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Judging white LED brightness.

Discussion in 'LEDs and Optoelectronics' started by Herschel Peeler, Apr 14, 2017.

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  1. Herschel Peeler

    Herschel Peeler

    401
    66
    Feb 21, 2016
    Is there a better way to judge LED brightness? I have a black plastic tube about 9 inches long and 8 ,, in diameter (internal) with a CdS cell at one end. I put the other end over the LED and measure the resistance of the CdS cell.
    My most powerful LED is said to be 220,000 mcd a 100 ,mA. That gives me 600 Ohms. Dark gives me about 15K ohms. A 18,000 mcd white LED gives me about 1,500 ohms. I use these as my references.
    I buy a lot of LEDs at bargain prices or by weight in bulk and try to judge brightness according to my black tube.
    Is there any other way to judge brightness of white LEDs?
     
  2. OBW0549

    OBW0549

    159
    118
    Jul 5, 2016
    A silicon PIN photodiode will give you an output which is highly linear and directly proportional to brightness, unlike a CdS cell. I've used an Osram SFH206K for that purpose (grading LEDs). Use it either with a current sensing resistor or with an opamp arranged as a transconductance amplifier.
     
    (*steve*) likes this.
  3. Herschel Peeler

    Herschel Peeler

    401
    66
    Feb 21, 2016
    Super! Shopping.
     
  4. OBW0549

    OBW0549

    159
    118
    Jul 5, 2016
    Minor nit: in my post above, I should have said "transimpedance amplifier," not "transconductance amplifier."

    The schematic below shows how I used the SFH206K in a setup to measure LED turn on/turn off time. The detector portion of the circuit uses a high-speed (100 MHz) LM6171 opamp because in this case I was interested in the LED's response time, but for light intensity measurements a FET input opamp (e.g., TL072) might be more appropriate, along with increasing R1 (in the detector circuit) to get more sensitivity.

    LED Drvr-Rcvr.png
     
  5. Herschel Peeler

    Herschel Peeler

    401
    66
    Feb 21, 2016
    Speed is not so much a concern for me, just relative brightness. I found the SFH206K at DigiKey for a buck and small change. Next time I place an order to them I'll get one. I didn't have anything similar in stock.
    No need for the 100 MHz op amp. An LM358 or such is probably what I'll use. A similar circuit was planned.

    Thanks again.
     
    Last edited: Apr 14, 2017
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