Physical Light units | High Definition RP | 10.0.0-preview.27
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    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. Note that for lights to behave properly when using PLU, you need to respect HDRP unit convention (1 Unity unit equals 1 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. Describes the total amount of visible light that a light source emits in all directions. When you use this unit, the amount of visible light is independent of the source's size meaning the illumination level of a Scene does not change depending on the size of a light source. However, highlights that a light source produces dim as the area of the light source increases. This is because the same power is spread across a larger area.

    A light source that emits 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.

    Nits (candela per square meter):

    The unit of luminance. Describes the surface power of a visible light source. When you use this unit, the overall power of a light source depends the size of the light source, meaning the the illumination level of a Scene changes depending on the size of the light source. Highlights that a light source produces conserve their intensity regardless of the size of the surface.

    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.

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