In
radiometry
Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as opposed to phot ...
, radiant exposure or fluence is the
radiant energy
In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic radiation, electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calcul ...
''received'' by a ''surface'' per unit area, or equivalently the
irradiance of a ''surface,'' integrated over time of irradiation, and spectral exposure is the radiant exposure per unit
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
or
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
, depending on whether the
spectrum
A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
is taken as a function of frequency or of wavelength. The
SI unit
The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of units of measurement, system of measurement. It is the only system ...
of radiant exposure is the
joule
The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...
per square metre (), while that of spectral exposure in frequency is the joule per square metre per
hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...
() and that of spectral exposure in wavelength is the joule per square metre per metre ()—commonly the joule per square metre per nanometre ().
Mathematical definitions
Radiant exposure
Radiant exposure of a ''surface'', denoted ''H''
e ("e" for "energetic", to avoid confusion with
photometric quantities), is defined as
where
*∂ is the
partial derivative
In mathematics, a partial derivative of a function of several variables is its derivative with respect to one of those variables, with the others held constant (as opposed to the total derivative, in which all variables are allowed to vary). P ...
symbol;
*''Q''
e is the radiant energy;
*''A'' is the area;
*''T'' is the duration of irradiation;
*''E''
e is the irradiance.
Spectral exposure
Spectral exposure in frequency of a ''surface'', denoted ''H''
e,''ν'', is defined as
where ''ν'' is the frequency.
Spectral exposure in wavelength of a ''surface'', denoted ''H''
e,''λ'', is defined as
where ''λ'' is the wavelength.
SI radiometry units
See also
*
Exposure (photography)
In photography, exposure is the amount of light per unit area reaching a frame (photography), frame of photographic film or the surface of an electronic image sensor. It is determined by shutter speed, lens f-number, and scene luminance. Exposure ...
*
Irradiance
*
Radiant energy
In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic radiation, electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calcul ...
References
{{Authority control
Physical quantities
Radiometry