Electroluminescent sheets can be any size, but the ones that David is talking about are about A4 in size and look and feel like a laminated manila folder. Being "electro-" and "luminescent" you won't be surprised to discover that you plug them into a power source and they glow.
[The power-brick (top) and Inverter (bottom) for a CeeLite™ Electroluminescent panel]
Flat, flexible, glowing A4 sheets are very useful in bibliography because they are moderately portable and you can use them to back-light a large page without moving the book it is in from the bookrest you are consulting it on (i.e., holding it up to a window or a light). Also, being a cool, large and uniform light source, unlike a incandescent bulb or a fluorescent tube, it is very easy to use them to illuminate (and photograph) large paper features such as watermarks.
I don't have a lot to add to Mark's post in terms of function and use of these sheets, but the supplier of these electroluminescent sheets, power supplies etc in Australasia is different from those he mentions, and so I suspect that my experience getting hold of the electroluminescent sheets will be of interest to Australasian bibliographers.
[Inverter and CeeLite™ Electroluminescent panel (back)]
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Shef Rogers visited Monash University in September 2008 to give a paper on Pope's Miscellaneous Poems and Translations (1712/1714). (The visit was organised by the Centre for the Book as a part of the Melbourne Bibliographical Circle series; details here.) He bought with him, and demonstrated the use of, an electroluminescent sheet that he had bought in New Zealand from the local representative of Flexalite.
Flexalite is—as their website will tell you—"the first and only authorised dealer of CeeLite™ LEC panels and Flatline Inverters in Australia." Unfortunately, CeeLite may be an award-winning product, but Flexalite is unlikely to ever become an award-winning retailer. Shef had some difficulty obtaining his CeeLite panel and Inverter in New Zealand, I had a lot of trouble obtaining my CeeLite panel and Inverter in Melbourne. I don't know of anyone else who has managed to get one in Australasia.
[Inverter and CeeLite™ Electroluminescent panel (front)]
The Melbourne distributor can only be contacted by mobile phone and was very "difficult" to reach by email (often not answering emails at all, or "answering" them at cross-purposes) and was only capable of sending orders by fax. I was unable to get the distributor to email me a jpeg or pdf purchase order or to post a hard copy. (It didn't help that Monash has effectively shut down its fax system and does not allow mobile calls from its phone system.)
After a very long series of frustrating emails I was eventually sent a pair of panels and inverters. I had asked for three panels—including one A5 panel—and two different inverters, one more powerful than the other. Some weeks after asking how I might return and replace the wrong inverter and obtain the A5 panel, I was sent the right inverter, but no A5 panel.
[CeeLite™ Electroluminescent panel (illuminated)]
At this point I gave up; and simply sent my money. A colleague—aware of my protracted and frustrating exchange with the local rep—was going to Sydney, which is where Flexalite is based, and was going to visit them in person. Unfortunately, when he turned up at their shop front he found that they were no longer there. They had moved! When I am next in Sydney I will try to locate their new premises and see if I can't get the "sample" (A5) panel.
Prices are similar to those reported by Matthew Davis ("the whole setup cost a little less than [US]$100"). A Ceelite Panel [AB100] 216 x 279mm [8.5 x 11 inches] is A$75.00; and a Ceelite Inverter [INV-US100-AB] is A$50.00: making A$125. The more powerful inverter [INV-FL300-AB], which puts out four amps instead of two, is the same price but makes no noticeable difference to the luminosity of the panels.
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[CeeLite™ Electroluminescent panel (illuminating a sheet of paper)]
I have shown before how useful these panels are in illuminating watermarks in books, so I thought I would use the Ceelite to illuminate a much more complex watermark on a loose leaf of paper.
The leaf concerned is a single-page letter dated 16 September 1732 from James Leake (bookseller in Bath 1721–64) to John Nourse (bookseller in the Strand, London 1730–80). The leaf (310 x 190 mm) was folded once to make four pages, only one of which was used for writing, then had the four edges folded in before being addressed and sealed. The letter is stamped "18 / SE" (for 18 September) and "BATH," which is where it was posted from.
[The front of a letter from James Leake to John Nourse]
The text of the address is:
To Mr Nourse
at the Lamb
near Temple Bar
[The back of a letter from James Leake to John Nourse]
The text of the letter is:
Mr Nourse Sept 16 1732
On Saturday by 9 in ye
morning Pray Send to Mr Hazard
One Set of Ovid neatly b[oun]d G[il]t
Back & The Lovers Marbl'd. I
have not yet examin'd your Catalogue
but shall soon when you may
to hear from
Y[ou]r Humble Ser[van]t
[The inside of a letter from James Leake to John Nourse]
As you can see, the watermark is a seal enclosing the Coat of Arms of the Kingdom of Great Britain (as it existed between The Acts of Union 1707 and 1801—see here) surmounted by the Imperial Crown of the Sovereign. The caption is the motto of the British Monarch Honi Soit | Mal Y Pense.
(BTW: This watermark appears in Philip Gaskell, A New Introduction to Bibliography (1972), 69 (Fig. 35), who cites Edward Heawood, Watermarks: Mainly of the 17th and 18th Centuries (1957), 448, where it is identified as "London, 1736." The letter is made up of one half of a "pot" sheet, the full sheet being at ca. 310 x 380 mm according to Gaskell (1972), 75, Table 3.)
Playing around with the contrast and colour-balance makes the watermark a little clearer (as here), and a high-definition camera with macro would probably be even more helpful. But, as you can see here, even the raw photograph off a bog-standard (and not very recent) digital camera provides a very clear image, certainly clear enough to make out all the features of the watermark.
[The watermark, view from inside the letter (watermark correctly oriented)]
More importantly, it is very easy to slip this panel under the page you are looking at, snap a picture and keep going. You can collate an entire volume like this and in a very short space of time build up a reference set of photographs of the watermarks in a volume, which you can then be used to compare to other copies of the same volume.
Of course, it is possible to record watermarks without the panels. But, like Matthew Davis, I have been reluctant to try to sketch/trace watermarks because I am "artistically incompetent," and these tracings take a long time to do. The resulting sketch would also be of no bibliographical value since my "Coat of Arms" would be indistinguishable from a Fool's Cap, a Fleur-de-lis or anything else for that matter. And the technological alternatives—like beta radiography—are expensive and available at few libraries. I suspect that this is why watermarks are still under-examined and under-recorded.
[The watermark, view from outside the letter (watermark reversed)]
As use of these cheap and relatively portable panels becomes more wide-spread I expect that bibliographers will be more inclined to record watermarks along with other bibliographical features. And as they become more widely used, and more evidence concerning the use of watermarks is recorded, the evidence that is collected will be of more use to the bibliographers, encouraging further recording etc.
[UPDATE: 2 July 2016: After all my pictures disappeared (again) I decided to give up on external hosts for large versions (1000px) of my image files and, for now on, will stick with the smaller images (500px), which Blogger is prepared to host.]