Physical Light units

HDRP uses Physical Light Units (PLU) for its lighting. These units are based on real-life light measurements, like those you see on light bulb packaging or a photographic light meter.

Units

Candela:

The base unit of luminous intensity in the International System of Units. For reference, a common wax candle emits light with a luminous intensity of roughly 1 candela.

Lumen:

The unit of luminous flux. Measures the total quantity of visible light a source emits. A light source emitting 1 candela of luminous intensity from an area of 1 steradian has a luminous flux of 1 lumen.

Lux (lumen per square meter):

The unit of illuminance. A light source that emits 1 lumen of luminous flux onto an area of 1 square meter has an illuminance of 1 lux.

Luminance (candela per square meter):

Measures the apparent brightness of light either emitted from a light source, or reflected off a surface, to the human eye. A light source that emits 1 candela of luminous intensity onto an area of 1 square meter has a luminance of 1 candela per square meter.

Exposure value (EV):

A value that represents a combination of a camera's shutter speed and f-number. It is essentially a measurement of exposure such that all combinations of shutter speed and f-number that yield the same level of exposure have the same EV. HDRP Lights can use EV₁₀₀, which is EV with a 100 International Standards Organisation (ISO) film.

Light intensities

Natural

Light measurements from natural sources in different conditions:

Illuminance (lux)	Natural light level
120 000	Very bright sunlight.
110 000	Bright sunlight.
20 000	Blue sky at midday.
1 000 - 2 000	Overcast sky at midday.
< 1	Moonlight with a clear night sky.
0.002	Starry night without moonlight. Includes airglow.

Artificial

Approximate light measurements from artificial sources:

Luminous flux (lumen)	Source
12.57	Candle light.
< 100	Small decorative light, such as a small LED lamp.
200 - 300	Decorative lamp, such as a lamp that does not provide the main lighting for a bright room.
400 - 800	Ceiling lamp for a regular room.
800 - 1 200	Ceiling lamp for a large brightly lit room.
1 000 - 40 000	Bright street light.

Indoor

Architects use these approximate values as a guide when designing rooms and buildings for functional use:

Illuminance (lux)	Room type
150 - 300	Bedroom.
300 - 500	Classroom.
300 - 750	Kitchen.
300 - 500	Kitchen Counter or Office.
100 - 300	Bathroom.
750 lux - 1 000	Supermarket.
30	City street at night.

For more examples of indoor light levels see Archtoolbox’s web page on Recommended Lighting Levels in Buildings.