"Mystery" antique coloured filters perhaps for microscopy?

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Iainp
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"Mystery" antique coloured filters perhaps for microscopy?

Post by Iainp »

Hi all. I bought a box of glass filters at an antique fair yesterday in Warwickshire, UK. The seller thought there were perhaps something to do with an optician, but I was fairly sure they were used for photographic purposes. He thought they were late 1800s (late Victorian Era) or early 1900s (Edwardian Era) but that's pure guesswork. They are certainly very old anyway.

There are 13 in the box, with slots for 3 more, presumably missing.

They are each made of two pieces of plain thick glass, 1.5" square, with a coloured sheet of gelatin or similar material sandwiched between.

Some have a number etched into the top right corner, and some have an additional L etched into the top left corner, and some have no markings at all. The colours range from deep cobalt blue, to a deep maroon, with rich oranges and greens along the way, but no yellow.

A puzzling feature of them is that they are all rather dark, with an exposure compensation required of around 25X. This seems a suspiciously high filter factor for Victorian / Edwardian photography.

I posted the below link to a photography forum, asking if anyone could ID them, and got lots of interesting suggestions but nothing definitive. Someone suggested that they are antique lantern slides, perhaps used as "mood" light colouring for stage shows, but given their small size and the fact they they are so dark, I'm not convinced. Another suggestion was that they might be " the light filters used by quacks for color light therapy. The best known of these quack devices was the Dinshah Spectro-Chrome" but again I couldn't see anything online quite like these.

Another contributor pointed out that they couldn't be Victorian colour taking-filters since spectrally sensitized emulsion was not invented, or at least used, until much later: "A red filter in this context only made sense the moment sensitisation for red was employed."

Eventually someone suggested they might be early microscope filters, which brings me here...

There are no other markings on the box or filters, apart from a small hand painted dot of red paint on one edge of each filter. If anyone has any ideas about what they are or how they would have been used, there are quite a few people who would like to know!
The images are here on my blog.
Thanks, Iain
https://fouragesofsand.blogspot.com/202 ... lters.html

PeteM
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by PeteM »

I doubt these were microscope filters. Sounds like you've already considered photo filters. That still seems somewhat likely -- perhaps from some early equivalent to Spiratone?? Early photographers were accustomed to long exposures.

Were they solid glass, rather than gelatin sandwiches, they might have been glass samples intended for use in making or repairing stained glass windows? They still might be samplesof some sort, if there's no sign of use.

It might help you to narrow things down if you can get a guesstimate on the age of the case. The pebbled black outer wrapping with a velvet interior look like precision instrument cases (micrometers etc.) from the late 1800's to early 1900's to me. If that outer wrapping is leather, more so. If an imitation (plastic) leather - newer. It's a bit of a surprise that someone went to the trouble of making an intricate padded case, but then didn't mark their brand in any way. Perhaps something missing from the inside cover or ??

Spectoscopy and colorimetry might have wanted comparison filters. Be interesting to pass a broad spectrum light through them and (if a friend has a spectrometer) see what passes through. On the odd chance they narrowly pass some range, could be they were part of a scientific instrument?

There were also lantern projectors using slides of about 2" by 2" and up in the US -- perhaps some of about 1.5 x 1.5"?? These would have had bright lamps (carbon arc, tungsten) and some were used to project backgrounds for theatrical stages. Seems a reach, though, that these would have been used for that. Should any of them have a faded burned-out spot toward the middle, that might give credence to their use in front of something like a carbon arc lamp.

FWIW, theatrical spotlights from 1960 on (likely earlier, my experience only goes back so far) used plastic (acetate etc. based) gels cut to much larger sizes and sandwiched (without glass) in metal frames.

Iainp
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by Iainp »

Thank you for the detailed ideas there Pete!
I had another response on the photography forum which ties in nicely with what you say, and the person has kindly offered to measure the spectra: "the spectrometer looks at each wavelength in turn to at least 5nm resolution (60 data points across the visual portion, but with UV & NIR available too)."
He wonders if the numbers corelate to the Kodak Wratten filters, and if so, date the filters to pre-1912, and they may have been used for printing, scientific purposes or possibly - he suggests - microscopy. The debate continues...

Examining the case more closely, it's neither plastic or leather, rather it's a kind of coated fine fabric, which I've seen on early 1900s cameras.
Just for interest, here are the scans of the filters:
https://fouragesofsand.blogspot.com/202 ... lters.html

chris_ma
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by chris_ma »

could also be color separation filters for early 3 color processes..

for those who don't know Sergey Prokudin-Gorsky, I highly encourage to check out his photos (and process)
https://en.wikipedia.org/wiki/Sergey_Prokudin-Gorsky
chris

PeteM
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by PeteM »

The most recently posted photos have an R-G-B look to them, with the sole exception of the one yellowish one. Might lead credence to the idea they were used in some color photo or printing process.

Duke
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by Duke »

Just from the size they look like early colorimetric filters for use in photometers (ie original "kolorimeter" or later SpeKol by Carl Zeiss Jena) for chemical analysis.
“Thoroughly conscious ignorance is the prelude to every real advance in science.” - JCM

Iainp
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by Iainp »

Thanks all. I'm sending the filters off for spectral analysis this week, and will post results here!
Iain

René
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by René »

Got a box with filters pretty much like yours, marked Wratten M filters, Kodak etc, Kingsway, London.
From the closure of a photographic division within an english horticultural centre. No markings on the filters, but with some written details in the box.
The centre was setup in the early 1900's.

Hope that helps,
René
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Iainp
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by Iainp »

René wrote:
Mon Nov 15, 2021 10:23 am
Got a box with filters pretty much like yours, marked Wratten M filters, Kodak etc, Kingsway, London.
From the closure of a photographic division within an english horticultural centre. No markings on the filters, but with some written details in the box.
The centre was setup in the early 1900's.

Hope that helps,
René
They really do look very similar, yes! Thanks René, very interesting. And there's still the puzzle of what red filters would have been used for in early 1900s.

I'm hoping to have some news about spectral analysis of these soon...

Iain

Chris S.
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Re: "Mystery" antique coloured filters perhaps for microscopy?

Post by Chris S. »

Guessing, here, but I suspect these are indeed contrast filters for black and white photography. (I think this is what Iain refers to as "colour taking-filters," but for others who may be unfamiliar with them, see: Using Coloured Filters in Black and White Photography.
Another contributor pointed out that they couldn't be Victorian colour taking-filters since spectrally sensitized emulsion was not invented, or at least used, until much later: "A red filter in this context only made sense the moment sensitisation for red was employed."
Agreed. If used as color contrast filters, they would not have been useful with the early orthochromatic emulsions. From a bit of reading, I see that panchromatic emulsions arrived about 1906, and improved over the next decade or so. Could these filters date from this period or a bit later?

For those who haven’t done studio photography with panchromatic B&W film (most commonly-used films were panchromatic), here’s an example of why such filters were often required: You need a product shot of a book that has a red title on a green background. When rendered into B&W on the negative, both red title and green background turn into the same shade of gray, or shades so close that the title is hard to see.

So you reach for a red filter and re-shoot. This lightens the red title and darkens the green background. Or you try a green filter, which darkens the red title and lightens the green background. In either case, you’ve selectively increased contrast to set apart features of importance. If you like one effect but find it too strong, other filter colors provide in-between effects. To guide your filter choice, consult a complementary color wheel; color filters transmit light that close to their position on the color wheel, and restrict light opposite their position on the wheel; colors further from these ideal points on the color wheel produce in-between colors produce intermediate effects.
René wrote:
Mon Nov 15, 2021 10:23 am
Got a box with filters pretty much like yours. . . . From the closure of a photographic division within an English horticultural centre.
For botanical photography in B&W, I could see real need for a decent-sized set of color contrast filters to depict small features, particularly in flowers. Here one might face difficult choices in trying to make petals, sepals, pistils and stamens appear distinctly. These might occur in several colors that ideally would each be photographed with a particular filter; in practice, one would probably try several filters, and go with the one that ends up showing the best compromise.

Considering the chart in René’s filter box, the code and color lines for some filters match current or recent use. For example, K2 as a yellow filter, and F as a dark red. A couple of sources: Cross Reference - Filters for Black-and-White Photography and Modern Photography’s 1958 B&W Filter Guide.

Also in that chart, the column marked "factor" likely "filter factor"—the multiplicative amount of light the filter blocks. These filter factors seem weirdly high, which lines up with Iain’s observation:
A puzzling feature of them is that they are all rather dark, with an exposure compensation required of around 25X. This seems a suspiciously high filter factor for Victorian / Edwardian photography.
Agreed—and also suspiciously high for the 1980’s emulsions I shot with. I have thoughts, here—neither of which feels fully satisfactory. One is that maybe the emulsions weren’t completely panchromatic, as we understand the term; were there emulsions in use for a while that captured the visible spectrum, but required strong filtration to do so? Another thought is that maybe the available light source wasn’t balanced to the film, and required filters to correct it? (René’s chart shows two different factors for some filters, suggesting use or either two emulsions or two kinds of light.)

Chris_ma, thanks for reminding us about Sergey Prokudin-Gorsky—his work is just wonderful. As far as early color separation work, the processes I can think of were three-color, as you said. So to my thinking, a set of eight filters (René’s set) or 13-16 filters (Iain’s set) suggests quite a different use?

Fun mystery!

--Chris S.

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