aspects of glare, namely peak glare illuminance and total glare dosage. Glare exposures corresponding to representative tungsten halogen (TH) and high intensity discharge (HID) systems were also examined. It was found that the shape of the glare profile had a significant effect on recovery time. A larger dose of glare (product of illuminance and exposure time) results in a longer recovery time. It was also found that discomfort ratings are dependent on glare profile, with greater discomfort being proportional to larger peak illuminances. Surprisingly, no effect of glare duration or dosage was found on discomfort. It was also found that under nominal aiming conditions the glare exposures representing TH and HID systems produced similar recovery times. However, with 1° of misaim upwards, the HID exposure produced significantly longer recovery times.

The Effect of Glare on Visual Performance under Reduced Headlamp Illumination

Experiments were carried out to determine the effect of dimming vehicle forward lighting systems on target detection. The experiments were undertaken with and without headlamp glare. It was determined that, if sufficient street lighting is available, headlamps can be dimmed to reduce glare without compromising driver performance.

Comment 1 on ‘A review of colour rendering indices and their application to commercial light sources’ by Xin Guo and KW Houser:

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