1. Point-by-Point Factors
Calculated Levels are Facing the Light
Fixture – With the exception of directly below

5. Nonrecoverble Light Loss Factor (NRLLF)
For Purposes of predicting maintained illuminance, light losses must be accounted. Some of these light losses are nonrecoverable because these are permanent and no manner of maintenance or relamping can recover them. There are four NRLLFs:
1- Voltage Factor: Fluctuations from nominal design voltage may result in light losses. This depends on lamp and ballast/transformer/driver combination, utility voltage consistency, wire sizes, and run distances. This could amount to a loss between 0% and 10%.

6. 2- Thermal Factor: Fluctuations from nominal lamp and ballast/transformer/driver design temperatures may result in light losses. This could amount to a loss between 0% and 20%.
3- Partition Factor: This is an issue in open offices. Furniture partitions block and absorb light. This could amount to a loss between 0% and 20%.
4- Ballast Factor: Flourescent ballasts are responsible for how many lumens the flourescent lamps actually produce. flourescent lamp lumen ratings by the lamp manufacturer are based on ballast factors (BFs) of 1 (means lamps produce 100% of their rated lumens). Any reduction in BFs such as 0.88 should be addressed in calculations, this could amount to 12% error.

7. Recoverble Light Loss Factor (RLLF)
RLLFs include all of light loss factors that can be recovered over the course of an installation’s lifetime. There are three RLLFs:
1-Lamp lumen depreciation (LLD): over the time that lamps are energized, they lose light output (5% to 10%). This loss is recovered when lamps are replaced.
2- Lumem dirt depreciation (LDD): As time progresses, dirt builds up on luminaire reflectors and lenses and on the lamp envelop. This might amount to (2% to 5%) over a few years. This loss is recovered when luminaries and lamps are cleaned.
3- Room surface dirt depreciation (RSDD): As time progresses, dirt builds up on room surfaces. This might amount to (1% to 2%) over a few years.

9. Lumen Method
The lumen or zonal cavity, method, of calculation is based on luminaire or lamp lumens. It’s the simplest method of determining a uniform luminaire layout based on horizontal illuminance criteria (typically at floor height for corridors and circulation spaces or desk height for work spaces). The illuminance on a large surface is equal to the total lumens falling onto that surface divided by the area of the surface, i.e., the total number of lumens available in a room divided by the area of the room.

10. The lumen method can be used in a number of ways, to find:
1- The initial average illuminance on a horizontal surface to be expected from a proposed layout.
2- The maintained average illuminance (average illuminance to be expected over time, compensating for dirt buildup and lamp depreciation factors ) to be expected from a proposed layout.
3- The quantity of specific luminaries required in a given space in order to achieve a specific average maintained illuminance on a horizontal surface.

11. There are several variables needed when undertaking lumen method calculations, these are: maintained illuminance criterion; area of the space; initial lamp lumens; recoverable light loss factors RLLFs; nonrecoverable light loss factors NRLLFs; and the luminairie’s coefficient of utilization (CU).

12. Room Reflectance Room comprised of Walls, Ceiling, and Floor.
Walls typically have Doors and Windows
All surfaces have a reflectance value to bounce light.

13. Light from Light Fixture bounces off of all surfaces.
Surfaces with less reflectance will bounce less light
Typical Reflectance Values:
*75% -90% White, Off White, Grey, Light tints of Blue or Brown
*30%-60% Medium Green, Yellow, Brown, or Grey
*10%-20% Dark Grey, Medium Blue
*5% -10% Dark Blue, Brown. Dark Green, and many wood finishes

14. Calculations using Lumens
Calculates the Average Illumination for a room.
Takes into account the room surface reflectance's – but assumes the surfaces are diffuse (not shiny!).
Assumes an empty room (without furniture).
Can also be used to determine the required Quantity of Fixtures needed for a target light level.
Does not determine light fixture layout or location – you must be following mnfrs spacing criteria.
1. You need Room Dimensions and the Fixture Mounting Height.
2. You need to select a Light fixture
3. Determine the rooms Room Cavity Ratio (RCR).
4. Look-up the fixtures Coefficient of Utilization for the RCR.
5. Calculate!

15. Coefficient of Utilization (CU)
• It defines the percentage of light output that is expected from a fixture
The value is determined by a CU table

16. Room Cavity Ratio (RCR)
Room Cavity Ratio is the volume between the Fixture and Height of Calculation
The RCR can vary depending on the height you want to calculate…as shown here with the calculation height at the floor.
Workplane height is typically 30-inches above the floor
A rooms RCR will always be between 1 and 10

19. The RCR can vary depending on the height of the fixture…
The RCR can vary depending on the height of the fixture….as shown here with Wall Brackets or Sconces.

20. RCR can vary depending on the height of the fixture…
RCR can vary depending on the height of the fixture….as shown here with Pendants.

21. RCR Example:
Room width=12 ft
Room Length= 15 ft
Ceiling height=10 ft

22. Lumen Method Formula
Initial average Illuminance from a given layout Ei
Maintained average Illuminance from a given layout Em
Number of Fixtures (Luminaires) required for a given room and a given illuminance target