Spectral Sensitivity – Iodized plates

[Pobboravsky]

Iodized plates are sensitive to blue light and UV radiation. Lenses absorb UV so that little to none of the UV contributes to the overall exposure.

 

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[Pobboravsky]

The graph in Figure 3 comes from Study of Iodized Daguerreotype Plates. It shows the absorption coefficient of silver iodide versus wavelength. Wavelengths from 350 to about 400 nanometers (nm)correspond to UV radiation. Wavelengths from about 400 to 450nm are seen as blue light. Only wavelengths that are absorbed by silver iodide can produce a photochemical reaction — i.e., can contribute to exposure of an iodized plate.

Since lenses generally block most, if not all, UV radiation, only blue light can expose iodized plates.

The graph shows that the absorption curve goes to zero for wavelengths greater than 450nm meaning that the plate is not sensitive to green and red light. The UV absorption (read, sensitivity) rises from 400 to 350nm.

[CasedImage]

Thanks Irv, I wonder what speed a pinhole daguerreotype would be like. Has anyone ever tried them?

www.CasedImage.com

[Pobboravsky]

Re: pinhole experiment to see how much UV contributes to the overall exposure.

I made a pinhole camera with a f/22 “aperture”. (1-mm aperture at a distance of 22mm from the plate) Pinhole apertures are usually much smaller than 1mm but I was not interested in sharpness – only in the contribution of UV to the overall exposure. Sharpness was not important because the test target was a gray scale with large steps. f/22 gave an exposure time of 5 minutes. At that time, the light source consisted of two banks of blue fluorescent tubes designed for illuminating newborns to correct jaundice. The manufacturer’s literature said that UV was filtered out for safety sake. A conventional “white” fluorescent tube was included to make subjects easier to view; blue light is not visually efficient, looks dim and subjects appear weird.

Two plates were exposed, one with a Wratten 2a over the aperture to block UV and other without the filter. The plate exposed with UV + blue light was twice as “fast” as the one exposed with blue light only. UV contributed an extra stop to the exposure for the conditions of this experiment.

This is a very imperfect experiment because the result depends upon how much UV radiation was in the light source. If the light source consisted of 100-watt incandescent bulbs there would be very little UV given off. If I was exposing by New Zealand sunlight UV could make a huge contribution to the overall exposure.

Unlike conventional lenses a pinhole does not stop any UV and is very cheap. One might say it is nothing, really. If others would like to experiment with faster “lenses” one could buy single element quartz biconvex lenses from Edmund Scientific or use an aluminized mirror lens; aluminum reflects UV and is free from the problem of different wavelengths focusing at different focal planes (chromatic aberration).

Irv

[CasedImage]

I think Mark Osterman has shot with the Wolcott style camera made by Ray Morgeweck which uses a mirror (not sure what type), I’ll ask Mark what the exposures were like.

www.CasedImage.com

[Andy Stockton]

Irv – What does the “missing left half” of the spectrum graph you posted look like? Does the silver iodide still have a lot of sensitivity in the UVB range (320 nm–280 nm)?

[Pobboravsky]

Andy,

I don’t know. The graph shows that sensitivity is rising at 350nm and one might expect it would get to 320nm but extrapolation can be misleading. I’m curious, why is UVB wavelengths of interest?

Irv

[Andy Stockton]

Well at my stage of progress it is likely just another diversion from the real work I should be busy with.

I think people are interested in anything that might decrease exposure times. Your suggestion of a simple quartz lens that passes UV fans the flames. Am I wrong in thinking that a lens that allows UVA and UVB would decrease exposure times? Both are plentiful in good old sunlight.

Of course the cost of a sophisticated quartz lens douses those same flames to a large extent. Then again maybe we are on the wrong track altogether (ref Mike Robinson’s comment about poor skin tones).

[Pobboravsky]

Andy,

If reducing exposure time is a priority the best bet is adding bromine sensitizing and mercury development. Even with an expensive quartz lens I suspect (I don’t know) you would gain just 1 or 2 stops. But if you go the quartz route I would be very interested in the results. If you try a single element quartz lens the image might be fuzzy because the UV and blue light might focus at different focal planes.

Irv

[photolytic]

Irv,

Has anyone done similar experiments to measure the sensitivity of silver bromide or mixed silver iodide/bromide coatings on Daguerreotype plates? The reason I ask this is because of the findings of Skiadnikiowitz et al (Technical Univ. of Dresden) published in their article “The wet Collodion Process- A Scientific Approach” in the Oct 1998 issue of The Journal of Imaging Science and Technology. I believe you gave me this article Irv.

They reported that the spectral sensitivity range of wet collodion plates coated with an AgI: AgBr mixture was 383- 530nm (Max @440nm) whereas AgBr alone was 390-500nm (Max @ 450nm) and AgI alone was UV -437 nm (Max @ 420-425 nm) The net result of adding bromides was increased sensitivity in the green region of the spectrum but no measurable increase in photographic speed. Similar results were found by Eder in 1912 and by Schlutz-Sellack in 1871.

[Pobboravsky]

John,

Thank you. That is more information than I had — how soon we (I)forget — assuming that I ever knew. This underscores why cdags is such an invaluable website. Great job, John

I don’t know of anybody measuring the spectral sensitivity of daguerreotypes.

Irv

[photolytic]

Irv, Do you still have access to the equipment that you used to measure the color sensitivity of AgI D-types for your thesis project? Edmund optics used to sell a calibrated color target transparency that contained the full spectrum of visible colors, all approximately the same density. As I recall it was over $1000 so I never was tempted to buy one. Presumabley one could place this target on a Dag plate, expose to a broad spectrum white light and measure the variations in density porportional to the spectral sensitivity of the plate. Ive done this using the Kodak Q60 color test chart but the colors it contains are not pure enough for a critical test.

 

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[Pobboravsky]

Hi John,

The graph of absorption versus wavelength of AgI came from:

Washburn. E. E., ed, International Critical Tables, N.Y.:McGraw-Hill, 1929, vol. 5, p. 270.

and

Berry, C.R., “Structure and Optical Absorption of AgI microcrystals”, Physical Rev., Vol. 161, no. 3, 15 Sept. 1967, pp. 848-851.

See reference 4 in the bibliography of Study of Iodized Daguerreotype Plates.

All the very best,

Irv

[photolytic]

Hi Irv,

Your estimates of the spectral sensitivity of iodized Dag plates were based upon absorption coefficients for silver iodide published in the international critical tables?

You used a Beckmann DK-2a spectrophotometer to measure total D-plate spectral reflectance over a wavelength range of 380 to 700nm. Since reflectance maxima were around 425nm you concluded that maximum spectral sensitivity was also at 425nm. Can you use a spectrophotometer to make the same measurements on a D-plate with an AgI/AgBr coating?

Skiadnikiowitz et al measured their AgBr/AgI spectral sensitivity with a spectrosensitometer equipped with a grating spectrograph. According to their description, wet plates were placed behind the exit slit of the monochronometer, and a step wedge traveled synchronously with the wavelength adjustment of the monochronometer in steps of 10nm. To calibrate the output of the spectosensitometer, they measured the relative magnitude of the radiant exposure as a function of wavelength and varied exposure times with the wavelength to achieve constant exposure energy for each wavelength. Could this be done with D-plates?

All the best,

John

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