Editing Retroreflector (section)

==Operation==
{{multiple image
|direction = vertical
|image1 = Observation angle retroreflector.PNG
|caption1 = Figure 1 – Observation angle
|image2 = Entrance angle retroreflector.PNG
|caption2 = Figure 2 – Entrance angle
}}
[[File:Balloon shadow.jpg|thumb|"Aura" around the shadow of a hot-air balloon, caused by retroreflection from dewdrops]]

Retroreflectors are devices that operate by returning light back to the light source along the same light direction. The coefficient of luminous intensity, ''R<sub>I</sub>'', is the measure of a reflector performance, which is defined as the ratio of the strength of the reflected light (luminous intensity) to the amount of light that falls on the reflector (normal illuminance). A reflector appears brighter as its R<sub>I</sub> value increases.<ref name="cpsc">U.S. Consumer Product Safety Commission [http://www.cpsc.gov/volstd/bike/BikeReport.pdf Bicycle Reflector Project report] {{webarchive|url=https://web.archive.org/web/20071005200344/http://www.cpsc.gov/volstd/bike/BikeReport.pdf |date=2007-10-05 }}<!-- Note: This is a public domain source and some text was copied directly. --></ref>

The ''R<sub>I</sub>'' value of the reflector is a function of the color, size, and condition of the reflector. Clear or white reflectors are the most efficient, and appear brighter than other colors. The surface area of the reflector is proportional to the R<sub>I</sub> value, which increases as the reflective surface increases.<ref name="cpsc" />

The R<sub>I</sub> value is also a function of the spatial geometry between the observer, light source, and reflector. Figures 1 and 2 show the observation angle and entrance angle between the automobile's headlights, bicycle, and driver. The observation angle is the angle formed by the light beam and the driver's line of sight. Observation angle is a function of the distance between the headlights and the driver's eye, and the distance to the reflector. Traffic engineers use an observation angle of 0.2 degrees to simulate a reflector target about 800 feet in front of a passenger automobile. As the observation angle increases, the reflector performance decreases. For example, a truck has a large separation between the headlight and the driver's eye compared to a passenger vehicle. A bicycle reflector appears brighter to the passenger car driver than to the truck driver at the same distance from the vehicle to the reflector.<ref name="cpsc" />

The light beam and the normal axis of the reflector as shown in Figure 2 form the entrance angle. The entrance angle is a function of the orientation of the reflector to the light source. For example, the entrance angle between an automobile approaching a bicycle at an intersection 90 degrees apart is larger than the entrance angle for a bicycle directly in front of an automobile on a straight road. The reflector appears brightest to the observer when it is directly in line with the light source.<ref name="cpsc" />

The brightness of a reflector is also a function of the distance between the light source and the reflector. At a given observation angle, as the distance between the light source and the reflector decreases, the light that falls on the reflector increases. This increases the amount of light returned to the observer and the reflector appears brighter.<ref name="cpsc" />