Introduction to photometric quantities | Photometry

Lighting technology  and radiometric quantities form the basis for the understanding and application of light and UV radiation in scientific, industrial and everyday contexts.

Light is the visible form of electromagnetic radiation, which lies in the visible spectrum from around 380 to 780 nanometers.

Lighting technology deals with the generation, distribution and application of light in order to achieve certain visual effects, use energy efficiently and improve the quality of life. These quantities are also called photometric quantities. The specialist field is called photometry.

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

1. Luminous flux Φ
2. Illuminance E
3. Luminance L
4. Luminous intensity I

Luminous flux

Luminous flux is a photometric quantity that describes the total visible light output emitted by a light source in all directions. It is measured in lumens (lm). The luminous flux indicates how much light energy is emitted by a source per unit of time and is a measure of the brightness of a light source.

• Formula symbol: Φ
• Unit: lumen, lm
• Definition: The V(lambda)-weighted radiant power.

The luminous flux is the total amount of visible light emitted by a light source. It takes into account all directions in which the light is emitted and is therefore a measure of the total brightness of the light source.

Example:

A conventional light bulb with a power consumption of 60 watts has a luminous flux of around 900 lumens. This means that the lamp emits a total of 900 lumens of visible light in all directions. However, an LED lamp, which also has a luminous flux of 900 lumens, only consumes around 10 watts. 

The luminous flux is measured with an integrating sphere.

Illuminance

Illuminance (E) is a photometric quantity that describes the luminous flux that strikes a surface. It is defined as the luminous flux per unit area and is measured in lux (lx). One lux corresponds to one lumen per square meter (lm/m²).

• Formula symbol: E
• Unit: lm/m², Lux, Lx
• Definition: Quotient of the incident luminous flux to the area size.

The illuminance indicates how much luminous flux falls on a certain surface and thus describes the brightness that is perceived on this surface. It is an important parameter in lighting design and technology.

Example:

On a sunny day, the illuminance outdoors can be around 100,000 lux. This is a very high illuminance level and ensures bright, well-lit conditions.

In a typical office, the recommended illuminance is around 500 lux to create a comfortable and productive working environment. This is often achieved by ceiling lights and desk lamps.
 

Luminance

Luminance is a photometric quantity that describes the impression of brightness that a light source makes on the human eye when viewed from a certain direction. It is defined as the luminous flux per unit area of the source and per unit solid angle. The unit of luminance is candela per square meter (cd/m²).

• Formula symbol: L
• Unit: lm/(sr m²), cd/m²
• Definition: Quotient of the emitted luminous flux and the product of the surface element and the solid angle.

The luminance indicates how bright a light source appears when viewed from a specific point. It takes into account the direction and area of the light source and is a measure of the perceived brightness.

Example:

A computer screen may have a typical luminance of around 300 cd/m². This means that the screen provides a comfortable and sufficient brightness for normal viewing.

Luminous intensity

Luminous intensity is a photometric quantity that describes the luminous flux emitted by a light source in a specific direction within a solid angle. It is measured in candela (cd), whereby one candela corresponds to one lumen per steradian (lm/sr).

• Formula symbol: I
• Unit: lm/sr, cd
• Definition: Quotient of the luminous flux emitted in a defined 

The luminous intensity indicates how much luminous flux is emitted from a light source in a specific direction. It is particularly useful for describing the directional distribution of light.

Example:

A powerful LED flashlight can have a luminous intensity of 1,000 candela to produce a focused beam of light that reaches far.