Atlantic Highway Sign - What is luminance? Back

By Dr. Susan Chrysler

Drivers see devices through both color and brightness, day and night. Daytime visibility is determined by both brightness and color contrast with the surrounding area; but at night, the primary factor is brightness - or luminance - contrast.

Luminance contrast can be achieved by externally or internally illuminating the device or, more typically, by relying on the vehicle headlights to illuminate the device. Since headlight illumination changes constantly, due to the type of headlight and distance from the device, we rely on retro reflective material on the device to redirect light back to the driver.

Retro reflection refers to the physical principle of returning light to its source. Light is returned in a cone, with virtually all of the reflected light within 3 degrees of the source. The amount of light returned depends on the geometry of the situation and the retro reflective properties of the material being used on the traffic control device. Two angles are most commonly used in describing the performance of retro reflective materials: entrance and observation angles.

The entrance angle (Figure 1) is formed between the light source, the face of the retro reflective device and the line perpendicular to the device. The entrance angle is affected by the offset of the sign from the roadway edge, the distance of the vehicle to the sign, the travel lane of the vehicle, the curvature of the roadway and the twist of a sign mounting.

The observation angle (Figure 2) is formed between the light source, the device and the observer - either a human eye or a light measuring instrument. Observation angle is affected by distance from the sign, the sign mounting position and driver eye height, such as a passenger car vs. Truck.

The coefficient of retro reflection, which is included in specifications, is the result of measurements at certain specified entrance and observation angles. It describes material performance at a specific point in space. For sign sheeting, the coefficient of retro reflection is designated as R A or S.I.A. and is measured in units of candelas per lux per square meter or cd/lx/m 2 . For road markings, it is designated as R L and is measured in units of millicandelas per square meter per lux or [(mcd)/m 2 /lx].

But viewing traffic control devices is a dynamic process, with constantly changing entrance and observation angles. What really matters to the driver is how bright an object appears. Luminance is the physical measurement made with a photometer and is expressed in candelas per square meter (cd/m 2 ). Luminance is the amount of light at the eye; the human visual system processes this light and perceives brightness.

An analogy to further explain the relationship of luminance to brightness is the relationship of temperature to heat. A day with 90 F. (32 C.) temperature (physical measurement) may feel comfortable to one person and hot to another (the perception of heat). This is similar to how the same sign (measured in luminance) may appear to a 30-year-old or a 60-year-old the perception of brightness may vary from bright to dim.

Perceived brightness - or luminance - is best thought of as a product of a system consisting of an illuminant (headlights), a reflector (a sign) and an observer (the driver). Changes in any part of this system will affect the resulting brightness to the observer. Changes to the geometry among the parts of this system will affect brightness as well.

Another factor that affects luminance is the contrast of brightness between a reflective device and its surrounding environment. For urban areas, with high ambient illumination, a sign may not be as easily visible as it would be in a dark, rural area.

The amount of luminance seen by a driver during the approach to a reflective device can be observed in graphs that contain measurements at various distance and show the effect of differing entrance and observation angles. Following are examples.

Figure 3 shows the luminance of encapsulated lens sheeting on a right shoulder mounted white sign for a passenger car vs. a truck. Separation between drivers' eyes and headlamps affects the observation angle, which affects luminance.

In Figure 4 we see how the choice of retro reflective material affects luminance. Sheetings are designed to perform in different ways with regard to entrance and observation angles. Note the luminance differences among Type A sheeting (engineer grade), Type B sheeting (High Intensity Grade) and Type C sheeting (Diamond Grade LDP).

Driver factors also affect perceived brightness. The single largest factor affecting drivers' vision is age. Research shows that older drivers need three times the luminance of younger drivers to see a sign at the same distance.