Measuring Light

Lumens

In simple terms, the lumen is a unit of luminous flux that is used to describe the amount of light produced by a light source or luminaire.

It is common to refer to the lumen output of a light source within a luminaire as the initial lumens and the lumen output of the luminaire as delivered lumens. This is important in the context of the concept of light source efficacy; light source lumens per Watt and luminaire efficacy; luminaire lumens per circuit watt. Lumens are expressed with the unit lm.

The lumen output of a light source or luminaire can be measured using photometric equipment.

Stoane Lighting luminaires range in output from 12lm all the way up to over 5000lm, the higher the lumen output the higher the LED power and the higher the heatsinking requirement, so higher lumen products are often bigger than lower lumen products.

LUX

Where lumens is an expression of the amount of light a luminaire or light source can produce, lux is an expression of the amount of light at a specific location.

Lighting designers will design lighting schemes with lux figures in mind (through regulation or best practice guidance) for specific locations and applications. 

Photometric files for luminaires can be used in lighting software such as Dialux and Relux to model lux levels in a space. Lux levels can also be measured using a light meter. The unit is lux.

Lumen maintenance and lifetime

Change in luminous flux of a light source or luminaire over time. LED light source output will reduce over time, naturally the result is that the same is true for the luminaires that contain them. How quickly this happens is in part dictated by the light source specification and design and perhaps more significantly the design, integration and build of the luminaire they are in. Higher LED temperatures can accelerate the lumen output depreciation so as manufacturers of luminaires,  Stoane Lighting focus on operating LEDs within the light source manufacturer specified temperature limits. It is common for requests to provide L70B50 hours of use information.

Here we find that LED lifetime and lumen depreciation are brought together. This is because under the LM80 testing and related TM21 lifetime calculations, lifetime is considered as the point where the luminous flux is 70% of what it was at the start (L70).

The B figure of B50 refers to a tolerance. It suggests that after the stated time, 50% of the samples (read light sources/luminaires) will be above 70% and the rest could be below. By contrast a B0 figure will describe a situation where none of the samples will fall below 70% relative flux. 

The time in the figure, e.g. L70 B0 at 50,000 hours describes in hours the time at which 70% relative flux is reached.

This is an imperfect way of establishing real luminaire lifetime because of course other issues can limit luminaire life. This is a topic industry associations are working on.