What is Lumen. What is Lux? Units of Measurement of Lighting

To understand lighting one should know the main measurements of lighting. To make it simple we will talk about only two units, the lumen and the lux.


A Lumen is the SI (Système international d’unités) unit of luminous flux. This is measurement of the light energy that is emitted from the light source. This depends on the type of the light source.

An Incandescent lamp of 40 W produces around 500 lumens. A fluorescent lamp of 32 W produces around 1000 lumens. An LED lamp of 3 Watts produces 500 lumens.

The cost and the life of the source are the two things that determine which to use.

Energy savings in lighting are the results of making a more energy efficient light source.


The Lux is the unit of illuminance and it is an indication of how much light is incident on a surface. One Lux is defined as an illuminance produced when one lumen is incident on one square meter area, so one Lux is one Lumen per square meter area.

The illuminance on a bright day is 100,000 lux. A televised football match requires an illuminance of around 16,000 lux. A kitchen requires around 500 lux. A public road at night requires around 30 lux.


The required illuminance of a surface depends primarily on the type or nature of the jobs or human activities on the illuminated surface.

Therefore, step 1 is to determine how much illuminance is required. Type of task, duration of usage, age, and number of people working at that area, precision level of the work, available natural lights are all some of the factors that decide the illuminance requirement.

Step 2 is to determine how many and what type of light sources are required to produce the required luminance. Three main factors have to be considered.

The type of the source: cost, colour, and life of the source are the main deciding factors to decide the type of light source.

Incident light energy follows the square law. The illuminance on a surface is indirectly proportional to the square of the distance making the distance from the source an important factor.

The co-efficient of utilization also is a factor that determines how much of the luminous flux from the source is actually available on the required surface. Reflectors, lampshades, and luminaires can increase or decrease the utilization factor. Good industrial fixtures can have utilization factors up to 90 %.