Comfort and visibility characteristics of spectrally-tuned high intensity discharge forward lighting systems

Recently, spectrally-tuned high intensity discharge (HID) sources were introduced for use in vehicle forward lighting systems. They use a dielectric coating to partially filter and redirect light emitted from an HID arc. When used in projector headlamp systems, these sources result in warmer color temperature light directed towards oncoming traffic and cooler color temperature light towards the passenger-side edge of the roadway. A series of calculation and field studies were performed to evaluate these sources. The potential of spectrally-tuned HID sources to reduce discomfort was calculated and compared to standard HID sources by considering the relative amount of short wavelength cone (SWC) photoreceptor energy emitted by each source. A field study was performed to validate these calculations and determine the absolute subjective ratings that the tailored HID sources would result in when viewed as glare sources under simulated nighttime driving conditions. Calculations were also performed to determine the relative potential of spectrally-tuned HID sources to improve peripheral visual performance when compared to standard HID sources, by considering the relative amount of rod stimulating energy emitted by each source. A field study was performed to validate the calculations and to determine the absolute response measures that these sources would result in under simulated nighttime driving conditions. The results indicate that spectrally-tuned HID lamps can increase comfort to other oncoming drivers, while at the same time, improving off-axis visual performance.