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Publication Number: FHWA-HRT-04-148
Date: December 2005

Enhanced Night Visibility Series, Volume XVII: Phases II and III—Characterization of Experimental Vision Enhancement Systems

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CHAPTER 2—METHODS

The characterization of the headlamps was performed in two efforts. The visible systems were measured with the assistance of a major automotive manufacturer at its photometry range. The ultraviolet systems were measured in cooperation with the University of Iowa at a corporate photometer range in St. Paul, MN. The testing was completed in multiple visits over several months.

APPARATUS

The VESs characterized included the following types of headlamps:

  • Halogen (i.e., tungsten-halogen) low beam (HLB).
  • Halogen high beam (HHB).
  • High output halogen (HOH).
  • High intensity discharge (HID).
  • Ultraviolet A headlamps (UV–A).
  • Hybrid UV–A with visible output (hybrid UV–A).

The apparatus for the visible light measurements is a turnkey goniophotometer that is owned by a major automotive manufacturer. The goniophotometer is designed to provide luminous intensity measurements using test methods that meet the requirements of recommendations J602, J575, and J1330 of the Society of Automotive Engineers (SAE).(2,3,4)

The primary apparatus used for the measurements made by the University of Iowa is a goniometer and a double monochromator. These instruments were used in concert to perform both a spectral (wavelength-by-wavelength) and spatial (differing-projection-angles) analysis of the various sources.

The goniometer consists of a large, flat plate that can be rotated in four axes of freedom. The headlamp was mounted on the front surface of the plate using the same mounting hardware as that used on the vehicles. Alignment of the headlamp was performed using both the angular freedom in the goniometer and the alignment capabilities of the headlamp itself. The automated control of the goniometer was then used to adjust the angle of view from the detector to the headlamp system. The detector for the system was either an illuminance meter or the double monochromator. These two detectors were located 15 m (approximately 49 ft) from the headlamps, and they could be swapped for different portions of the testing.

The double monochromator was used to establish the spectral characteristics of the light sources. The output of the source, whether UV–A or visible light, was determined based on a spectral analysis of the flux. The spectral characteristics of the headlamp sources were measured from a wavelength interval of 200 to 800 nm with a 2-nm bandpass.

VISIBLE LIGHT SYSTEM MEASUREMENT

The visible light headlamps were measured following SAE Recommended Practice J575 chapter 4, section 5.(3) Each headlamp was mounted on the goniophotometer and aimed according to either the mechanical aiming pads or the visual methods, depending on the nature of the headlamp being tested. The luminous intensity of each headlamp was measured from 10° up to 15° down and from 45° to the left to 45° to the right. Measurements were taken in 0.1° increments in both the horizontal and vertical planes. All headlamps were measured while the bulbs were steadily burning and powered according to their respective power requirements.

UV–A SYSTEM MEASUREMENT

The UV–A sources were spectrally and spatially characterized using the goniometer and the double monochromator. During the measurement process, a complete spectral power distribution, from 200 to 800 nm in 2-nm intervals, was measured at the center point of the beam (0,0). To obtain a full matrix of distribution angles, a less detailed spectral measurement was performed at all of the subsequently tested orientations. The measurements were performed for all the combinations of vertical angles, −20°, −10°, 0°, 10° and 20°, and horizontal angles, −20°, −10°, 0°, 10° and 20°. In all, 25 measurements were made. All units were powered according to their respective power requirements.

 

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