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In radiometry, radiant flux or radiant power is the radiant energy emitted, reflected, transmitted or received, per unit time, and spectral flux or spectral power is the radiant flux per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. 

## Flux

Flux describes the quantity which passes through a surface or substance.

## Frequency

Frequency is the number of occurrences of a repeating event per unit of time.

## Heat flux

Heat flux or thermal flux, sometimes also referred to as heat flux density or heat flow rate intensity is a flow of energy per unit of area per unit of time.

## Hertz

The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second.

## International Organization for Standardization

The International Organization for Standardization (ISO) is an international standard-setting body composed of representatives from various national standards organizations.

## International System of Units

The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system, and is the most widely used system of measurement.

## Joule

The joule (symbol: J) is a derived unit of energy in the International System of Units.

## Luminous flux

In photometry, luminous flux or luminous power is the measure of the perceived power of light.

## Normal (geometry)

In geometry, a normal is an object such as a line or vector that is perpendicular to a given object.

## 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).

## Photometry (optics)

Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye.

## Power (physics)

In physics, power is the rate of doing work, the amount of energy transferred per unit time.

## Poynting vector

In physics, the Poynting vector represents the directional energy flux (the energy transfer per unit area per unit time) of an electromagnetic field.

In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic and gravitational radiation.

Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light.

In radiometry, radiosity is the radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area, and spectral radiosity is the radiosity of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength.

## Watt

The watt (symbol: W) is a unit of power.

## Wavelength

In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.

## References

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