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Camera alternative to PAR

PBM3000

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Joined
19 Jul 2017
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Location
Hampshire
I note that the 'fashionable' way of measuring light levels is PAR. When I changed my Roma 240's T8s to a 59w LED I was concerned that I wouldn't be able to match the old light levels with the new (as I wanted a start point as close as possible to the original setup - then increase/decrease from there without 'upsetting' the plants).

But I didn't have a PAR meter. What to do?.......

I used my DSLR camera's in-built light meter.

I took a reading of the old tank's light levels at substrate which, in my case, was 1.3 seconds @f8 (ISO 100). When the LED's were subsequently installed it was just a simple case of reducing the output, in my case to 70%, to match the original light reading.

I'm guessing that I can't be the first to have thought of this method but thought I might pass on my 'find'.

I'm also curious about creating (or finding) a table that matches DSLR meter readings with PAR/LUX so non PAR meter owners can assess light levels. Given that camera light meters should in theory all be standardised (at '18% grey' and hence 'ISO') surely this methodology is viable?

Any thoughts?
 
Nice idea and at a first pass is fine. If plants look good under your new light then that is fine.

However you don't know the spectral response of your DSLR meter unlike a PAR meter that has a known and calibrated spectral response. Thus you can't really compare light levels with the DSLR light meter.

Oh and of course you are measuring reflected light, rather than direct. The reflected light will contain lots of green, if you have lots of plants.

Generally replacing T8 tubes with T5 LED's tubes gives a light (lumens) output higher than T8 but lower than a proper T5 HO tube.
 
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Fair point on the spectral response although could you get at least some indication from the RGB values?

Random image from web:
img_13.jpg
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Fair point on the spectral response although could you get at least some indication from the RGB values?
Yes, it's been done, but as Ian points out, the devil is in the details of the "some indication". The camera sensor has a specific response to real world but that response curve is known only to the sensor and camera manufacturers. Then there is another response curve that relates the sensor response to the actual image produced by the camera. I read a paper on this where NEC of Japan used a Nikon DSLR to do this, but of course they have access to all the data. They were doing this to find a cheaper way to control lighting in nurseries. According to the results, they were able to get to within 5% of a PAR meter..

In order to do the calibrations though, they had to use equipment costing 10x the value of a PAR meter.
It was an exercise for real eggheads and I thought by the end of the paper they would show the histogram from that specific camera model re-mapped with PAR on the Y axis, but no. I might actually contact the author for that, but he isn't going to donate the information...

Cheers,
 
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