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Photo Forum / Film Photography / Darkroom / August 2005Blue channel wavelength??
I'm thinking about the possibility of building a simple densitometer along the lines of that outlined in Phil Davis' _Beyond the Zone System_. I'd like to use some sort of blue LED as the light source so I can look at negatives processed with PMK. Does anyone know the approximate wavelength of the blue channel on something like the X-Rite 361? Perhaps there's an LED close by (in wavelength). Being an electrical engineer by day, I know how to build a highly stable LED-base light source, I just need the wavelength. Thx. steve > I'm thinking about the possibility of building a simple densitometer > along the lines of that outlined in Phil Davis' _Beyond the Zone [quoted text clipped - 6 lines] > how to build a highly stable LED-base light source, I just need the > wavelength. Hi, I dont know much about it I have to admit, but it sounds interesting... Do you have to measure the desity at a particular wavelength? Couldn't you pick any wavelength, or check which wavelengths were best? Scott. for BW photography blue channel wavelength is not critical. Absorption curve for AgX is quite broad. Any blue LED will do, OTOH why not green LED mimicing spectral responce of a human eye (give or take)? *Stephan Goldstein wrote: *> I'm thinking about the possibility of building a simple densitometer *> along the lines of that outlined in Phil Davis' _Beyond the Zone *> System_. I'd like to use some sort of blue LED as the light source *> so I can look at negatives processed with PMK. *> *> Does anyone know the approximate wavelength of the blue channel *> on something like the X-Rite 361? Perhaps there's an LED close *> by (in wavelength). Being an electrical engineer by day, I know *> how to build a highly stable LED-base light source, I just need the *> wavelength. *> * *Hi, * *I dont know much about it I have to admit, but it sounds interesting... *Do you have to measure the desity at a particular wavelength? Couldn't *you pick any wavelength, or check which wavelengths were best? * *Scott. > I'm thinking about the possibility of building a simple densitometer > along the lines of that outlined in Phil Davis' _Beyond the Zone [quoted text clipped - 6 lines] > how to build a highly stable LED-base light source, I just need the > wavelength. I would guess you could determine that from Kodak's Wrattan Filter book. These are called Status Filters. For blue, you might wish to look at their #94 filter, with #93 for green and #92 for red. This is the passband for the filter. It peaks at about 450nM, at about 8.5% transmittance. For the light source, I think they assume a 3200K incandescent source. If you do use an incandescent source, you should use a #301 filter too, but I infer you will not be using such a source. While the #94 filter is considered "narrow band", It is pretty wide compared to a monochromatic source. I do not know about LEDs, but I imagine they would not duplicate the curve of the #94 and #301 when viewing a 3200K incandescent source. If you do not match this latter, your readings will not correspond to those of other workers. Whether this is important to you, only you can decide.  Signature.~. Jean-David Beyer Registered Linux User 85642. /V\ PGP-Key: 9A2FC99A Registered Machine 241939. /( )\ Shrewsbury, New Jersey http://counter.li.org ^^-^^ 10:35:00 up 48 days, 4:28, 3 users, load average: 4.11, 4.13, 4.09 > I'm thinking about the possibility of building a simple densitometer > along the lines of that outlined in Phil Davis' _Beyond the Zone [quoted text clipped - 6 lines] > how to build a highly stable LED-base light source, I just need the > wavelength. Steve, The wavelength(s) will depend upon your paper selection. I've included links to the spectral sensitivity data for a few Ilford products. Note that their single grade papers are primarily exposed by blue light, while multigrade papers are affected by blue and green light for exposure and contrast control. Single grade papers by other manufacturers may have their emulsion sensitized to blue and green to provide increased speed without an effect on contrast. Using an LED with an output centered on 425 nm will work well for Galerie. You might consider blue and green channels for other papers. Also, the density of the image will affect local contrast. A pyro negative exposed to OD 1.0 may be used to test local contrast reduction. The yellow filtration usually results in a reduction of one paper grade with MG papers. Although it is a bit more complex, a two channel approach with inclusion of factors for different MG filters (spectra) may be solved with a fairly simple model. Ilford Galerie Sensitivity: 350-500 nm; peak 425 nm <http://www.ilford.com/html/us_english/pdf/galerie_fb.pdf> Ilford Ilfospeed RC Deluxe Sensitivity: 350-500 nm; peak 425 nm <http://www.ilford.com/html/us_english/pdf/ilfospeed.pdf> Ilford MG FB Warmtone Sensitivity: 375-550 nm; peaks 425 & 525 nm <http://www.ilford.com/html/us_english/pdf/warm_tone.pdf> Ilford MG IV FB Sensitivity: 375-550 nm; peaks 425 & 525 nm <http://www.ilford.com/html/us_english/pdf/mgivfb.pdf> |
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