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5.1 KiB
Markdown
92 lines
5.1 KiB
Markdown
# Understand physical light units
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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).
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## Units
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<a name="Candela"></a>
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#### Candela:
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The base unit of [luminous intensity](Glossary.md#LuminousIntensity) in the International System of Units. For reference, a common wax candle emits light with a luminous intensity of roughly 1 candela.
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<a name="Lumen"></a>
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#### Lumen:
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The unit of [luminous flux](Glossary.md#LuminousFlux). 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.
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A light source that emits 1 [candela](#Candela) of [luminous intensity](Glossary.md#LuminousIntensity) from an area of 1 steradian has a luminous flux of 1 lumen.
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<a name="Lux"></a>
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#### Lux (lumen per square meter):
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The unit of [illuminance](Glossary.md#Illuminance). A light source that emits 1 lumen of [luminous flux](Glossary.md#LuminousFlux) onto an area of 1 square meter has an illuminance of 1 lux.
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<a name="Nits"></a>
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#### Nits (candela per square meter):
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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.
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A light source that emits 1 candela of [luminous intensity](Glossary.md#LuminousIntensity) onto an area of 1 square meter has a luminance of 1 candela per square meter.
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<a name="EV"></a>
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#### Exposure value (EV):
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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 **EV<sub>100</sub>**, which is EV with a 100 International Standards Organisation (ISO) film.
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## Light intensities
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### Natural
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Light measurements from natural sources in different conditions:
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| Illuminance (lux) | Natural light level |
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| ----------------- | ------------------------------------------------- |
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| 120 000 | Very bright sunlight. |
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| 110 000 | Bright sunlight. |
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| 20 000 | Blue sky at midday. |
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| 1 000 - 2 000 | Overcast sky at midday. |
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| < 1 | Moonlight with a clear night sky. |
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| 0.002 | Starry night without moonlight. Includes airglow. |
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### Artificial
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Approximate light measurements from artificial sources:
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| Luminous flux (lumen) | Source |
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| --------------------- | ------------------------------------------------------------ |
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| 12.57 | Candle light. |
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| < 100 | Small decorative light, such as a small LED lamp. |
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| 200 - 300 | Decorative lamp, such as a lamp that does not provide the main lighting for a bright room. |
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| 400 - 800 | Ceiling lamp for a regular room. |
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| 800 - 1 200 | Ceiling lamp for a large brightly lit room. |
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| 1 000 - 40 000 | Bright street light. |
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### Indoor
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Architects use these approximate values as a guide when designing rooms and buildings for functional use:
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| Illuminance (lux) | Room type |
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| ----------------- | -------------------------- |
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| 150 - 300 | Bedroom. |
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| 300 - 500 | Classroom. |
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| 300 - 750 | Kitchen. |
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| 300 - 500 | Kitchen Counter or Office. |
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| 100 - 300 | Bathroom. |
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| 750 lux - 1 000 | Supermarket. |
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| 30 | City street at night. |
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For more examples of indoor light levels see Archtoolbox’s web page on [Recommended Lighting Levels in Buildings](https://www.archtoolbox.com/materials-systems/electrical/recommended-lighting-levels-in-buildings.html).
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### Lighting and exposure diagram
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The following cheat sheet contains the color temperature values and light intensities of common real-world [Light](Light-Component.md) sources. It also contains [Exposure](Override-Exposure.md) values for different illumination scenarios.
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