Thread: How to measure Lumen or lux

You see many statements about how many lumen a certain solution is, and it may be correct in the lab.

But not having a lab, but having a Traxide loom, a Defender, Cibie Super Oscar Lights and 75w HID , plus a smart phone app, what next?

From what i can gather, it is measured from a reflected surface. But how far from the reflective surface, (white wall) for the vehicle and how far for the measuring device.

Since so many people make claims about their luman output, i sure someone knows how to measure it correctly.

thanks.

This is a complicated one to get your head around?.. Lumen is a measure of light energy (Flux)?.???? Lux is the measure of light per unit area, which is the same creature as Foot Cd (imperial version) ?. 1 Lumen per m^2 is 1 lux?. so you need the projected area to determine the brightness? Brightness is achieved by having the lumens packed into a small area? so the data below is completely indeterminate? Not to mention that the quoted Lumens are not the effective Lumens available? So from the data below you cannot work out how bright the light is? just the total light produced.. which will be about 40% less than what is quoted?..

The best way to think of Lumens and Lux?. is think of it as pressure? That is the Lumen (Light Flux) is the pound and Lux is the pound per square inch? PSI?.. The higher the light pressure?. the brighter the object?? so the data must show the area at distance and the total number of lumens falling on that area to determine how bright the light will be?. They don't do this for good reason?? the laymen thinks lumens are brightness? not so?. if they are spread wide it will be low and the reverse if packed in tight (focussed).

As for LED?.. the truth is far from what is advertised?.. nearly all LED bars are advertised as Raw Lumen output not net?? a brief explanation is below??.


Whilst some of this is regarding LED, much applies to all..

LED lamp output values (Lumens) can vary dramatically depending on which values are being quoted. The following information may assist selecting the right product for the right application.
RAW vs Effective Light Output
There are inherent losses associated with any lamp (LED, Halogen, HID or Incandescent).
The main losses are related to 3 items: Thermal Losses, Optical Losses and Assembly variation.
Thermal Losses:
The greatest challenge is Thermal management of LED's. LED Manufacturers typically measure the light produced by their LEDs after 25 milliseconds (ms). That is equivalent to a flash bulb. It gives a lumen number that is the absolute maximum value. However, LEDs produce less light as they get hotter. LEDs generate a tremendous amount of power in a relatively smaller area.
As the LEDs are powered for longer and longer periods of time, they typically get hotter and hotter depending on the thermal management system. It's not unusual for LEDs to get over 100?C. For vehicle applications, most specifications require that the lamp be measured at 10 minutes and 30 minutes to make sure that the LEDs temperature has stabilized.
This will result in the LED producing 10 - 20% less light than it's advertised value.
Optical & Assembly Losses:
As light travels though an object (Lens), it loses intensity depending on the clarity of the lens material. This is due to inherent losses internal to the material and to losses at the boundary of the part as the light travels from air through the lens and back to air. These losses are present whether the light source is an LED or bulb. The losses associated with the lens material and optics again can vary from 10 - 20%. There are also losses associated with assembly and manufacturing variation. The LEDs output varies as the assembly tolerances may diminish the overall performance from the lamp from theoretically LED maximums.
Simply adding the expected values of the LED light sources together will never result in a lamp that shines that amount of light on the ground.
RAW
The RAW lumen output is calculated by simply taking the number of LED's In a lamp and multiplying by the LED rated output.
Example: 8 LED's rated at 100 lumen per watt
8 x 100 = 800 Lumen
Effective
The effective lumen output is calculated by taking the RAW lumen value and subtracting the Thermal, Optical and assembly losses.
Example:
RAW Lumens = 800
Less - Thermal, Optical & Assembly Losses = 40%
Effective Lumens = 480 Lumens