Question

How do I calculate how many lumens I'll need to replace the fluorescent bulbs in my kitchen with LED recessed lights?

Asked by Diane Korsower
Arcata, CA

The kitchen area is 10' x 16' with a 7 1/2 foot ceiling. The existing fluorescent lights say the produce 2250 lumens per light, which gives 18,000 lumens total for the 8 bulbs. Even the best recessed downlight residential LED's provide maybe 600 lumens, which would suggest I'd need THIRTY of them in my little kitchen.This can't be right, can it?

Answer

Lumen is the measurement of visible light and using this factor to calculate the amount of necessary illuminance for a specific space is the incorrect approach to take.

Footcandles (FC) not lumens

For determining the proper lighting levels for a specific space you must calculate the required footcandles (fc) at the workplane.

  • For a kitchen the workplane is typically 36” above finish floor also known as counter top height.
  • Typical fc required at the workplane for a kitchen is between 50-100 fc. 

Caution: complex calculations ahead

The actual calculations are not straight forward and require knowing the candlepower of the lamps which you intend to use plus additional information supplied by the manufacturer. 

Below are the calculations you will need in determining the correct fixtures for the space. 

For individual lamp location:

  • Source aimed at target perpendicular to the source: fc=I / D2 where I=candle power and D=distance from source.
  • Source aimed at an angle to a horizontal surface: fc=I / D2 x cos (angle) 

For an area:

  • fc=(Number of lamps x initial lamp lumens x LLF x CU) / area
  • LLF= light loss factor =LLD x LDD
  • LLD (lamp lumen depreciation) and LDD (lamp dirt depreciation) and are specific to type of lamp and fixture.
  • Initial lamp lumens are published by the lamp manufacturer.
  • CU= coefficient of utilization and published by the product manufacturer and located on the product data sheet.

You also will have to calculate the RCR (room cavity ratio):

  • RCR=[5(height above workplane to source)(length + width of room)] / length x width.

Sorry for being so technical, but unfortunately there is no easy way to do an accurate comparison.

I would recommend you contact a lighting supplier to help you determine the required lamps for your specific area. 

Tagged In: led

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