Page 143 - Pipeline Rules of Thumb Handbook

130 Pipeline Rules of Thumb Handbook

FLOODLIGHTING CONCEPTS

This material is intended as a helpful guide in the evaluation of lighting requirements and is intended to serve only as a
guide.

Terms

Candela or Candlepower—Light sources do not project the the lumens are falling. A density of one lumen per square foot
same amount of light in every direction. The directional char- is one footcandle. One footcandle is equal to 10.76lux.
acteristic of a lamp is described by the candlepower in spe- Lux, lx—The SI unit of illuminance. This metric measure-
cific directions. This directional strength of light or luminous ment is based upon the density of lumens per unit surface
intensity is measured in candelas. area similar to footcandle, except one lux is one lumen per
Lumen—Light quantity, irrespective of direction, is mea- square meter. One lux is equal to 0.09 footcandle.
sured in lumens. Lamp lumens are the quantity of light pro- Light Loss Factor, LLF—These factors are used to adjust
duced by a lamp. Average light level calculations use total lighting calculations from laboratory test conditions to a closer
lamp lumens as a basis and then adjust for all factors that approximation of expected field results. The I.E.S. Lighting
lower this quantity. The amount of useful light in a floodlight Handbook, 1984 Reference Volume, defines LLF as follows:
beam is measured in beam lumens. “a factor used in calculating illuminance after a given period
Footcandle, fc—Specifications are usually based on density of time and under given conditions. It takes into account tem-
of light or level of illumination which is measured in foot- perature and voltage variations, dirt accumulations on lumi-
candles. Footcandles are the ratio of quantity of light in naire and room surfaces, lamp depreciation, maintenance
lumens divided by the surface area in square feet on which procedures and atmosphere conditions.”

Floodlighting calculations

Floodlighting encompasses many variations. Since the loca- The most commonly used systems for floodlight calcula-
tion of the floodlight relative to the object to be lighted can tions are the point-by-point method and the beam-lumen
be in any plane and at any distance from the source ...flood- method.
lighting application is often considered the most complex and
difficult of all lighting techniques.

Point-by-point method

The point-by-point method permits the determination of EI D 2 (1)
=
footcandles at any point and orientation on a surface and the
degree of lighting uniformity realized for any given set of
conditions.
In such situations, the illumination is proportional to the
candlepower of the source in a given direction, and inversely
proportional to the square of the distance from the source.
See Figure 1.
Footcandles on Plane Candlepower of Light Ray
( Normal to Light Ray) = Distance in Feet From
Source to Point-Squared Figure 1. Inverse Square Law.