RTI capture of reflective materials

[Anna Serotta]

Greetings RTI community! 

A colleague of mine is interested in documenting the surface of a 19th c. Japanese magic mirror.  In general, such objects are cast in high tin bronze with a design (usually of a Buddha or inscription) on one side and a shiny smooth reflective mirror surface on the other side. When a strong light is shone on them they reflect the cast image from the back side onto a white wall. The overall reflective surface of these mirrors is slightly convex. The predominating theory on how these mirrors work is that there are slight inequalities in curvature, the thicker portions being slightly flatter than the thinner ones, and even sometimes actually concave. So, in theory, RTI should be able to show the image that is reflected because of these slight differences is concavity.

We did two RTI captures the other day of this object, both of which show scratch marks in the mirror’s surface (from use, wear, etc.) but we were unable to see any of the slight inequalities in curvature that we were looking for.  So either the surface features we’re looking for don’t exist, or the capture process is problematic.  The surface is extremely shiny (it is, after all, a mirror) and we're wondering if anyone on this Forum has had success imaging something with similar surface qualities, and if so, is there anything we might do differently in the capture process to improve our data?  We tried processing the data both as LRGB and RGB ptms and also as a RTI file with the HSH fitter--none of these gave us what we were looking for, and we still feel like we have incomplete data. 

Thanks!

Anna Serotta
Contract Conservator
Sherman Fairchild Center for Objects Conservation
The Metropolitan Museum of Art

[Tom]

 

Anna,

 

RTI and PTM was not designed to handle mirrored surfaces. The per-pixel reflectance functions are arbitrarily complex, so low dimensional representations like polynomials and spherical harmonics are just not appropriate.

 

Even if you coated the mirror to make it diffuse, I would not expect RTI to work well at characterizing *very* low curvatures. You might consider making geometric measurements of a laser bouncing off the mirror surface to measure the curvature directly.

 

Sorry,

 

Tom Malzbender

http://www.hpl.hp.com/personal/Tom_Malzbender/

[Clifford Lyon]

> RTI should be able to show the image that is reflected because of these slight differences is concavity

The cast image?  It seems as if you'd need to model more than the surface; you'd also need a model of the inscribed side, b/c it is the interaction between the light reflected at the surface and the light refracted through the surface to the inscription that makes the magic...  Or did I misunderstand?

[Lindsay MacDonald]

Hi Anna. Tom Malzbender is quite right, that RTI is not really
appropriate for a flat surface with very low frequency undulations.
But my guess is that if you coat the mirror with white powder
and take some images with a raking light at a very low angle
you will be able to see any deviations from planar. I did this
last year with some airplane wing surface material and was able
to detect depressions of only 10 microns.

Lindsay MacDonald
Dept of Geomatic Engineering
University College London