OT: Ways to read hidden lettering?

[N_Cook]

Situation is wishing to read some black gloss-painted letters over white
emulsion painted plaster, but for a few decades has been covered by
wallpaper. 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.
3 ideas so far, any suggestions relating to these , or any other ideas?

1/ A 3W <>.9micron wavelength IR torch bought and on the way, in
conjunction with a webcam that I've removed the IR filter from and
checked with IR zappers as responsive.
2/ I'll also try a vitreous resistor set in a chromed cone in
conjunction with a keyfob pyro thermometer, for 10 to 25 micron IR
"interogation". The wallpaper is striped so it may be possible to move
the pyro, parallel to the lines, if the wallpaper dyestuff interferes
with any response. Recording the temperature change with X and Y , if
any differentiatable changes become apparent.
3/ high power directed white light and neutral density film over a
camera lens.
I'll make a test bed of some black painted letters of the relevant size
on plaster covered with some wallpaper. Letters are known to be about
65mm high and letter thickness of black paint about 6mm, as they were
stencilled. Wallpaper is simple uniform thickness, not anaglypta.

Hiring a pro IR video camera is out of the question as too pricey and
may not work for this use anyway . I'm still looking for somewhere that
might hire (for some reasonable fee) one of the low res <>60x80 pixel IR
video cameras

[John Robertson]

Why not make your own IR camera:

http://www.picturecorrect.com/tips/how-to-build-your-own-infrared-camera/

Lots online when you search for "Make your own IR camera"

John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

[N_Cook]

I've only tried my hacked IR filter-less camera with IR zappers, not
tried to take a pic of anything. I'd not considered adding an IR-pass /
visible cut filter, as in that file. I wonder what material that is mase
of, polythene? I know that passes the IR of PIR use, whatever wavelength
that is.
I also assume the IR of such a modified camera without germanium lens is
only sensitive to about 1 to 3 micron IR

[N_Cook]

from
http://www.instructables.com/id/Floppy-Disk-IR-Camera-Hack
it seems floppy disc material, passes IR and blocks visible for this purpose

[et...@whidbey.com]

On Fri, 24 Apr 2015 08:16:57 -0700, John Robertson <sp...@flippers.com>
wrote:

Thanks for posting that link John. It really makes me want to buy
another camera to modify. Since I have a machine shop any physical
mods and special filter mounts will be easy for me. In fact, I built a
filter/lens mount for my Sony DSC-120 Cybershot camera. This has
allowed me to mount telephoto and fisheye lenses to it.
Eric

[et...@whidbey.com]

Wasn't it you who tried taking an IR filter out of a camera and found
that it would not focues without the filter in place? John, posted a
link in a reply to your post . The article the link points to says
that some cameras will need the IR filter replaced with a piece of
glass the same physical size for the camera to work properly. You
should read the article. It has some pictures that were taken with
various filters that only let in certain wavelengths of IR and blocked
all visible light. I can see how this might work for you project.
Eric

[N_Cook]

guilty, that was on a close to scrapper camera, just used as a trial
run, not worried if I wrecked it. The current one I've removed the IR
blocking filter from, was a thin sheet , not a block of material like
the one of my previous posting

[Mike]

Hmmm, those terahertz airport scanners are rumoured to be able to see
through clothing, so perhaps they would work. May be difficult to beg,
steal or borrow one, though...

Mike.

[N_Cook]

I wonder how many people had accidently lost their "X-ray" Sony
camcorder so they could not surrender it .
http://www.liveleak.com/view?i=faf_1385072214

[Dave Platt]

In article <mhdrpt$p42$1...@dont-email.me>, N_Cook <div...@tcp.co.uk> wrote:

>I've only tried my hacked IR filter-less camera with IR zappers, not
>tried to take a pic of anything. I'd not considered adding an IR-pass /
>visible cut filter, as in that file. I wonder what material that is mase
>of, polythene?

You can make an acceptable IR-pass, visible-block filter using a thin
piece of black-colored acrylic sheet plastic ("Plexiglas"). The black
color is apparently a dye, not a pigment.

I've taken some acceptable near-IR photos using this technique, with
an old Canon digital camera whose sensor lacks an IR filter. It
requires an exposure of several seconds but it does work.

[Ecnerwal]

In article <mhdksj$sro$1...@dont-email.me>, N_Cook <div...@tcp.co.uk>
wrote:

> Situation is wishing to read some black gloss-painted letters over white
> emulsion painted plaster, but for a few decades has been covered by
> wallpaper. 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.

If you can hack the old tech, IR film (of the roll in the camera, take
pictures you can't see, process in a darkroom with wet chemistry sort)
might be a way to access near-visible IR photography on a lower budget.

Before you rip out the IR filter, check the camera (or various cameras)
with the source - many cell phone cameras (at least) can "see" enough
near-IR to show near-IR sources without removing a filter.

Visible light, digital camera and image post-processing MIGHT do it.

Different but similar application with old letters (literary estate) and
blacked out parts was sorted by scanning at high color depth and
separating brown-ish-black from blue-ish-black as the ink written in and
the ink blacked out with were different. Not my project, went to a talk
at a conference where it was presented.

--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.

[et...@whidbey.com]

On Fri, 24 Apr 2015 08:16:57 -0700, John Robertson <sp...@flippers.com>
wrote:

John-I got all jazzed up reading the article you posted the link to.
Then I looked at some more. Then a little later I made a trip to the
hardware store and stopped in the thrift store a few doors down to
look for a camera. Dropped 8 bucks and came home with two Cannon ELPH
cameras. I don't know yet if eather camera works but for 8 bucks it
was worth the risk.
Eric

[N_Cook]

I've just butchered a 3.25" floppy disc and quite a nice sepia tone to
the outdoor scene I was looking at it through, not tried on the camera yet.

[N_Cook]

"photoshopping" I can see has possibilities here as I'm only interested
in reading the text , ie differentiatable colour or luminance levels,
not pictures of foliage against black skies or anything artistic.

[Martin Brown]

To be honest I think your best bet is to illuminate the scene with
filtered longwave IR (and no visible) and use an amateur astronomy CCD
camera time exposure to try and image through the wallpaper. How well it
works depends on how much pigment is in the wallpaper. The paper should
be moderately transparent to IR but the pigments may not be.

These cameras are relatively common and your local astronomy club might
well have someone who is game to give it a try. The difficulty is that
there is a limit to how well glass lenses transmit IR. The other problem
is that you will need to do it with a quality longpass filter as you are
in the noise >> signal regime on this one.

The FLIR addon for an iPhone5 would be another option.
http://www.amazon.com/FLIR-ONE-Thermal-Imager-iPhone/dp/B00K0PXFB6/ref=sr_1_1?ie=UTF8&qid=1429954794&sr=8-1&keywords=flir

Possibly the one with the best chance of success if you don't know
someone who owns a genuine FLIR camera.

--
Regards,
Martin Brown

[N_Cook]

agreed one of those add-ons or the low res FLIR cameras , but just for a
one-off job , cannot justify the prices or even to hire one for one day.


I've now got a test patch of wall with painted on gloss black 3 letters
of the right size, will wallpaper over it on Monday.

[RodK]

So does exposed & developed 35mm film.

[Martin Brown]

ITYM *unexposed* developed slide film which is a good cheap IR long pass
filter since the dyes block most of the visible light.

Using it for viewing eclipses or looking at the sun can cause serious
eye damage since it makes the visible light level tolerable whilst still
allowing enough near IR through to cause retinal burns.

--
Regards,
Martin Brown

[Phil Hobbs]

Exposed colour negative film is OK too. The main difference is the
strong orange cast of the film stock, but you don't care in an IR filter.

Cheers

Phil Hobbs

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

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

[N_Cook]

No discussion of the small halogen bulb or vitreous resistor set at the
apex of an insulated chromed brass cone for a long wavelength IR
illuminator, in conjunction with one of these
http://www.maplin.co.uk/p/pocket-infrared-thermometer-n19fr
keyfob IR thermometer I use for locating hotspots on circuit boards.
Now I have a test bed of hidden letters i can have a go at different
settings. But any suggestions of what sort of wattage and temperature
may show the greatest contrast of white and black under paper?

[N_Cook]

some background data, for after the wallpaper glue is set over my test
patch.
The wallpaper I'm using is about 0.23mm thick and optical wavelength
opacity about 8 stops (compared with neutral density filters) , and
again in optical about 10 stops for the floppy disc material

[N_Cook]

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
12 ohm vitreous resistor, >200 deg C on pyro thermometer,nothing glowing
to sight, to register an image that you can tell the orientation of the
resistor. Still waiting on IR torch, may try cone of heat and pyro
thermometer but would be nice to get a direct image rather than having
to plot pyro readings

[N_Cook]

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.

[Syd Rumpo]

On 24/04/2015 15:45, N_Cook wrote:

> Situation is wishing to read some black gloss-painted letters over white
> emulsion painted plaster, but for a few decades has been covered by
> wallpaper. 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.
> 3 ideas so far, any suggestions relating to these , or any other ideas?

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

Cheers
--
Syd

[Jasen Betts]

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


--
umop apisdn

[et...@whidbey.com]

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

[N_Cook]

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
15mm from the paper.

[nova...@gmail.com]

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?

[Ron D.]

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.

[N_Cook]

On 29/04/2015 16:33, Ron D. wrote:
>
>
> 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
1 deg F difference between black and white areas, about 17 degrees F
above ambient. I'll have to give it a proper tryout , moving the probe
consistently in , decided on preliminary results, the cool or preheated
direction and take a grid of readings after a specific number of
seconds. My moulded up combination mount is edged with right-angle faces
so I can run it along the horizontal face of a plastic spirit level and
along the vertical wall face. The axial offset between pyro and heater
cone is a bit larger than 90 degrees because of the horizontal
separation of both. Take readings every 1/4 inch , then raise the level
1/4 inch etc.
I'll also try shining a 150W photoflood filament lamp from a fixed spot
and run the pyro around , angled so not directly in the beam and not in
its shadow either.

[Ron D.]

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.

[N_Cook]

150 W photoflood just showed the irregularity of illumination

[Phil Hobbs]

On 4/30/2015 8:17 AM, N_Cook wrote:
> 150 W photoflood just showed the irregularity of illumination

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

[N_Cook]

On 30/04/2015 13:55, Phil Hobbs wrote:
> On 4/30/2015 8:17 AM, N_Cook wrote:
>> 150 W photoflood just showed the irregularity of illumination
>
> You could probably replace the wallpaper in less time than you've
> already spent on this!
>
> Cheers
>
> Phil Hobbs
>

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.

[mrob...@att.net]

In sci.electronics.repair Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:
> You could probably replace the wallpaper in less time than you've
> already spent on this!

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

[N_Cook]

On 01/05/2015 06:57, Charlie+ wrote:
> On Thu, 30 Apr 2015 20:38:32 +0100, N_Cook <div...@tcp.co.uk> 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

[N_Cook]

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

[Leif Neland]

Efter mange tanker skrev Phil Hobbs:

> On 4/30/2015 8:17 AM, N_Cook wrote:
>> 150 W photoflood just showed the irregularity of illumination

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

> Cheers

> Phil Hobbs

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."

--
https://www.paradiss.dk
Ting til konen eller kæresten.
Eller begge.

[N_Cook]

On 02/05/2015 07:23, Charlie+ wrote:
> On Fri, 01 May 2015 07:47:11 +0100, N_Cook <div...@tcp.co.uk> wrote as
> underneath :
>
> snip

>
>>> 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
>

> 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+
>

No. If my reduced apaeratures , close coupled heater in cone and pyro
thermometer does not resolve 6mm wide lettering I may have another go.
Perhaps indirect bounce from a large distributed heater, using some
sort of diffusing surface to bounce off.

[Jasen Betts]

On 2015-05-01, N_Cook <div...@tcp.co.uk> wrote:

> 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

plane mirror and sunlight (heliostat)

--
umop apisdn

[N_Cook]

Now that is an interesting idea, for this application , may require 2
mirrors, but feasible.

[N_Cook]

I got back to this project yesterday. I' d tried oblique lighting with
NIR and modded webcam before and not seen anything. This time I could
make out lettering under pattern-free areas but not convincingly under
patterning. But I remembered I'd painted letters
NCE
V
I could quite plainly see the N and V and the curve of the C but also
curve in the E area. I dug out the paper I'd written down the letters
and it was
NCQ
V
So without patterning, in the real application, it may be possible,
"brass-rubbing" optically.
I'll also have a go with mark3 close-coupled heater cone and cut down
pyro thermometer,tomorrow , on a grid of readings and repeat for
different temperatures and hopefully improve on the 0.7 deg F differential.
I tried the modded webcam , with no floppy disc filter,viewing a Weller
magnastat soldering iron barrel. The heater area glowed bright and
dulled to nearly no discernable pixels , and the thick part of the
barrel stayed at some intermediary luminosity, brightening and dimming
but observable in space by its conical shape at all times , with a 700
deg F magnastat in place.

[N_Cook]

On 17/05/2015 08:02, Charlie+ wrote:
> On Sat, 16 May 2015 20:04:48 +0100, N_Cook <div...@tcp.co.uk> wrote as
> underneath :
>

> Thanks for continuing your experimentation story! C+
>

I've now got some convincing AVI of the N followed by a circular letter
not complete as an O , could be C or G . With floppy disc material over
the webcam lens , so I can work in room light and laptop screen is black
with no IR torch on. With all controls on the webcam driver set to min
brightness and max for contrast, saturation and mid white balance. As
only 5 frames per second, sweeping the NIR LED torch gives a better
chance of reading the letters then snapshots.
In the real application the context of words should make distinguishing
the likes of Q and C, or E and F easier.
In the next hour or two I'll give the heater and pyro method another go
over 1/4 inch steps horizontal and vertical at a few heater settings,
but stick to 3 seconds per position, for optimum of speed of capture and
one level of consistency. Could increase to 10 seconds or so if
required for greater resolution later, over questionable letters.

[N_Cook]

Got nowhere on the heater cone and pyro.
I even tried a rubber hot water bottle with hot water ,held against the
test wall for 10 minutes. Loads of temp variation but nothing to do with
black paint, just change of heat capacity of different thickness of
plaster over brick presumably.
So I'm wondering if even an expensive thermal imaging camera and some
sort of diffuse and even heat source would work either, unless just
flashed over the paper and not hanging around enough to deep heat.
The NIR webcam is relying on "brass-rubbing" relief in all probability.
The \ part of the N is evident in raking white light but not the rest of
the N or any of the next letter or any of the other 2 letters so bit of
a mystery why the NIR+webcam is picking up so much differentiation.
With no patterning on the paper it would probably resolve the likes of C
and G, Q and O, E and F ,P and R, B and 8 etc.
I tried polarizing filters as well
I've run out of ideas, what I would like to try was a long IR, 5 to 10
micron LED to direct heat energy onto a small spot, the pyro seems to be
reacting to heat radiating off the close by metal heater cone rather
than heat bouncing off the paper. All the parabolic reflectors I have
are too big to direct a little halogen bulb or similar "point" heat source.

[Carl Ijames]

Put a small halogen bulb, 5-20 watts and 12 VDC like those used for under
cabinet kitchen lighting, in one end of a short piece of copper tubing. The
inside of new tubing is pretty shiny in the visible and should be a great
reflector at 10 um, and something like 1/4 or 3/8" tubing a few inches long
should give a pretty good spot that size when held against or almost
touching the wall paper. Probably comparable to a small parabolic reflector
but with a closer working distance. The longer the tube the more directed
the beam, and the more light lost to the walls. I would start with 1.5 or
2" of tubing but you might need to try even shorter. Good part is it's
cheap :-).

Have you called a few of those companies that use ir cameras to map heat
leakage from a house, to see if they would be willing to try a quick
experiment for free in return for whatever publicity you can give when you
examine the final target?

-----
Regards,
Carl Ijames carl.ijames aat deletethis verizon dott net

"N_Cook" wrote in message news:mja3p3$lbh$1...@dont-email.me...

[N_Cook]

Great minds ...
I'd just removed a 6V 10W halogen bulb from its reflector, I was going
to roll up some polished tin-plate, soldered on the outside.

[N_Cook]

I'll have to abandon the close coupled heater and pyro thermometer. Even
overdriving the 6V bulb to 8V and 8 deg C nominal over ambient on the
pyro thermometer , nothing convincing found. That 8 deg C is the reading
on the scale with my cut down brass cone, probably corresponds to 15 deg
C or more for the proper cone that it is properly calibrated to.
Whether a FLIR device and heat source would work, is a bit questionable.
But the IR filter removed web cam and 5 degree or so raking angle NIR
torch, video image recorded to laptop, is worth trying.

[Carl Ijames]

I came across something that sounds pretty intriguing and might be within
your budget, a long wave IR camera chip that responds to 7-14 um IR and
delivers 80x60 pixel images at about 2 frames per second. Here is where I
found it: https://www.sparkfun.com/products/13233, they have links you can
follow for more info. This plus a Raspberry PI and their free demo software
gets you an IR video camera for under $400 (not including a TV monitor) that
stands a good chance of being able to read your hidden lettering. Watch the
video, it's very impressive. After you finish your project I'm sure you
could sell it on craigslist for most of what you paid for it :-).

[N_Cook]

The off the shelf FLIR 60x80 camera is not much more in cost.
Next week I'll take my modded webcam + NIR torch to the site and have a
go on site. I'll also take high quality pics of the wallpaper along with
architects colour swatches and ruler. If my NIR webcam does not show
anything then the next pricewise plan of campaign is look into
custom-made wallpaper (usually for kids bedrooms) and permission to
remove and replace a patch of wallpaper. Luckily the relevant patch of
wall is isolated from the rest of the room , so an absolutely colour and
texture matched patch should not be noticeable.

[N_Cook]

I'm hoping, if not fully readable image, to have enough image to show
something is under there, intriguing enough to get permission to remove
a section of wallpaper.
As it should be very localised text then knowing where it is then
perhaps simply remove the patch of wallpaper and move one of the
pictures to cover the spot, may even be possible. Because of the
significance of the text they may wish to have it accessible, but not
necessarily on show, for the odd academic visitor.

[et...@whidbey.com]

Since you first brought this subject up I have been intrigued with
modding a camera in order to see NIR. I did this mod to a Canon SD
300 Powershot camera. It not only sees NIR but also near UV. I have
been using the camera to look through stuff that is opaque to my eyes,
in both NIR and NUV. Is it possible that NUV might go through the
wallpaper?
Eric

[N_Cook]

A assume it was near UV, 20 LED cluster only labelled UV , numerous
materials flourescing under it. I returned to the test wallpaper patch
and tried straight on and oblique with various visibly opaque or going
on opaque "filters" over the webcam lens , passing the UV image, mylar,
polyester , black polythene, CD in overprinted-free area of label,
floppy disc material blocked the uv illuminated image. Could see not the
slightest hint of any letters under the paper wiht any of them, or
absent filter, but could see the wallpaper printed design in each case,
but nothing in the background room light illumination.

[Rich Webb]

The best value (not the lowest price) in consumer thermal IR imagers
is still the FLIR E4. It's only 80x60 out of the box but anyone active
on s.e.d or s.e.r can probably follow the thread over at
http://www.eevblog.com/forum/testgear/flir-e4-thermal-imaging-camera-teardown/
and unlock the 320x240 native capability.

With the advent of the Lepton and similar low resolution sensors it's
unlikely FLIR (or anyone else) will release a 320x240 imager that's
software-crippled to a lower res anytime soon so this is pretty much
it for bargain sensors, until maybe the next gen.

It's also possible to add a ZnSe close-up aux lens which makes it
quite useable for PCB inspection. FLIR doesn't supply them, so it's
ebay for the lenses and 3D printing for the holders. Works, though.
https://www.dropbox.com/s/cca4mu28b11ilpt/FLIR.jpg?dl=0

[N_Cook]

The trouble with thermal imaging cameras is I'm not sure they would work
for this purpose. I've not found an image of something that is natively
at the same temperature to its surroundings , hidden under something
visibly opaque. I'm assuming some sort of search heat source would be
required to highlight hidden black from white, but I've not found a pic
of someone doing that . My coupled use of non contact pyro thermometer
and external heat sourse seemed to just show differences in the thermal
capacity of the underlying structure. to differentiate , it would
probably require a very specific wavelength of IR, specific for picking
up paint v plaster difference.

[Arfa Daily]

>> The FLIR addon for an iPhone5 would be another option.
>> http://www.amazon.com/FLIR-ONE-Thermal-Imager-iPhone/dp/B00K0PXFB6/ref=sr_1_1?ie=UTF8&qid=1429954794&sr=8-1&keywords=flir
>>
>>
>> Possibly the one with the best chance of success if you don't know
>> someone who owns a genuine FLIR camera.
>>
>
> agreed one of those add-ons or the low res FLIR cameras , but just for a
> one-off job , cannot justify the prices or even to hire one for one day.
>
>
> I've now got a test patch of wall with painted on gloss black 3 letters of
> the right size, will wallpaper over it on Monday.
>

I was going to suggest the FLIR one. But how about the local fire brigade ?
They like to get involved in 'projects' and have probably the most advanced
thermal imaging cameras available. They can see bodies through walls, so how
about 'illuminating' the area with infra red from the opposite side, and
then seeing if the letters 'obstruct' the infra red passing through the wall
enough to be able to read them with such an item ?

Arfa

[N_Cook]

Events have overtaken this project. Since my last visit they've removed
the wallpaper, not surprising as a horrible green.
On the negative side the walls had been emulsioned with white emulsion
paint some years before the wallpaper went on.
On the positive side , as standard white emulsion, it should be easier
to get permission to strip back this top layer of paint, and easily
repaint if nothing there.
I've tried oblique NIR on it and nothing seen. A youngster around
reckoned he could see a letter under the paint with his young eyesight
but not myself or the NIR video capture with waving torch. But we now
know exactly where to strip back the paint as 2 bits of the text, top
and bottom of it, were not painted over in effect, which shows there is
likely more text for any doubting Thomases. Top bit not covered and
lower bit emerged with routine daily cleaning rubbing back the paint
enough to make out some letters. I've tried diluting none
methyl-chloride paint stripper 1 to 3 with meths and that gently removes
emulsion but leaves any gloss paint underneath, untouched.
Just need permission now