The Physics of Retroreflectivity
Terminology
There are three types of reflection. Every surface is a reflector. If the light hits a sheet of paper, it is reflected in all directions. This is diffuse reflection. With a mirror, the light reflects from the surface at the same angle (angle of incidence equals the angle of reflection). This is specular reflection. With a retroreflector, light is reflected back in the direction that it came from.
In order for retroreflectivity to work you need light source, a target, and a receptor. Light goes from headlamps, to the sign, and reflects back to the headlamps.
Measurement
The light coming out of the car headlamps is luminous intensity, the light hitting the sign is the illuminance. The illuminance decreases with the square of the distance. Luminance is what the brightness the driver sees.
Light is reflected back in a cone shape. The light is brighter in the center, which is called the illumination axis. As you move further away from the axis, the reflected light gets dimmer.
Geometry defines how bright a sign looks. The target axis is the line that is perpendicular to the sign. The observation angle is the axis of light reflected back from the sign to your eyes. There are two lines, one for each eye, but since your eyes are so close together, then it is like having one line. The observation angle is the angle between the observation axis and illumination axis. The entrance angle is the angle between the illumination axis and the target axis or the line perpendicular to the sign. Entrance and observation angles are the two most common geometry measurements.
Influencing Factors
You need to have four things in order to see retroreflective devices at night. The four things are a sign (or target), headlamps which produce illumination source, a driver that is the receptor, and the vehicle. The size of the vehicle matters. There are specific aspects for each of these four elements that affect how bright a sign appears. For the sign, it depends upon the location and orientation of the sign with respect to the headlights and driver. It also depends upon how efficient the retroreflective sheeting is at reflecting light back to the source. The headlamps establish how much light is directed toward the sign. A sign can reflect only the light that reaches it. Each driver is unique and has their own characteristics for the luminance needed to see a target, their visual capabilities, and their contrast sensitivity. Finally, the size of the vehicle determines how far the driver is sitting from the illumination axis (which defines the observation angle). The further the driver is from the illumination axis, the dimmer the sign will appear.
When a driver sees a sign at night, he is viewing the sign luminance, or brightness. The driver is not “seeing” retroreflectivity. A sign’s luminance can range from dim to very bright. Sign sheeting can range from engineering grade to microprismatic, with microprismatic sheeting making the sign brighter. The current trend for sheeting is toward microprismatic materials, which are the newer materials. For headlamps, the old sealed beam headlamp provides a brighter sign. Modern cutoff headlamps direct less light at the sign, so the sign looks dimmer if all other factors are equal. Signs are also brighter for younger drivers. As we get older, we need more light to see a sign. Finally, the larger the vehicle, the less bright a sign appears. This is because of the larger observation angle caused by the greater distance between the driver and headlamps.
In summary, current trends in sheeting use is the one trend that is promoting brighter signs. For the other three elements, trends are all toward signs appearing less bright – newer headlamps are directing less light toward signs, we have more older drivers, and vehicles are getting larger.
