Lumens, Lux, and Foot-Candles


Light output vs. Light intensity


Lumens is a measurement of light output.  Lux and Foot-candles are a measurement of light intensity.  Lumens, Lux, and Foot candles are closely related, but often confused as to the meaning and how to use the information to make lighting decisions.


Understanding Lumens

Lumens are generally indicated on specifications of lighting fixtures or light sources such as bulbs, tubes, strips, or other products.  Lumens are a measurement of the total light output of a device at the light source.


Understanding LUX and Foot-Candles

Lux (lx) and Foot-Candles (fc) are measurements of light intensity or light density.  Light intensity is measured at a specific location, such as at the desktop, or workbench, or the work surface of tools. 

  • LUX is a measurement of Lumens per square meter. 
  • Foot candles is the imperial equivalent and is a measurement of lumens per square foot.  
  • ​One LUX (lumen per square meter) is equal to 10.7639 foot-candles (lumens per square foot)
  • The further away from the light source, the less intensity.  The dropoff in intensity is exponential, not linear.


Light Intensity vs. Fixture Height


In the three light intensity map images to the right, you will see the same 20ft x 20ft floor plan with fixture height at 8ft, 10 ft, and 12 ft heights.  All three have the same nine units of 48 Watt shop light fixtures in the same location on the ceiling.  


The numbers in the light intensity map indicate foot-candle

intensity at the surface measured (work surface height

where tools and workbenches exist and at the floor height

where there is no worksurface).


Please note the following differences in intensity levels:

Table Saw:

---8ft = 85fc, 85fc, 85fc

---10ft= 80fc, 75fc, 79fc

--12ft=  69fc, 66fc, 70fc


Jointer:

---8ft = 74fc, 79fc

---10ft= 63fc, 70fc

---12ft= 54fc, 61fc


Miter Station (Left Wall)

---8ft = 65fc, 75fc, 78fc, 66fc
---10ft= 51fc, 61fc ,60fc, 54fc
---12ft= 42fc, 50fc, 55fc, 47fc


At Floor Upper Right Corner

---8ft = 40fc
---10ft= 36fc
---12ft= 33fc


The color legend indicates the foot-candle intensity
that corresponds each color on the light intensity
map.  Red=100 foot-candles, black= 0 foot candles.