OT: Ways to read hidden lettering?

webcam without IR filter and floppy disc material placed over the lens, moved .3mm farther from the CCD for focusing. Does show the sunny cloudless sky as darker than a nearby sunlit slate roof. Following in a dark room. No response to a cup of boiled water . Requires 5watt into a

IR torch arrived and I gave it a go, various power levels from a power supply, different angles and distances and webcam filter settings but even for the letter I deliberately put under a blank spot of wallpaper, saw nothing convincing. Just thought of another idea, blank off around the torch output so I can place directly on the paper and see if penetrating IR shows anything nearby to the torch.

I wonder if some fairly benign liquid - maybe something Freon-like - would make the paper temporarily translucent? Probably not, but who knows...

Cheers

Webcam won't respond to thermal infrared, only near infrared, like IR LED.

But the webcam may be able to see near IR reflected off of the wall behind the wallpaper. Eric

I've now tried UV shows nice bright blue flourescence of the paper but nothing relating to the letters. Alsop tried 2 and 3 thicknesses of floppy disc material, to use brighter near IR, but still no ghostly apparition. Tomorrow I will try my modified "keyfob" thermal IR thermometer with a 3W illuminator made from a resistor in a conical metal corded lamp switch drop handle. Needs about 3W shone on the paper only, no plaster or letters under, at 45 degrees, with pyro at 90 degrees, so give 5 to 10 degrees over ambient, its receiving cone about

How bout ultrasound

And how about using theIR filtering lenses from vcr's tv's and remote controls etc. ?

How about hitting the wall with an Infreared heat lamp?

Will the "old trick" of very thin paper (tissue/tracing paper) and a pencil work.

You put the paper down and use a pencil, The edges generally get defined.

So like brass-rubbing? or last message written on the notepad business. I'll give it a go when I've finished with the other methods. I had tried very oblique/acute ? lighting and saw nothing emerging.

My close-coupled pyro and resistor heater cone seem to be showing about

Yea, tack the paper down and take a pencil and go perpendicular to the line coloring in the detected edges. So, you want a solid large width line.

If it works, you might want to consider artist's charcoal.

You could probably replace the wallpaper in less time than you've already spent on this!

Cheers

Phil Hobbs

-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics

hobbs at electrooptical dot net

Ah but it feels like proper , but alternative, research. With the pyro and heater cone sensor as it is I can reliably determine where a 65mm broad black line is (included with my test text), a differential of about .7 deg F over gloss black or emulsion white painted plaster under plain bits of wallpaper and about .5 deg differential where the paper is patterned. I now have a good idea of improvements for mark 2. Insulating the heater cone because the heat is heating up the mount and transfering to the paper by conduction. So instead of running the sensor horizontally (heater and pyro are set horizontal so I can read the LCD) I have to make runs vertically. Slowly moving the sensor is adequate to zero in on a 65mm wide solid black band. So the first part of the quest is achievable, the text/s is accompanied by a 65mm black band/s, so should be able to determine where exactly the text or possibly texts are, as it is immediately above the black lines. Mark 2 would have a cutdown pyro cone , so monitoring less area of paper and compensated with the resistor mounted in a cutdown cone and it mounted in kiln cement rather than epoxy so I can run the heater hotter. Also create a reference sliding face perpendicular to the wall and vertical as well as the original just horizontal and wall contact face.

From N_Cook's previous postings, I recall that this has something to do with suspected lettering under wallpaper in a historically listed building - he can do all the non-destructive testing/probing he wants, but he can't lift up the wallpaper in that building. Therefore, he has made a hopefully similar example at home, and is refining his technique.

Matt Roberds

On Thu, 30 Apr 2015 20:38:32 +0100, N_Cook wrote as underneath :

NC Fraid I havnt read the whole thread! Have you tried heating a patch of wall above ambient then and taking thermal image as the patch of wall cools after heating source removed? Tiny differances in emissivity might be detectable? C+

The trouble there is getting even heating to better than +/-0.5 deg F, tried with a photoflood bulb and uneven to something like +/-2 deg F

Confirmed, originally 15th century but mainly 17th century, the part I'm interested in is early 20C text on a 19C alteration

You have forgotten the context:

"Setting is a commercial premises that permission to non-invasively explore would be possible but steaming off wallpaper and remedial work (no matching new wallpaper found) is not permitted."

On Fri, 01 May 2015 07:47:11 +0100, N_Cook wrote as underneath :

snip

Yes I can imagine that might be a practical problem on a large area but did you detect any test letter at all with this method? If so then a method shoud be possible, if not then its a deadend! There is no cheap and cheerful X-Ray method! C+