Applying Research on Night Vision to Forward Lighting

Automobile Headlamps

Because night driving is a visual task that relies on peripheral vision at low light levels, automobile head lamps should be evaluated using a mesopic spectral sensitivity function. The comparisons are especially interesting for new high intensity discharge (HID) head lamps, which allow designers to alter the lamp spectrum in dramatic ways to improve luminous efficacy and achieve desired color appearances. The Vertical Luminous Exitance Graph 1 illustration uses standard photometry, or the photopic spectral sensitivity of the eye for an HID head lamp distribution.

Using head lamp intensity and spectral data provided by OSRAM SYLVANIA, the LRC applied its model for mesopic visual sensitivity to calculate vertical illuminance down the roadway. The contours change when the mesopic sensitivity of the eye is taken into account. The spectrum of the source is that of an existing HID head lamp. (Vertical Luminous Exitance Graph 2)

The graphs show contours of equal vertical illuminance for objects in front of the head lamp at a height above the roadway equal to the mounting height of the head lamp. All three graphs are for head lamps of identical lumen output and distribution. The differences result from using a mesopic spectral sensitivity and changing the spectrum of the light source. Using a hypothetical lamp spectrum that is spectrally better tuned to mesopic sensitivity, Vertical Luminous Exitance Graph 3 illustrates that objects further down the roadway would be more effectively illuminated for off-axis detection.

Sponsor: OSRAM SYLVANIA