Need help determining units of light for old light meter

Discussion in 'Casual Photo Conversations' started by stemked, Apr 10, 2012.

  1. stemked

    stemked Moderator

    Hi Folks.
    I am teaching a lab on photosynthesis and need to replace a piece of equipment that measures light, hopefully with my old trusty Weston Master IV light meter. I know how to use the instrument but what I need are the actual units I will receive when I use the meter. Obviously I can set ASA which informs me at what Shutter speed to use at a given f stop. But how can I convert this into useable units for the experiment? Any reasonable straight-forward equation and what units should I report?
    Sorry, I know I can look this up, but I'm trouble shooting right now.
    Many thanks.
  2. Some of the old meters measured directly in "footcandles". These would be the ideal unit of standard measure I would think? There is an easy formula to convert EV to foot candles. an EV of 10 equals 240 foot candles.

    Footcandles = 2^EV / ISO * 0.032
  3. It's been a long time since I --- however:
    I believe the Weston outputs EV setting (exposure value).
    In an old notebook, I wrote:
    EV x 2.5 = Lumens
    Lumens รท 10.764 = Ft. Candles
  4. Both previous answers had it almost. For flat sensor illuminance meter:

    2^EV * 2.5 = Lux (at ISO100)

    Lux = FC x 10.76
    For example:

    10 EV = 2^10 * 2.5 = 1024*2.5 = 2560 Lux
    2560 Lux = 2560/10.76 = 238 FC
    But it depends on light meter, as you can read from here:
  5. EV are logarithmic units - foot candles and lumens are linear units, so the conversion isn't quite that simple.
    Google is your friend. The first hit on the search, {conversion ev to footcandles}, is the very useful chart by Sekonic:
    Many of the other hits from that search may also be useful to the OP.
    Tom M
    PS - I didn't see the very nice response from the previous poster as I was editing my own response. Good info there.
  6. There is an easy formula to convert EV to foot candles. an EV of 10 equals 240 foot candles.​
    LV, not EV. EV is Exposure Value which is only a function of shutter speed and aperture. LV or Light Value is dependent on actual light level.
    At ISO 100 the two have the same numbers.
    A small pedantic point, but one worth mentioning.
  7. stemked

    stemked Moderator

    Wow! Thanks Chang!
  8. When I was visiting a research lab some years ago, one of the techs there produced a "standard foot candle". It was a wax candle in the shape of a foot.
  9. This is a late and rather different reply. The fundamental problem is that the spectral response of photosynthesis ( from 400- 700 nm but with peaks in most species in blue and red where chlorophyll absorbs most strongly) is quite different from that of the selenium cell in the Weston (one big peak in the green, with tails out inot blue and red). So you can calibrate the weston for photosynthesis measurements for a single light source, say daylight, but it will not hold the calibration if you then change to a different light source. One approach would be to simply look up the light intensity in photosynthesis-friendly units (photons per square metre per second for example, from 400-700 nm) at midday for your latitude and longtitude and time of year, read the incident light with the Weston, and so calibrate it, then using the EV scale just adjust for different light intensity readings. I did this once years ago, cross-checking with a 'proper' light meter, but have since parted with both the Weston and the calibration.

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