User's Guide ________________________________________________________
14 _____________________________________________________ M210283en-A
0205-012
Figure 1 LM21 Background Luminance Sensor
Concept of Runway Background
Illumination
In order for the runway lights to be seen, they have to
illuminate the eye to a level above the threshold for
detection.
In RVR (runway visual range) calculations, the visual
threshold of illumination is called the ET. The main factor
of the illumination threshold is the brightness of the
background against which the light is viewed. This
brightness is measured by background luminance sensor.
Table 3 Definitions for Illumination Terms
Definition Description
Illuminance The luminous flux received on a unit area
of a surface (lux)
Luminance The photometric brightness of a surface,
that is, the luminous intensity of any
surface in a given direction per unit of the
projected area (candelas per square
meter)
Luminous flux The time rate of flow of light (lumen)
Luminous intensity The luminous flux per unit solid angel in a
given direction (candelas)
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