Recently, there has been a proliferation of commercially available lamps with spectral light output differing from conventional halogen lamps for use in vehicle headlighting systems. For the last 20 to 30 years halogen lamps have been used as the standard source in most headlamp applications. These lamps produce the familiar blackbody-like continuous-spectrum output light. In the last ten years, high-intensity discharge (HID) light sources have come onto the market place with their characteristic discrete spectra and higher correlated color temperature (CCT). Even more recently, coated “blue” halogen lamps have become available which reduce the amount of long-wavelength light and shift the light output to higher CCTs. Currently, halogen lamps are under development that have glass envelopes doped with neodymium, which acts similarly to the coated lamps in reducing long-wavelength light and shifting the CCT to higher values. With all of these new headlamp sources being used with different spectral light output the question arises about the appearance of common roadway objects under different headlamp illumination.
A laboratory study is presented that examines subjective color preferences under different types of headlamp illumination. Side-by-side and sequential viewing of targets illuminated by headlamp bulbs was performed by subjects. Neodymium, HID, “blue”-coated, and conventional halogen lamps were compared. Since traffic signs are among the most important colored objects along the roadway, common sign materials were used to make up the test targets. Several color viewing properties were examined, including color naming, specific color comparison preference, overall color comparison preference, and overall preference. The implications of the results to driving performance and aesthetics are discussed.