#illumination_unit  a measure of illumination
  supertype:  #light_unit__lightunit  a measure of the visible electromagnetic radiation
  subtype:  #footcandle  a unit of illuminance on a surface that is everywhere 1 foot from a point source of 1 candle
  subtype:  #lambert__l  a cgs unit of illumination equal to the brightness of a perfectly diffusing surface that emits or reflects one lumen per square centimeter
  subtype:  #lux  a unit of illumination equal to 1 lumen per square meter; 0.0929 foot candle
  subtype:  #phot  a unit of illumination equal to 1 lumen per square centimeter; 10,000 phots equal 1 lux

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