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Federal Highway Administration Research and Technology
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This report is an archived publication and may contain dated technical, contact, and link information
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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The purpose of this document is to provide an archive of data about the vision enhancement systems (VESs) used during the Enhanced Night Visibility (ENV) project. All of the data collected about the photometric, radiometric, and spectral aspects of the headlamps are cataloged in this document; however, while information on the headlamps associated with the experimental infrared systems is presented, data on the infrared systems themselves are not included in this volume. A more indepth look at the technical specifications of the infrared systems is found in ENV Volume XIII, Comparison of Near Infrared, Far Infrared, High Intensity Discharge, and Halogen Headlamps on Object Detection in Nighttime Clear Weather.


The Enhanced Night Visibility project tested and compared 12 different VESs for their ability to provide object visibility to drivers. Twelve different headlamp types were used on eight experimental vehicles or rack systems to provide different visibility conditions during the Enhanced Night Visibility project. This volume characterizes all of these headlamps, with the exception of the infrared systems.

To fully characterize the output of the VESs used in the Enhanced Night Visibility project, two aspects of the individual headlamp systems were tested: spectral and radiant emissivity. The following paragraphs give more detail on the characterization procedure along with definitions to clarify the discussion.

Definitions and Nomenclature

Most of the definitions used in this project are adapted from the Illuminating Engineering Society of North America Handbook (2000).(1) Following is a list of definitions and formulas used in the analysis of headlamps in this project:

  • Light and radiant energy. Radiant energy travels in electromagnetic waves. Light is electromagnetic radiation that is perceptible to the human eye. The human visual spectral response function, V(lambda symbol), is used to weight the radiant energy from a source to its human perceptual response. Because the ENV project deals with sources that provide both visible and nonvisible radiation, the spectral distribution (energy per unit wavelength) of the lamp must be characterized with photometric (associated with visible light) and radiometric (associated with radiant energy) measurements.

  • Spectral power distribution. The spectral power distribution (SPD), denoted as P(lambda symbol), of a light source is the energy output per unit wavelength. It is measured using a spectroradiometer, which separates the broadband radiation emitted from a source into discrete wavelength intervals. The radiant energy from the source is measured in radiant Watts (W) for each interval. The width of the wavelength interval is usually fixed in nanometers (nm). This series of measurements is referred to as the spectral characterization. For visible light sources, the measurement is usually made through the visual range of the electromagnetic spectrum (380 to 800 nm). For ultraviolet sources, the measurement may include a range from the visible spectrum down to 100 nm. Ultraviolet A (UV–A), however, is defined as the region from 315 to 400 nm.

  • Headlamp or luminaire. This term generally refers to the source of electromagnetic radiation (lamp), all means of optical control (reflectors and lenses), electrical control (wiring, terminals, and ballasts, as necessary) and the housing containing all of the components.

  • Lamp. The lamp is the light- or radiation-generating source in the system. Typically, the lamp is small and removable to facilitate replacement of a failed lamp.

  • Lens. The lens is the clear portion of the system that all of the light passes through. The lens typically provides primary control of the beam pattern of the emitted radiation from the luminaire. Headlamps use prismatic light control surface treatments, or lenticles, in the lens face to control the direction of the light or radiation emission.

  • Reflector. The reflector is the surface behind the lens that redirects the radiation from the lamp through the lens, helping to control direction of the radiation. The reflector is usually highly polished.

  • Optics. The optics system is defined as the combination of the lens and the reflector and possibly baffles. The term refers to all parts of the luminaire that control the radiation or light output.

  • Housing. The housing is the shell around the lamp and the optics, providing protection for the system. This is typically a plastic assembly encompassing both the lens and the reflector and includes a mounting base.

  • Measurement units. The definitions of the measurement units used in this report are found in figure 1 through figure 5. Photometric and radiometric terms are defined. In the formulas, phi symbol = radiant or luminous flux from a source, omega symbol = solid angle, theta symbol = angle of observation, A = area of a surface that reflects or emits electromagnetic radiation, P(lambda symbol) = SPD of a source, V(lambda symbol) = human visual spectral response function, lambda symbol = wavelength, I = radiant or luminous intensity, E = irradiance or illuminance, L = radiance or luminance, and k = maximum spectral luminous efficacy in lumens per Watt.

  • Radiant flux. Figure 1 provides the equation to calculate the total radiant flux, in Watts, from a source:

    Equation. Radiant flux. Click here for more detail.
    Figure 1. Equation. Radiant flux.

  • Luminous flux. The luminous flux, measured in lumens (lm), is the radiant flux of a source evaluated in terms of the human visual response. The coefficient k, the maximum spectral luminous efficacy, provides for conversion from Watts to lumens. For photopic vision, k is defined as having a value of 683 lm/W. Figure 2 provides the equation to calculate luminous flux:

    Equation. Luminous flux. Click here for more detail.
    Figure 2. Equation. Luminous flux.

  • Radiant and luminous intensity. The intensity of a source is the flux per unit solid angle, propagated in a given direction. Figure 3 provides the equation to calculate the radiant or luminous intensity, as appropriate. For radiant intensity, the unit is Watts per steradian (W/sr), while luminous intensity is measured in candela (cd). One cd is equal to one lumen per steradian.

    Equation. Radiant or luminous intensity. Click here for more detail.
    Figure 3. Equation. Radiant or luminous intensity.

  • Irradiance and illuminance. The density of the flux falling on a surface is defined as the irradiance or the illuminance. Figure 4 provides the equation to calculate the irradiance or illuminance, as appropriate. For irradiance, the unit is Watts per square meter (W/m2). The unit of illuminance is the lux (lx), which is equal to one lumen per square meter.

    Equation. Irradiance or illuminance. Click here for more detail.
    Figure 4. Equation. Irradiance or illuminance.

  • Radiance and luminance. The flux reflected or emitted from a surface or through a projected area that is propagated in a given direction per unit solid angle is defined as the radiance or luminance of the area. Figure 5 provides the equation to calculate the radiance or luminance, as appropriate. The unit of radiance is Watts per steradian square meter (W/sr m2), while the unit of luminance is candela per square meter (cd/m2).
    Equation. Radiance and luminance. Click here for more detail.
    Figure 5. Equation. Radiance and luminance.

  • Intensity profile. The radiant or luminous intensity from a source is a measure of the directional aspects of the emitted radiation. The measurement of the intensity profile of a source is referred to as the spatial characterization of the source. The spatial characterization is performed using a goniometer and either a radiometer or photometer, as appropriate. The goniometer positions the source relative to the photometer or radiometer so that the flux emitted in a given direction or series of directions may be measured. The results are usually tabulated by horizontal and vertical angles relative to a defined orientation and then plotted.

    Two plots that are often generated from luminous intensity data are the isocandela and isoilluminance plots. Isocandela plots show areas of common luminous intensity through an imaginary plane in front of the measured source. An isoilluminance plot is generated by placing the source (theoretically or in actual practice) at some position and orientation relative to a surface and illustrating the areas of equal illuminance provided by the source onto the surface.

    Because the ENV project used VESs that emitted both visible light and UV–A radiation, combining a goniometer with a spectroradiometer provided measurement of the full range of spectral distribution at each measurement angle. This permitted generation of both radiant and luminous spatial characterizations from one data set.

    Note that the combination of a goniometer with a photometer is often identified as a goniophotometer. The term goniophotometer is used in this document to identify a system dedicated to the measurement of a luminous intensity profile.


The purpose of this volume of the ENV report is to define the performance of the various headlamps used to create the vision enhancement systems tested as part of the ENV studies. This volume provides a reference point for all the other volumes in the ENV series.


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