# Introduction to radiometric quantities | radiometry

Radiometric quantities are measured quantities that refer to the total electromagnetic radiation. In technical applications, this is used as UV radiation or IR radiation. The field is called radiometry. Radiation is classified according to the wavelength of the radiation.

The following diagram shows the four radiometric quantities. The table contains a visual representation, mathematical definition, unit and brief explanation for each quantity.

- Radiant flux Φ
- Irradiance E
- Radiance L
- Radiant intensity I

# Definition and explanation of radiometric measurands

**Radiant flux**

Radiant flux , also known as radiant power, is the total amount of energy emitted by a radiation source in all directions per unit of time. The unit of radiant flux is the watt (W).

- Formula symbol: Φ
- Unit: W
- Definition: The total emitted radiant power.

Example:

A standard incandescent lamp with a power consumption of 60 watts emits around 57 watts as radiant flux, with the rest being lost as heat. These 57 watts are distributed across the visible and infrared spectrum.

The radiation flux is measured in an integrating sphere.

**Irradiance**

Irradiance is a physical quantity that describes the power density of the radiation hitting a surface. It is defined as the radiant power per area incident on a surface element. The unit of irradiance is watts per square meter (W/m²) or milliwatts per square centimeter (mW/cm²).

- Formula symbol: E
- Unit: W/m², mW/cm²
- Definition: The radiation hitting a surface element regardless of the direction.

The irradiance indicates how much energy in the form of radiation falls on a certain surface per unit of time. It does not matter from which direction the radiation comes.

Example:

The average irradiance of the sun on the earth's surface on a clear day is around 1000 W/m². This means that every square meter of the earth's surface receives 1000 watts of radiation.

The irradiance is measured with a radiometer.

**Radiance**

Radiance is a physical quantity that describes the radiant power emitted by a radiation source per unit area and per unit solid angle. It is measured in watts per square meter per steradian (W/(m²-sr)).

- Formula symbol: L
- Unit: W/(sr m²)
- Definition: Quotient of radiated radiant flux and the product of surface element and solid angle.

The radiance takes into account both the area of the radiation source and the solid angle at which the radiation is emitted.

Example:

The sun has a high radiance because it emits a large amount of radiant energy into space over a relatively small area.

**Radiant intensity**

Radiant intensity is a physical quantity that describes the flow of radiation in a particular direction per unit solid angle. It is used to indicate the intensity of radiation in a specific direction. The unit of radiant intensity is watt per steradian (W/sr).

- Formula symbol: I
- Unit: W/sr
- Definition: Quotient of radiated radiant flux in a defined direction and the solid angle element.

The radiant intensity describes the amount of energy emitted by a radiation source in a specific direction within a solid angle. It is a measure of the directional dependence of the radiated power.

Example:

An LED with an output of 1 watt could have a radiant intensity of 10 W/sr if it emits the light at a solid angle of 0.1 sr. This means that the emitted energy is strongly concentrated on a small solid angle.