Useful Formulas - Calculating Average Light Level Throughout a Space
(3 Formulas)

• Average Maintained Illumination (Footcandles) = (Lamps/Fixture x Lumens/Lamp x No. of Fixtures x Coefficient of Utilization x Light Loss Factor) ÷ Area in Square Feet

• Average Maintained Illumination (Footcandles) = (Total Lamps x Lumens/Lamp x Coefficient of Utilization x Light Loss Factor) ÷ Area in Square Feet

• Average Maintained Illumination (Footcandles) = (Lamps in One Fixture x Lumens/Lamp x Coefficient of Utilization x Light Loss Factor) ÷ Area in Square Feet/Fixture

• Zonal Cavity Method (Determining Cavity Ratios)
Room Cavity Ratio (for regular rooms shaped like a square or rectangle) = [5 x Room Cavity Depth x (Room Length + Room Width)] ÷ (Room Length x Room Width)

• Room Cavity Ratio (for irregular-shaped rooms) = (2.5 x Room Cavity Depth x Perimeter) ÷ Area in Square Feet

• Ceiling Cavity Ratio = [5 x Ceiling Cavity Depth x (Room Length x Room Width)] ÷ (Room Length x Room Width)

• Floor Cavity Ratio = [5 x Floor Cavity Depth x (Room Length x Room Width)] ÷ Room Length x Room Width

Room surface reflectances can be predicted in a new design or measured in an existing facility. If existing facility:

• Room Surface Reflectance (%) = Reflected Reading ÷ Incident Reading

• Reflected Reading = Measurement from a light meter holding it about 1.5 feet away from the surface with the sensor parallel and facing the surface.

• Incident Reading = Measurement from a light meter held flat against the surface and facing out into the room.

The Zonal Cavity Method. For each cavity, identify…

hcc (ceiling cavity depth) in feet
hrc (room cavity depth) in feet
hfc (floor cavity depth) in feet
Reflectances of walls, ceiling and floor expressed as a %

Lumen Method
Required Light Output/Fixture (Lumens) = (Maintained Illumination in Footcandles x Area in Square Feet) ÷ (Number of Fixtures x Coefficient of Utilization x Ballast Factor x Light Loss Factor)

• Light Loss Factors
Light Loss Factor (LLF) = Ballast Factor x Fixture Ambient Temperature Factor x Supply Voltage Variation Factor x Lamp Position Factor x Optical Factor x Fixture Surface Depreciation Factor x Lamp Burnouts Factor x Lamp Lumen Depreciation Factor x Fixture Dirt Depreciation Factor x Room Surface Dirt Depreciation Factor

• Lamp Burnout Factor = 1 - Percentage of Lamps Allowed to Fail Without Being Replaced

• Calculating Number of Lamps And Fixtures And Spacing

• Required No. of Fixtures = (Desired Average Maintained Light Level x Area in Square Feet) ÷ (Lumens/Lamp x Lamps/Fixture x Coefficient of Utilization x Light Loss Factors)

• Required Lamps = Required Lumens ÷ Initial Lumens/Lamp

• Maximum Allowable Spacing Between Fixtures= Fixture Spacing Criteria x Mounting Height

• Fixture Spacing Criteria: See the manufacturer's literature

• Mounting height: Distance in feet between the bottom of the fixture and the workplane

• Spacing Between Fixtures = Square Root of (Area in Square Feet ÷ Required No. of Fixtures)

• Number of Fixtures to be Placed in Each Row (Nrow) = Room Length ÷ Spacing

• Number of Fixtures to be Placed in Each Column (Ncolumn) = Room Width ÷ Spacing

For the above two formulas, round results to the nearest whole integer.

• Spacingrow = Room Length ÷ (Number of Fixtures/Row - 1/3)

• Spacingcolumn = Room Width ÷ (Number of Fixtures/Column -1/3)

If the resulting number of fixtures does not equal the originally calculated number, calculate impact on the designed light level:

• % Design Light Level = Actual No. of Fixtures ÷ Originally Calculated No. of Fixtures

To calculate fixtures mounted in continuous rows:

• Number of Luminaires in a Continuous Row = (Room Length ÷ Fixture Length) - 1

• Number of Continuous Rows = Total Number of Fixtures ÷ Fixtures Per Row

Figure 2. Calculating average light level at a point throughout a space.