Page 144 - Pipeline Rules of Thumb Handbook

Electrical Design 131

When the surface on which the illumination to be determined Then b equals the angle between the light ray and a per-
is tilted, the light will be spread over a greater area, reducing pendicular to the plane at that point.
the illumination in the ratio of the area of plane A to the area
of plane B as shown in Figure 2. This ratio is equal to the
cosine of the angle of incidence; thus:

of
Candlepower Light Ray
Footcandles on Plane B =
Distance in Feet From
Source to Point-Squared
x Cosine of Angle b
E = ( I D ) ¥ Cosine b (2) Figure 2. Cosine of Angle Law.
2

Beam-lumen method

The beam-lumen method is quite similar to the method Coverage. It is recommended that sufficient point-by-point
used for interior lighting except that the utilization factors calculations be made for each job to check uniformity and
must take into consideration the fact that floodlights are not coverage.
usually perpendicular to the surface and therefore not all of Light Loss Factor (LLF). The maintenance or light loss
the useful light strikes the surface. factor is an allowance for depreciation of lamp output with
Beam lumens are defined as the quantity of light that is age and floodlight efficiency due to the collection of dirt on
contained within the beam limits as described as “beam lamp, reflector, and cover glass. The total factor may vary from
spread.” Beam lumens equal the lamp lumens multiplied by 0.65 to 0.85 depending on the type of lamp and luminaire
the beam efficiency of the floodlight. used and may include losses due to lamp orientation, or “tilt.”

Design procedure

Step 1—Determine the level of illumination. See Table 1 Utilized Lumens
for some typical levels of illumination (fc). The basic formula CBU = BL (4)
is:
The exact CBU can be determined graphically by projecting
¥
¥
N BL ¥ CBU LLF the outline of the area to be lighted upon the photometric
fc = (3) data and totaling the utilized lumens. This procedure is
A
detailed in the I.E.S. Lighting Handbook. See Table 3.
where
N = quantity of luminaires Table 1
A = area in square feet Typical Illumination Levels
BL = beam lumens
CBU = Coefficient of beam utilization Area fc Area fc
LLF = Light loss factor
Building exteriors Storage tanks 1
Entrances Pump areas 2
Step 2—Determine type and location of floodlights. Active 5 Parking 1
Regardless of light source there are industry standards on Inactive 1 Substation 2
Vital locations 5 Storage yards
beam spreads. See Table 2.
Catwalks 3 Active 20
Step 3—Determine the coefficient of beam utilization.
Stairs 5 Inactive 1
This factor, CBU, written as a decimal fraction, is expressed
Barge or truck unloading 5
in the following ratio: