Photometric and Radiometric Exposure
Photometric or luminous exposure Hv is the accumulated physical quantity of visible light energy (weighted by the luminosity function) applied to a surface during a given exposure time. It is defined as:
where
- Hv is the luminous exposure (usually in lux seconds)
- Ev is the image-plane illuminance (usually in lux)
- t is the exposure time (in seconds)
The radiometric quantity radiant exposure He is sometimes used instead; it is the product of image-plane irradiance Ee and time, the accumulated amount of incident "light" energy per area:
where
- He is the radiant exposure (usually in joules per square metre (J/m2))
- Ee is the irradiance (usually in watts per square metre (W/m2))
- t is the exposure time (in seconds)
If the measurement is adjusted to account only for light that reacts with the photo-sensitive surface, that is, weighted by the appropriate spectral sensitivity, the exposure is still measured in radiometric units (joules per square meter), rather than photometric units (weighted by the nominal sensitivity of the human eye). Only in this appropriately weighted case does the H measure the effective amount of light falling on the film, such that the characteristic curve will be correct independent of the spectrum of the light.
Many photographic materials are also sensitive to "invisible" light, which can be a nuisance (see UV filter and IR filter), or a benefit (see infrared photography and full-spectrum photography). The use of radiometric units is appropriate to characterize such sensitivity to invisible light.
In sensitometric data, such as characteristic curves, the log exposure is conventionally expressed as log10(H). Photographers more familiar with base-2 logarithmic scales (such as exposure values) can convert using log2(H) ≈ 3.32 log10(H).
Table 1. SI photometry units- v
- t
- e
| Quantity | Symbol | SI unit | Symbol | Dimension | Notes | |||
|---|---|---|---|---|---|---|---|---|
| Luminous energy | Qv | lumen second | lm⋅s | T⋅J | units are sometimes called talbots | |||
| Luminous flux | Φv | lumen (= cd⋅sr) | lm | J | also called luminous power | |||
| Luminous intensity | Iv | candela (= lm/sr) | cd | J | an SI base unit, luminous flux per unit solid angle | |||
| Luminance | Lv | candela per square metre | cd/m2 | L−2⋅J | units are sometimes called nits | |||
| Illuminance | Ev | lux (= lm/m2) | lx | L−2⋅J | used for light incident on a surface | |||
| Luminous emittance | Mv | lux (= lm/m2) | lx | L−2⋅J | used for light emitted from a surface | |||
| Luminous exposure | Hv | lux second | lx⋅s | L−2⋅T⋅J | ||||
| Luminous energy density | ωv | lumen second per metre3 | lm⋅s⋅m−3 | L−3⋅T⋅J | ||||
| Luminous efficacy | η | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | ratio of luminous flux to radiant flux | |||
| Luminous efficiency | V | 1 | also called luminous coefficient | |||||
| See also: SI · Photometry · Radiometry · (Compare) | ||||||||
- v
- t
- e
| Quantity | Symbol | SI unit | Symbol | Dimension | Notes | |||
|---|---|---|---|---|---|---|---|---|
| Radiant energy | Qe | joule | J | M⋅L2⋅T−2 | energy | |||
| Radiant flux | Φe | watt | W | M⋅L2⋅T−3 | radiant energy per unit time, also called radiant power. | |||
| Spectral power | Φeλ | watt per metre | W⋅m−1 | M⋅L⋅T−3 | radiant power per wavelength. | |||
| Radiant intensity | Ie | watt per steradian | W⋅sr−1 | M⋅L2⋅T−3 | power per unit solid angle. | |||
| Spectral intensity | Ieλ | watt per steradian per metre | W⋅sr−1⋅m−1 | M⋅L⋅T−3 | radiant intensity per wavelength. | |||
| Radiance | Le | watt per steradian per square metre | W⋅sr−1⋅m−2 | M⋅T−3 | power per unit solid angle per unit projected source area. confusingly called "intensity" in some other fields of study. |
|||
| Spectral radiance | Leλ or Leν |
watt per steradian per metre3 or watt per steradian per square |
W⋅sr−1⋅m−3 or W⋅sr−1⋅m−2⋅Hz−1 |
M⋅L−1⋅T−3 or M⋅T−2 |
commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and either wavelength or frequency. |
|||
| Irradiance | Ee | watt per square metre | W⋅m−2 | M⋅T−3 | power incident on a surface, also called radiant flux density. sometimes confusingly called "intensity" as well. |
|||
| Spectral irradiance | Eeλ or Eeν |
watt per metre3 or watt per square metre per hertz |
W⋅m−3 or W⋅m−2⋅Hz−1 |
M⋅L−1⋅T−3 or M⋅T−2 |
commonly measured in W⋅m−2⋅nm−1 or 10−22W⋅m−2⋅Hz−1, known as solar flux unit. |
|||
| Radiant exitance / Radiant emittance |
Me | watt per square metre | W⋅m−2 | M⋅T−3 | power emitted from a surface. | |||
| Spectral radiant exitance / Spectral radiant emittance |
Meλ or Meν |
watt per metre3 or watt per square |
W⋅m−3 or W⋅m−2⋅Hz−1 |
M⋅L−1⋅T−3 or M⋅T−2 |
power emitted from a surface per wavelength or frequency. |
|||
| Radiosity | Je or Jeλ | watt per square metre | W⋅m−2 | M⋅T−3 | emitted plus reflected power leaving a surface. | |||
| Radiant exposure | He | joule per square metre | J⋅m−2 | M⋅T−2 | ||||
| Radiant energy density | ωe | joule per metre3 | J⋅m−3 | M⋅L−1⋅T−2 | ||||
| See also: SI · Radiometry · Photometry · (Compare) | ||||||||
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