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LED's more efficient than CFL's?

Discussion in 'Lighting' started by Roland Paterson-Jones, Sep 21, 2003.

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  1. I posed this question some time back$ou7$#link1

    However, I have finally taken the trouble to work the numbers myself for
    Nichia model NSPL510S, which is nominally rated at 2500 mcd (typical) and
    2*phi-half of 50 degrees.

    The conservative calculation (see below) gives 85 lumens/watt, which
    surprised me hugely. Typical incandescent is apparently 17 lumens/W by
    comparison, and CFL's apparently about 4 times that, i.e. about 70 lumens/W.

    Can someone please check my logic and workings.

    Data is from
    You will need to register to download the file, but it is free.

    I used the directivity graph to plot the minimum (relative) intensity
    according to half-angle for various discreet angles
    (5,10,15,20,25,30,40,45,60,75). I ignored angles beyond 75 degrees, since
    the intensity dropped below 2%.

    Then, I used the formula, 2*PI()*(1-COS(PHI/180*PI())) in Excel to calculate
    the steradians within a beam of half-angle PHI. Note that 'PHI/180*PI()'
    simply converts from degrees to radians.

    The steradians covered by each band (e.g. between 5 and 10 degrees) is then
    simply the difference between the 5 and 10 degree total steradians.

    Then I used the formula lumens = candelas * steradians to calculate the
    total lumens output in each band. This is a conservative figure, since I use
    the minimum intensity for each band.

    Then the total lumens output of the LED is the sum of the lumens over all
    bands. Here are the precise workings in CSV (comma-separated) - import it
    into your favourite spreadsheet for a better view:

    ,Luminosity (mcd),2500,10,95%,,0.10,0.07,0.61,0.82
    ,Current (mA),20,15,98%,,0.21,0.12,1.05,1.86
    ,Voltage (V),3.6,20,80%,,0.38,0.16,1.19,3.05
    ,Power (W),0.072,30,25%,,0.84,0.25,0.57,4.56

    Note the total lumens is 6.14, and for a typical power rating of (20mA *
    3.6V = ) 0.072W, this represents a little over 85 lumens/W.

    The last column is the cumulative lumens output from the centre of the beam
    out. Note that a little less than 2/3 of the light is generated within
    phi-half (3.99 lumens in 25 degrees).

    Roland and Lisa Paterson-Jones
    Forest Lodge, Stirrup Lane, Hout Bay
    mobile: +27 72 386 8045
  2. Nick Birrer

    Nick Birrer Guest

    I imported your data into Excel and verified the sr/band, but
    the values I got for lumens/band were considerably less.
    Here's your csv with a few columns added on:

    Voltage (V),3.6,20,0.8,,0.38,0.16,1.19,3.05,,0.165,0.330,0.847


    What are we doing different to calculate lumens? I'm using

    2.5*(sr/band)*(pct intensity).

    I get a total of 1.7 lumens and 23.7 lumens/watt

  3. Nick Birrer

    Nick Birrer Guest

    That explains the 3.6x discrepancy!
    I looked at your %intensity estimates and I get about the same
    looking at Wilycon's chart. Although the intensity is a lot
    lower at high angles, the solid angle subtended by the
    corresponding 'band' is a lot larger which would explain the
    suspicious lumens. I'm curious about the 30ma you used for the
    current - 20ma would yield an even more inflated lumen value.

    A couple of notes on the lumen estimate-

    In order for the estimate to be a valid lower bound
    (conservative), the radiation pattern has to be axially
    symmetric. I've looked at some LEDs with a magnifier and noted
    that the emission region is square which suggests lack of
    axial symmetry. I'm not an LED expert, but I assume this also
    applies to the LEDs in question.

    The other necessity for this estimate to be a valid lower
    bound is that the intensity monotonically decrease with
    increasing off-axis angle. This appears to be the case from
    the intensity vs. angle slice shown in the .pdf, but again,
    not being an expert here I'll leave it for others to verify
    whether this is true or not.

    I didn't check the Nichia specs - didn't wanna play the silly
    register game. However, the numbers seem more in line with
    what I would expect given what I've read about LED luminous


    P.S. It looks like my news software wrapped some of the lines
    in the .csv I previously posted. Apologies for the hassle if
    anyone tried to import this.
  4. Nick Birrer

    Nick Birrer Guest

    Doh! Let's try this bit again-
    How about lumen/watt instead...
  5. 23.7 lm/W is a very respectable number for white LEDs.
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