[argyllcms] Re: Absolute light meter calibration?

  • From: Richard Kirk <richard@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 11 Aug 2015 09:30:06 +0100

On 10 Aug 2015, at 22:55, Ben Goren <ben@xxxxxxxxxxxxxxxx> wrote:

Over the weekend I found a US Geological Survey researcher who supplies
reflectivity spectra of the Moon to satellite operators who can likely get to
me date-and-location-specific data that'll let me use the entire Moon as a
calibrated color target, which is exactly what I need for the project that
sparked the exercise.

That in turn gets us back to bolometers, and Lord Rosse's experiments to
measure the temperature of the moon.

I still might play around with Richard's suggestion of a bolometer, but
likely not for calibrating light meters.

The most useful property of bolometers is they measure the energy of any
radiation in absolute terms of energy. This is also a problem with bolometers,
as we will probably want to exclude all the non-visible wavelengths. We can use
a dichroic to reduce the light to just the visible wavelengths, and then use a
bolometer, but then we have compromised our absolute measurement by introducing
some spectral property that we have to measure with something else. The other
problem with bolometers is that they are so insensitive, so you have to make a
really fancy one using vacuum cryogenic techniques, or content yourself with
measuring the sun, and dropping the intensity by a thousand or so for your
reference instrument.

I have a Bentham standard light source (www.bentham.co.uk). This is extremely
stable - I have used it to do spectral densitometery on rolls of film test
patches and the variation is a fraction of a percent over several hours with
their power supply. However, the lamp looks a lot like a standard car light
bulb, only frosted. This suggests you might be able to make something similar.
You will still have to calibrate it to check your readings.

A better absolute source of light would be bremsstrahlung radiation.
Synchrotrons are usually giant buildings but it should be possible to make a
desktop standard light source. Needles to say, I didn't get the go-ahead to
build one.

I have fooled with chemoluminescence, and electroluminescence as standards, but
I did not get good results. I have toyed with having a high-efficiency light
source in a calorimeter, and using that to measure the energy in the emitted
light (a bit like a bolometer but inside-out). In the end what worked best was
to get one of the large Hamamatsu photodiodes
), and have the NPL characterize it. The result was pretty close to the
Hamamatsu spec, so this might get you within 5%. The sensitivity goes well
beyond the visible, so you may want to use a dichroic mirror if you are looking
at something like a CRT, but most displays should be cold enough to give a
sensible response.

Using a diode and looking up the response is not an absolute measurement, but
it is the best thing I could find on a budget. I am sure there is a better
solution out there.

Good luck.
Richard Kirk
FilmLight Ltd, Artists House, 14-15 Manette Street, London W1D 4AP
Tel: +44 (0)20 7292 0400 Fax: +44 (0)20 7292 0401

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