Light level or Illuminance, is the amount of light measured in a plane surface (or the total luminous flux incident on a surface, per unit area). The work plane is where the most important tasks in the room or space are performed.

Measuring Units of Light level

Illuminance is measured in Foot Candles (ftcd, fc, fcd) or lux in the metric SI system. A foot candle is actually one lumen of light density per square foot; one lux is one lumen per square meter.

  • 1 lux = 1 lumen / sq meter = 0.0001 phot = 0.0929 foot candle (ftcd, fcd)
  • 1 phot = 1 lumen / sq centimeter = 10,000 lumens / sq meter = 10,000 lux
  • 1 foot candle (ftcd, fcd) = 1 lumen / sq. ft = 10.752 lux

Calculating Illuminance

Illumination can be calculated as, I = Ll Cu LLF / Al

where,
I = Illumination (lux, lumen/m3)
Ll = Lumens per lamp (lumen)
Cu = Coefficient of utilization
LLF = Light loss facter
Al = Area per lamp (m2)

Example of illumination:
10 incandescent lamps of 500W (10,600 lumens per lamp) are used in an area of 50m2. With Cu = 0.6 and LLF = 0.8. Now, find the illumination.

As we know that illumination can be calculated as, I = Ll Cu LLF / Al
=> I = (10 x 10600 lumens x 0.6 x 0.8) / 50m2
... I = 1018 lux

Common light levels of outdoors from natural light sources

Condition

Illumination (ftcd)

Illumination (lux)

Sunlight

10,000
107,527

Full Daylight

1,000

10,752

Overcast Day

100

1,075

Very Dark Day

10

107

Twilight

1

10.8

Deep Twilight

0.1

1.08

Full Moon

0.01

0.108

Quarter Moon

0.001

0.0108

Starlight

0.0001

0.0011

Overcast Night

0.00001

0.0001

Common and Recommended Light Levels for Indoors

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