Sun print exposure meter

[Salmonoid]

I have been toying with the idea of an exposure meter that would measure the light during a sun exposed contact print. It would buzz when the correct amount of sunlight has been reached. It would automatically adjust for clouds passing over head, or different seasons. I have the concept, and prototype in process at the moment. I know I want one for myself, but was wondering if anyone else would be interested in it.

[Loose Gravel]

This can be done with a MetroLux and probably other closed-loop timers. It would be good to put the appropriate spectral filter over the sensor, depending on whether you are printing silver or platinum, etc.

[gbock]

I'd be interested. Please keep me posted.

Gerhard

[Salmonoid]

Based on the poll and the difficulty of designing this device, at this time I will not go forward with this project. Thanks to all who replied, this information was very helpful to me.

[htmlguru4242]

This is really a good idea; why not continue with it? It's probably not THAT entirely hard in the grand scheme of things ...

If you could build in proper light amounts for the exposure of different materials, somehow account for the overall density of the negative; then it's just a cumulative timer / counter of the amount of light reaching the meter; I'd imagine that straight measurement of the light reaching the meter would be enough to account for seasons, clouds and the like ... Just a suggestion.

[Ole]

I didn't see this poll until right this minute. Of course I'm interested!

[mark]

WOuldn't this meter have to measure the amount of UV as opposed to amount of light?

[David A. Goldfarb]

I haven't tried this with my Metrolux yet, but it is supposed to work. I'm not sure, however, if the Metrolux has enough range. My ideal negs for albumen require an exposure in the shade of about an hour, maybe 20 minutes in direct sun.

[juan]

I, too, just saw this thread. I'm interested, too.
juan

[Loose Gravel]

MetroLux exposures can be made very long by changing the counting mode to 10x. This allows for exposures of 9999 seconds or 167 minutes. This can be extended by 2x by using percent at 200%. This can be extended further (usually) by adjusting the calibration number on the photo sensors or by applying ND to the photo sensor. This gets you over 5 hours without any difficulty.

The MetroLux photo diode (and probably all diodes for this application) has a minimum response of 320 nm and a max of 730 nm. This max is way outside the response of any photo paper, silver or plat or other. However, it may be workable without filtration if the light (visible) is directly correlated to the UV. It probably is over a short period, but not day to day, as suspended particles in the atmosphere would greatly affect the ratio of visible to UV.

Adding a blue filter would help this correlation for two reasons. First and most is that the silicon photodiode is exceptionally responsive to red light, which you don't want. Second, by using blue, it eliminates the red leaving the blue and/or UV which are more closely correlated and have similar response.

Should the filter need to pass only UV, then the problem is more difficult for me. No doubt some combination of Rosco gels could get you a strong UV pass, but these are not spec'd much below 340 nm. I don't know how short of wavelength these 'other' processes 'see'. However, glass blocks the very short UV unless it is special or coated.

Should shorter wavelengths be required by the photo sensor, photo diodes that are enhanced are available to see to 190 nm. These are a little more expensive, but not prohibitive.

My guess is that nothing very fancy is required. In my dealings with alt process folks, standard silicon diodes have always been sufficiently repsonsive. The good news, with the sun, you have plenty of energy, plenty of photons.