Exposure time vs. properties of glass

[Dan Pelland]

Another important point came to my attention regarding contact printing frames. That is thickness and type of glass. How does exposure time compare for these variables, other factors being the same? Does anyone have (or know of) data?

 

[Dave Parker]

In real world applications, the glass contained in the contact print frames will not affect the exposure times of the contact print, most glass will allow light to pass through with virtually no effect, very few types of glass actually absorb light, unless it has been specifically designed to absorb certain parts of the light seen or unseen light spectram.

One thing that will affect contacts is how dirty the glass is, and I am not talking about observed dirt, but the un-observed items that can be present on the surface of the glass, such as if the contact printer is located in an enviorment with smokers, this over a very short time can leave an virtually unnoticed film on the glass, that will change the properties of the light for exposure.

Dave Parker
Satin Snow Ground Glass

 

[Kirk Keyes]

Does anyone have (or know of) data?

Dan, check out this link: Dead Link Removed

Look for the magenta line that is labeled "BoroFloat". That's the most common type of glass that you will find these days. (The closer to "1.0" on the transmittance scale, the more transmittance there is.) You can see that the glass has a little absorption in the visible wavelengths (400-700 nm) down to about 350-360 nm in the Ultraviolet and then it starts to absorb more until it absorbs almost all the light below 310 nm.

So for all practical purposes, float glass is pretty transparent, and in the photographically useable ranges, it typically only absorbs a few percent of the light that falls on it. Looks to be about 3 percent according to this graph. Remember that a piece of glass that is twice as thick as another piece, will absorb twice as much light. But since we are dealing with percent levels of absorbtion, you should not see much of a difference in printing times.

So figure out the difference in thicknesses of the glass you are interested in, and you can then figure out how much more exposure will be needed.

Kirk - www.keyesphoto.com

 

[Kirk Keyes]

PPG Starfire:

According to this page, http://www.ppg.com/gls_commercial/documents/PPG Glass Product Data - monolithic.pdf

in the 6mm thickness, Starfire transmits 91% visible and 85% UV, whereas the clear glass (possibly float glass) is listed as 89% and 66% respectively. If you are doing a process where you need UV transmission, the Starfire may be worth it.

 

[sanking]

PPG Starfire:

According to this page, http://www.ppg.com/gls_commercial/documents/PPG Glass Product Data - monolithic.pdf

in the 6mm thickness, Starfire transmits 91% visible and 85% UV, whereas the clear glass (possibly float glass) is listed as 89% and 66% respectively. If you are doing a process where you need UV transmission, the Starfire may be worth it.

I did a chapter (or Appendix) on UV light sources for the 2nd edition of Dick Arent's book on platinum and palladium printing. As part of the research for the chapter I tested the UV transmission of a number of glasses, including ordinary crown or plate glass and Starfie. Ordinary crown glass in 1/4" thick size absorbs about 30% of the UV light over about 380nm. In the same thickness Starfire transmits a maximum of about 10% more UV light in this wavelength than ordinary float glass. In practical printing tests of pt./pd. this difference translated into a slight speed gain for the Starfire of about 1/3 of a stop.

Is the gain in printing speed worth the expense? Your decision. I think it might be if I were building a contact printing frame from scratch. However, in my own case I have considered the issue and determined that the slight speed gain is not worth the trouble in time and expense to replace the float glass in my existing frames.

Sandy

 

[Kirk Keyes]

As part of the research for the chapter I tested the UV transmission of a number of glasses, including ordinary crown or plate glass and Starfie.

Sandy, did you do this empirically using photo (alt or otherwise) papers?

 

[sanking]

Sandy, did you do this empirically using photo (alt or otherwise) papers?

Yes, empirically using several different mixes of pt./pd. ranging from 50/50 palladium/platinum, to 75/25 palladium/platinum to 100% palladium. The printing tests were done with 3/8" thick glass.

I also measured transmittance of the two types of glass using the densitometer in UV mode, either to check the empirical results or vice-versa.

Sandy

 

[Dan Pelland]

Thanks for the info everyone. I looked for Sandy's contribution in my old P&P Printing book. For one thing, I found I'll have to get the 2nd edition. As for the frames, I'll stick with regular float glass.