Lux vs Lumen

Lumen and Lux are two photometric units in the SI system of units. They are closely related to each other and, in simple language, measure how bright a light source appears in two different contexts. These measurements are important in light sources and other cases where the intensity of the light plays a role.

Lumen is the SI unit of the luminous flux, which is a measure of the total amount of visible light emitted by a source. It is the perceived power of the light from a source. The lumen is defined as the luminous flux emitted by a light source of one candela of intensity over a solid angle of one steradian.

Therefore, 1lumen(lm)=1cd/sr.

In simple, if a point light source emits one candela of luminous intensity through a solid angle of one steradian, then the total luminous flux into the solid angle is known as a lumen. This is a measure of the total number of packets (or quanta) of light produced by a light source.

The light outputs of the projectors are usually measured in lumens. Also, lighting equipment such as lamps are commonly labeled with their light output in lumens; in some countries, this is required by law.

Lux is the SI unit of measurement of illuminance, i.e. the total luminous flux incident on a unit surface area. It is a measure of how much the incident light illuminates the surface and gives an indication to the perception of the intensity of the light by a human eye. Lux is defined as the number of lumens per unit area.

Therefore, 1 lux=1lm/m2

In this definition, the effect on luminous flux by the spread in an area is taken into account. Therefore, the illuminance is inversely proportional to the area (illuminance obeys inverse square law).

Consider a light source with a luminous flux 100 lumens at 1 meter distance from the source. At 2 meters away, the luminous flux is the same, which is 100 lumens, but the area over which the light spread has changed. Therefore, the illuminance at 2 meters is one fourth of the value at 1m, which is 25lux. Further away the illuminance is even lower.

Therefore, the illuminance is of importance to sensors, camera, and other equipment that need a minimum brightness to operate properly. In most equipment, this critical number of lumens is measured and noted on the product.

Lumen vs Lux

• Lumen is the measure of the luminous flux and is defined as the luminous flux from a light source of one candela through a solid angle of 1 steradian.

• Lux is the measure of the illuminance and is defined as the number of lumens per square meter.

• Lumen measures the amount of light (photon output) from light sources weighted by the luminous function, to account for the sensitivity of the human eye.

• Lux measure how bright a light appears. Lux takes into account the spread of light over an area.

• If measured from a fixed source, number of lumens stays constant, and the number of lux decreases over the increasing distance.