What is the difference between candela, lumen and lux?

Candela, lumen, and lux are all units of photometric measurement, but they measure different properties of light. The following are the specific differences between these three units:

Definition and Use

Candela (cd)
Definition: Candela is the basic unit of light intensity in the International System of Units, defined as the light intensity of a light source in a specific direction with a luminous flux of 1/683 watt per steradian emitted within a unit solid angle.
Use: In lighting engineering, candela is used to determine the luminous intensity of a light source in a specific direction, and is often used to measure the output power of a laser, the radiation flow, and the intensity of sunlight.

Lumen (lm)
Definition: Lumen is a unit of luminous flux, which indicates the total amount of light emitted by a light source per second. It describes the overall brightness level of the light source.
Use: Lumen is usually used to describe the brightness of lamps or light sources, such as light bulbs, projectors, etc. When purchasing lamps, lumen value is an important reference indicator.

Lux (lux, lx)
Definition: Lux is a unit of illuminance, which means that the illuminance received per square meter is 1 lumen. It describes the distribution and brightness of light over a specific area.
Use: Lux is often used to measure the light level in indoor or outdoor environments, such as the brightness of lights in living rooms, bedrooms, study rooms and other places. It can also be used to evaluate the effect of lighting equipment in actual use.

Relationship and difference

Relationship: Lumen and lux are both units derived from the definition of candela. Lumen measures the total luminous flux of a light source, while lux measures the distribution and brightness of light over a specific area.
Difference:
Different measurement objects: Candela measures the luminous intensity of a light source in a specific direction; Lumen measures the total luminous flux of a light source; Lux measures the distribution and brightness of light over a specific area.
Different application scenarios: Candela is often used to measure the output power, radiation flow rate and intensity of sunlight of lasers; Lumen is often used to describe the brightness of lamps or light sources; Lux is often used to measure the light level of indoor or outdoor environments.

In summary, the three units of light measurement, candela, lumen and lux, each have their own unique definitions, uses and measurement objects. In practical applications, it is necessary to select appropriate units for measurement and evaluation based on specific needs and scenarios.