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Figure 1 - Illuminance vs. Luminance

 
  
 
GSA
Courtroom Lighting Criteria Evaluation
Task 1 Report
3
Definitions and Background
The following terms are used in this report, and may be helpful in understanding the
analysis.
3.1
Lighting Units and Quantities
Illuminance is the amount of light coming from a light fixture that lands on a surface. It is
measured in Footcandles (Lux in the metric system). A typical office has an illuminance of
between 30 to 50 footcandles (300 to 500 lux) on desktops. Horizontal illuminance
describes the amount of light landing on a horizontal surface, such a desk, and vertical
illuminance describes the illuminance landing on a vertical surface, such as a wall or a
face.
Luminance describes the amount of light leaving a surface in a particular direction, and
can be thought of as the measured brightness of a surface as seen by the eye.
Luminance is expressed in Candelas per square foot, or more commonly, Candelas per
square meter (Cd/m). A typical computer monitor has a Luminance of about 100 Cd/m.
The following two diagrams illustrate the difference between illuminance and luminance.
Figure 1 - Illuminance vs. Luminance
The image on the left shows a light projecting onto a light colored wall, and the image on
the right shows the same light projecting onto a dark colored wall. In both cases, the
measured illuminance on the wall is the same, since the same amount of light is landing
on the surface. However, the wall in the image on the left has a higher luminance value
than the wall in the image on the right since the measured brightness is higher.
The color rendering index of a light source is a measurement of how well a light source
represents color compared to an ideal source. Color rendering index (CRI) is measured
on a scale of 0 to 100. As a gauge of measure, sunlight has a CRI of 100, typical
fluorescent office lighting has a CRI of 80 to 85, and parking lots vary from 20 to 65.
3.2
Key Ratios in Lighting
By measuring luminance, we can compare the brightnesses of areas in a room, and
determine Luminance Ratios. Luminance ratios help quantify the relationship between lit
elements in a room, and are also sometimes known as contrast ratios.
The reflectance of a surface is the percentage of light landing on a surface that is reflected
outward. A typical white wall has a reflectance of around 70%, while a dark wood surface,
such as cherry, has a reflectance of about 20%. Reflectance of room surfaces is critical in
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Issue  March 1, 2006


   


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