LED Light Meter

[trevwhite]

Hi

I want to measure the Lux output from some LED lights. So far when looking for a meter I have found that LED meters need to be configured to the LED wavelength you want to measure. I think they call this the colour value of white lights where you get the warm white or the cold white types.

I found that for RGB leds a light meter is basically useless and you need very expensive equipment to measure it. Anyone got any experience of LED light meters?

At the moment I just want to measure white LED lights but later on I will be needing to measure RGB versions.

Thanks

Trev

[Rick Law]

Hi

I want to measure the Lux output from some LED lights. So far when looking for a meter I have found that LED meters need to be configured to the LED wavelength you want to measure. I think they call this the colour value of white lights where you get the warm white or the cold white types.

I found that for RGB leds a light meter is basically useless and you need very expensive equipment to measure it. Anyone got any experience of LED light meters?

At the moment I just want to measure white LED lights but later on I will be needing to measure RGB versions.

Thanks

Trev

You are mixing two different things.  This may help you clear it up.

Warm white, cool white, etc. are descriptions of the color emitted by a light source.
Color temperature is a quantification of the color emitted by a light source.
-  Warm white = 2000K to 3000K, K for Kelvin
-  Color temperature 2000K means when you heat a "perfect black body" to 2000 degrees Kelvin, that's the color light it emits.
- "Perfect black body" is like an ideal opamp, more a theoretical concept than physical.
-  Kelvin is celsius offset by 273.  So 10 degree celcius is 283 degree Kelvin.  Kevin at 0 degree is a temperature that in theory all molecular motion stops.

Wave length/frequency of light color versus color temperature
- A light can be a combination of light of different wave lengths.
- So, wave length of a light describes a specific property of light wave (how fast is the EM wave waving).  We perceive that as a certain color.
- Only a laser would put out a monochromatic light (of a single wave length).
- Color temperature by definition is what a black body puts out when heated.  Per definition, it doesn't care if it is a combination of multiple waves.

Lux/Lumens are units to measure brightness.
-  Lumens replaces Candle Power.  Lumens is a unit to measure how much light a source emits.  This is independent of direction and how much it is spread.  So to truly measure lumens, you need a sphere to collect all the light emitted by an LED.
-  Lux is how much brightness it is at a certain point.  As defined, it is a one lumen of light entirely projected onto an area of one square meter.

* * *

Lux meter has been around for a long time - in photography. 

A Lux Meter could in theory be calibrated to any specific wave length or set of wave lengths.  In general, most if not all Lux Meter you buy will be human visual range.

I hope this will help with clarification.

Rick

[Someone]

I found that for RGB leds a light meter is basically useless and you need very expensive equipment to measure it. Anyone got any experience of LED light meters?

Sound like you have a good grasp of the basics, the reason typical photographic light meters arent suitable is that they use one (or just a few) light sensors and make assumptions about the spectrum of the light. Lux is a photometric unit and is weighted to match the eyes response:
https://en.wikipedia.org/wiki/Luminosity_function
So if you hit a meter designed for daylight with a narrow wavelength range source such as an LED (including white LEDs) then it wont read the right number. The good news is there are ever cheaper instruments available that will do the full weighting of different spectrums:
http://www.sekonic.com/oman/products/c-7000/overview.aspx
Unless you want to do your own calibrations thats about as cheap as it gets at the moment.

[MacMeter]

Here is another choice, not cheap. As mentioned above, you don't need anything fancy to read simple LUX.

http://sensing.konicaminolta.us/products/cl-70f-illuminance-meter/

The CL-70F CRI Illuminance Meter is an entry-level solution for the measurement and evaluation of the illuminance, color temperature, and color rendering index (CRI) of various illumination sources such as LEDs and fluorescent lamps. It is most commonly used in restaurants, museums, studios, and theaters for accurate light measurements. Its high-resolution CMOS sensor captures and displays the spectral power distribution of current and future generation light sources including LEDs, HID, Halogen, and OLEDs providing unparalleled color measurement accuracy.

[Someone]

Here is another choice, not cheap. As mentioned above, you don't need anything fancy to read simple LUX.
http://sensing.konicaminolta.us/products/cl-70f-illuminance-meter/

Looks like a rebadge of the Sekonic unit (as they have several models based on the same hardware), good to have some other outlets/distributors to get competitive pricing from.

[MacMeter]

I'm not sure about rebadging, as in the past they were always separate companies, but a lot has changed over the years, mergers etc. I always preferred Minolta meters over Sekonics. While my Minolta 3, (old model), can read color temps, it's pretty useless when it comes to LED's, sodium vapor, mercury vapor sources etc. Any source with spikes. Now with most film and TV production using digital based cameras, there is usually at least one or more "critical" monitors usually costing around 8-10 thousand dollars. Setting up and maintaining all the image control settings is the job of the DIT tech. (Digital imaging technician). Since the final recorded is ultimately more important then what any color meter says, the critical monitor is the best tool for making final lighting decisions. Unfortunately, they are less portable, more expensive, and require AC power. So there is still a need for an accurate hand held meter that can read the newer light sources like LED's, but not quite as necessary as before, assuming you have access to a critical monitor while on the shoot itself.