polarized light set up?

[Melissa McGrew]

Dear RTI colleagues,

I am hoping for some advice on using low tech polarizers to potentially enhance the surface topography of early (c. 1850s) low relief decorative paint on wall and ceiling surfaces that have been subsequently overpainted numerous   times.  The project unfortunately does not include the time or budget for RTI set up and documentation, however correspondence with a lighting engineer has resulted in a recommendation to apply a polarizing film (linear light polarizing gel)  to our portable light source (Makita LED square headed flashlight set up at a raking angle to the surface) as well as a circular polarizer to our SLR camera (Nikon D70) to help enhance and then to photograph the low relief pattern (not visible from the straight on view with ambient light).  I have found some excellent photos of such a technique being successfully used for enhancing fossil reliefs, but have not been able to find any case studies utilizing this technique for documenting painted wall and ceiling overpainted surfaces.  Can anyone relay any (even opinion-based) suggestions for setting this up (e.g.polarizer types, lens type, closing out ambient light - this may or may not be completely possible) or for even saving us some investment, by alerting me that this set up is not likely to provide a good product? The walls are currently painted opaque light yellow while the ceilings are currently white.


Many, many thanks for any suggestions,

Melissa McGrew
Conservator,
BCA, Inc.
mmc...@bcausa.com

[Taylor]

Hi Melissa,

From what you're describing, it sounds like it will be difficult to get enough polarized light on the surface to overcome the ambient light, especially if you're capturing large areas, unless you can block out the ambient light sources.  Another disadvantage of the method you're describing is that the details you can see are limited to what is revealed by the raking light directions you choose for the captures.  A good reference for using polarized light, raking light, and specular illumination (and other techniques) is the AIC Guide to Digital Photography and Conservation Documentation (2nd edition, pp. 115-121).  If you don't have access to a copy, I can quote some additional suggestions.  I haven't used polarized light for photography, but am familiar with using it for microscopy.

Given these challenges and your limited budget, specular illumination (axial or oblique) might have advantages for revealing surface topography, because you would have more flexibility to use a more powerful light source.  However, RTI has significant advantages over both methods by giving you more flexibility with various viewing algorithms, virtual control of the light direction from your office computer, and the flexibility to use a more powerful light source with neutral density filters to overcome ambient light.  Given the relatively low cost of RTI (using readily available consumer equipment) and its advantages for revealing textural details, you might want to reconsider using RTI for the higher quality of data you can get with a limited budget. 

The cost for RTI will depend, in part, on the size of the areas you want to photograph, the level of resolution you want, and time and personnel available.  The RTI setup isn't likely to take longer or be more expensive than the polarized light setup, but it requires more images (30-50) for the same area.  Once you're set up, however, the image capture sequence only takes about 10 minutes with two people.  The only additional expense would be for remote triggers, which can be rented at relatively low cost, and sufficient data storage and some processing time.  It's definitely possible to do RTI solo, but it helps to have an assistant.   You can do RTI tethered to a computer or untethered.  I've captured about 40 Gb of RTI images of a painting in a day, working solo and untethered, at a surface resolution of 500 ppi.

Best of luck,
Taylor Bennett

[Lindsay MacDonald]

Hi Melissa,

That's a very good question. Polarising filter pairs used in this way
are an excellent way to reduce glare and unwanted reflections from
glossy surfaces, especially at low angles. I would suggest a linear
polariser on the camera as well as on the light, but it would be best
to experiment with both linear and circular types to see which works
better for your application.

My experience for revealing the topography of surfaces, however,
is that polarisers are not needed. The RTI method works very well
if you use a good localised source of light (such as flash) at a low
raking angle to the surface, and suppress the ambient illumination
as much as possible.

You can purchase a ring-mounted polarising filter (PL for linear or
CPL for circular) for your camera from Amazon. Cheap and cheerful
ones (Ex-Pro brand) are available for about $10, better ones from
Hoya, Cokin or B+W will be up to $100. Choose the correct diameter
for the front screw-in ring on your lens.

You can purchase an acetate sheet of polarising filter for your lamp from:
http://www.leefilters.com/lighting/colour-details.html#239&filter=tf
It costs about $80 for an A3-size sheet.

Good luck!

Lindsay MacDonald
Dept of Geomatic Engineering
University College London