# Testing LED Light Output?



## War Eagle (May 16, 2011)

Hello! New to the forum and have a question: how do you measure the light output from an LED source? I understand (maybe not correctly) that a digital light meter is not calibrated for measuring LEDS. Is this true and if so, is there an inexpensive way to measure led light output?

I am working on an led taillight project and want to ensure the led conversion puts out the same amount of light as the incandescent bulbs which are being replaced.

Thanks for the advice!


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## Oznog (May 16, 2011)

"lumens" vary by color. A light meter is calibrated for white light of a particular temp, and going to specific colors will not yield correct lumen values.

Now, given a specific color, your red incan taillight and red LED light should be the same- but the color is not going to be exactly the same, and human eye response has a very steep slope from orange to red. "Sunset orange"/"red-orange" scores like 30% higher visual lumens than a true "red", for the same light power. 

Angle will be important.

Honestly, what you're gonna have to do is power them up side-by-side and walk 50 ft away and see which one's brighter. Nobody says it have to be THE SAME. Just "bright" is good.


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## Harold_B (May 16, 2011)

Well, the DOT and SAE might have something to say about it if you plan to put the design into a production vehicle. If that's the case you could start your shopping list here: http://www.sae.org/images/books/toc_pdfs/HS-34_2010.pdf

Most engineering companies that design tail lights have dedicated equipment that is purpose built for testing indicator lights.


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## Oznog (May 16, 2011)

If you wanted to do that, you could use a solar cell to determine how many photons struck per second (the solar cell has a milliamps-per-photon/sec transform which is pretty constant regardless of wavelength, in the visual range). Then look up how many lumens than corresponds to for that rate of photons per unit of area at that distance.


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## SemiMan (May 16, 2011)

Light meters have a color corrected filter and whether you hit them with monochromatic light or wide spectrum light in theory does not matter. From a practical standpoint, most cheap meters are relatively accurate (5-10%) from 500 - 650nm, but tend to be quite inaccurate, i.e. 25% off below 450nm, and start to get inaccurate at the upper end 650nm as well but not as bad as the blue.

Are you prepared to measure over 100+ angles ? .... that is what the specs call for.

Semiman


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## Optical Inferno (May 18, 2011)

The SAE will definately have issues with this light. If you are planning on putting this light into a vehicle that is on the road, get ready for legality issues. Trust me when I say light output will not be your issue. 
As Harold B said, start with the SAE requirements for tail-lights and pay special attention to their angular requirements. After you think you have something that is street legal, send it to Calcoast ITL. for certification at the tune of about $1000-$2000.
In short your best bet would be to buy an entire assembly that has already been certified from a reputable dealer.


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## bshanahan14rulz (May 18, 2011)

Technically, what you are planning is illegal, and we would never endorse illegal activity.

Now, for a similar array for a more legal use, you also want to keep in mind that certain colors are more efficient than others, i.e. red-orange looks brighter than red LEDs. 
Also, you don't want TOO bright.
If you can't get the help there, 

Lastly, with regards to your ORIGINAL post, I used to be a member at HIDPlanet.com's forums; they were pretty into those types of car modifications. I'm not active anymore, but they had a pretty active LED section there. Might stop by there and see about your original question.


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