One kilolux is exactly 1,000 lux. The prefixed unit exists because daylight is thousands of times brighter than indoor lighting, and writing outdoor illuminance in plain lux means four- and five-digit numbers.
About the units
The lux is one lumen per square metre. It covers indoor lighting comfortably — offices, homes and workshops all fall between 100 and 1,000 lx — but outdoor daylight runs from about 1 klx on a heavily overcast day to 100 klx in direct summer sun. The kilolux keeps those figures to one or two digits. Note that illuminance measures light arriving at a surface weighted for human vision, so it says nothing directly about the energy in the light: photosynthetic and photovoltaic calculations need radiometric units in watts per square metre instead, and the conversion between them depends on the spectrum and cannot be reduced to a factor.
The exact factor
The factor is exactly 1,000. Reference values in kilolux: direct sunlight at noon 32 to 100 klx; full daylight in shade 10 to 25 klx; overcast midday 1 to 2 klx; sunrise or sunset 0.4 klx. Indoors even a brightly lit room rarely exceeds 1 klx, which is why moving from outside to inside feels dark for a minute — the eye is adapting across more than an order of magnitude. Camera exposure calculations and daylight autonomy studies in building design both work at this scale.
Where you meet this conversion
Outdoor and daylight measurement. Daylighting design in architecture uses kilolux for external illuminance and lux for the resulting internal levels, with the daylight factor expressing the ratio between them. Horticulture and greenhouse design measure available light in kilolux, though plant research prefers micromoles of photons per square metre per second because photosynthesis responds to photon count rather than to perceived brightness. Solar resource assessment, road lighting design and photographic work under sunlight all use the unit.