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led luminosity meter

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MrDEB

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Dealing with different leds and comparing luminosity and flux etc and wondering just how much current has an effect on leds.
Wondering if anyone has built an led brightness meter?
Thinking of using a pic connected to two or three separate LDR's enclosed inside a 12 inch long or longer tube to shield the led output from ambient light.
The output of the three LDR's are to get an average of the leds brightness. Use an LCD display and maybe divide the ADC output into 1024 samples.
obtain a baseline then adjust the current as per data sheet and compare using PWM vers constant on.
This idea is something I have been pondering off and on lately as I am redesigning my bike turn signal device, re-configuring the led driver circuitry as well as the leds to use. Original design utilized bi colored leds but needed a brighter matrix so am going to make my own led matrix but only using red leds. Project would be too expensive using bi colored leds.
 

OBW0549

Active Member
Dealing with different leds and comparing luminosity and flux etc and wondering just how much current has an effect on leds.
No need to wonder; googling the phrase, "led brightness vs current" yields a wealth of information. Bottom line: LED luminosity varies approximately linearly with current.

Be aware that the sensitivity of LDRs varies greatly across the visible spectrum, and the response to red light will be much less than the response to green, which is where CdS cells reach their peak sensitivity.
 

MrDEB

Well-Known Member
am wondering if adding the op amp filter would help equalize the difference between red and green etc.
I did run across a circuit where two op amps are used (LF 357 or LF 411)
 

OBW0549

Active Member
am wondering if adding the op amp filter would help equalize the difference between red and green etc.
I did run across a circuit where two op amps are used (LF 357 or LF 411)
Unfortunately, nothing you can do electronically is going to alter the fact that the LDR responds differently to different wavelengths of light.
 

ronsimpson

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When testing LEDs; the dispersion angle makes a great difference in the readings. You can get 5 degree or 180 degree LEDs. One seems much brighter. They have the same piece of silicon inside.

I made a number of these things. It collect all the light from any angle and measures the total light.

 

KeepItSimpleStupid

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The eye responds differently to different wavelengths too. Pre-select your LEDs by color and then by forward voltage. You should be good to go.
 

Dick Cappels

Active Member
Meters like that can be calibrated against another instrument, but because of the variation in response of people's eyes across the visible spectrum and the response to CDS cells is nothing like people's eyes the calibration is only good for the type of light source for which it was calibrated.
upload_2017-2-11_21-44-50.png<= Cadmium based sensor sensitivy curve
Fortunately there are some sensors that have spectral filtering that results in a pretty close response to the human luminance sensitivity curve so it would be much better for measuring the lumiinance (or illuminance) of various types of light sources.
upload_2017-2-11_21-41-49.png <=== CIE human sensitivity curve
upload_2017-2-11_21-40-48.pngBH1680FVC sensitivity curve


Here is one such sensor at about US$0.24 each. http://www.es.co.th/detail.asp?Prod=017609995 A datasheet can be downloaded from that page.
 

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KeepItSimpleStupid

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Light sources like "arc lamp sources" when they age, they put less out in the blue. We needed an AM 1.5 global (sunlight) source and we calibrated based on the spectral response of our house made detectors. When we no longer had enough "blue", it was time for a lamp change. Essentially a combination of lamp hours, change in lamp power, calibrated intensity (normal) and calibrated intensity (somewhat blue). We had some cells calibrated at a government lab and then we calibrated many (about 10) of our own. Calibration always consisted of the last source used + one calibration cell, lamp power for 100 mW/sqcm.
Since our detectors were "naked" with no filters, damage was possible. The recycling kept everything in check. If it didn't work, you had to have three calibration standards that agreed. We only had about 4 of the "blueish" sensitive ones. We had systems that could measure the spectral response of the sensor, but we did not have something to measure the spectral response of the lamp output. The detector can measure the "integrated" response of it's sensitivity.

So, if we had a home made device that had more of blue response, then the other wavelengths could be mechanically filtered out.

There were filters that shaped the arc lamp output to AM1.5G. One of those filters were about 1mm thick and 3-4" in diameter. Close to a hot bulb and a pretty clunky electric shutter that I had to calm down. That lens was about $1000.00 USD.
 
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