Convert lux to lumens and the lumens per watt concept

Lumen is a unit that measures how much visible light a source emits and is always listed on lamp boxes. The best light bulb is one that emits more light while using less electricity. LED lamps are more efficient than older incandescent bulbs because they produce the same amount of light with less power. When comparing LED bulbs with the same power rating, the one with higher lumen output uses higher-quality LEDs.

To measure lumens, we use an integrating sphere (Ulbricht sphere), which is large and expensive. To determine a lamp’s power, we also need a watt meter. Dividing lumens by watts gives lumens per watt, showing how much light each bulb produces per watt. We can then compare a 20-watt lamp from one brand with a 10-watt lamp from another by knowing how much light each delivers per watt.

There are two units used to measure visible light: lumen, which measures the total amount of visible light emitted, and lux, which measures the amount of visible light that falls on a specific surface area.

Definition of lux from Wikipedia:

The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre.

Lumen definition from Wikipedia:

The lumen (symbol: lm) is the unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time, in the International System of Units (SI).

These two definitions demonstrate how distinct these two units are; however, the definition of lux also states that lux and lumen can be equal if all the light from the source or lamp is concentrated on a one-meter square.

Watch the video below to understand the rest of the story, but if it’s difficult for you, continue reading:

We use a sphere of light because it’s challenging to direct a lamp’s light without waste or reflections. When we place the lamp inside this one-square-meter sphere, any point on its surface shows the lux reading equals lumens.

If we set the sphere’s area (S) to one square meter, the radius (r) is calculated using the formula for the surface area of a sphere: S = 4πr². Solving for r, we find that the radius is approximately 28.20 cm. This means the lux measurement at a distance of 28.2 cm from the light source is equivalent to lumens.

Of course, this approach is flawed and completely inaccurate. The area must be absolutely dark to prevent any distracting light from interfering with the test or reflecting anywhere. For example, if you look at the comments on the same video, several people suggest ways to improve the accuracy of this measurement. There are two other methods: DIY Sphere and pipe tube. If you click on the links and read about them, the pipe technique is simpler but has a positive error, which may not be significant given the approach’s low cost.