UFC 3-530-01
22 August 2006
Chapter 7, "Exterior Applications".
2-5
ILLUMINANCE. Illuminance refers to the light level, or amount of light falling
on a surface. It is measured in lux or footcandles. Horizontal illuminance refers to the
amount of light falling on a horizontal surface. This type of illuminance is often
measured on a desk, work surface, or floor. Vertical illuminance refers to the amount of
light falling on a vertical surface such as white boards, signs, and walls. Vertical
illuminance on peoples' faces is also important for identification at entries and security
checkpoints.
2-5.1
Traditionally, illuminance has been the basis of lighting design. However, we
"see" brightness; we don't see lighting levels or lux. Since the revision of the IESNA
guidelines, new standards regarding design must be followed. The Lighting Handbook
chapter on "Quality of the Visual Environment" (QVE) has added many other design
factors besides illuminance. It is important to review all of the design criteria issues in
order to prioritize issues. In many cases illuminance is no longer a top priority. Lighting
wall and ceiling surfaces is usually more important than providing high levels of
horizontal illuminance. In order to provide flexibility and interest in a space, light ceiling
and wall surfaces with lower ambient lighting levels. Provide higher illuminance levels
with individualized task lighting.
2-5.2
There are three different types of visual responses: Photopic or our day
vision (3 cd/m and higher), Scotopic or our night vision (.001 cd/m and below) and
mesopic or a combination of night and day vision (.001 cd/m to 3 cd/m). (Lighting
Handbook page 1-6). The majority of exterior lighting is designed in the mesopic range.
2-5.2.1 Photopic sensitivity peaks at 555 nm in the green-yellow range. Scoptoic
vision sensitivity peaks at 507 nm more in the blue light range. Mesopic vision varies
between these values depending on the lighting level. As the lighting levels become
lower, lamp sources with more blue light become more effective in nighttime vision.
2-5.2.2 Since lamp lumen ratings are all based on photopic sensitivity, they need to
be adjusted for nighttime applications. "Photopic and scotopic lumens must be
determined from the spectral power distribution of the light source" (Lighting Handbook
page 1-6.) In addition, photopic luminous efficiency function applies to visual fields of
size 2 degrees or less. (Lighting Handbook page 2-1). This means that only tasks that
are on-axis or one that is focusing straight ahead apply to the photopic lamp lumen
ratings. Any task that is in our peripheral vision does not. Peripheral vision shifts to
shorter wavelength sensitivity. (Lighting Handbook page 3-9).
2-5.2.3 There are numerous research projects evaluating the most effective method
of determining mesopic lumen ratings for lamp sources. All of these methods show a
significant effectiveness of a white light source such as metal halide over a high-
pressure sodium light source. An example used by Dr. Mark Rae (editor of the Lighting
Handbook) shows that for a typical roadway luminance of 0.3 cd/m2, a 400 watt HPS
produces 135 lux (13.5 footcandles) and consumes 400 watts plus ballast watts. A
metal halide system with equal visibility produces 86 lux (8.6 footcandles) consuming
335 watts plus ballast watts. In another study involving military facilities, both
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