Infrared Video

["J.Wood" <shopper@xxxxxxxxxxxxxx>]

The current issue (May/June 1997) of Archaeology (Archaeological Institute
of America) has an article "Imaging Maya Art" which discusses using infrared
video. The murals at Bonampak, Mexico,  have become covered with salt
crystals which obscured the images.  Infrared video was used to view the
murals through the crystals.  The technical aspects are:

        "The infrared video technique we used at Bonampak is ideal for
resurrecting faded images no longer visable to the naked eye.  For those
interested in learning how the murals at Bonampak were recorded using
infrared, here are the technical specifications:  We videotaped the Bonampak
murals with a Hamamatsu C2741-05ER infrared Vidicon camera connected to a
C2400 controller.  The camera's spectral response extended to a wavelength
of 2.2 u in the mid-infrared.  The images were stored using a Panasonic
AG-DS550 S-VHS recorder. 
When the camera was operated in the black-and-white mode the specfied
signal-to-noise ratio was greater than 50 dB.  The combined resolution of
camera and recorder was approximately 400 lines.

        The camera could record an area about five inches high by six inches
wide.  Two floodlights were used to illuminate each segment.  During the
January 1996 expedition the camera was equipped with a 700 nm long-pass
infrared filer yielding a wavelength response from 0.7 to 2.2 u.  During the
May 1966 expedition we improved image contrast by equiping the camera with a
1.0 u long-pass filter yielding a pass band of 1.0 to 2.2 u.

        All images from the first expedition were obtained by scanning the
murals left-to-right and top-to-bottom with the camera mounted on a tripod.
This covered about a 48-inch square segment at each tripod setting.  During
the second expedition, we used a rail system to bring the camera lens closer
to the mural surface, eliminating angular distortions at the perifery of
each image.

        Determining how each frame will fit into a final composite
photograph is a multistep process.  A grid is placed over each digitized
image.  Four to 20 specific features that appear in pairs of overlapping
images are then marked.  Their coordinates are plugged into program that
computes how the images must be manipulated to match up the features in the
image pairs.  To increase the accuracy of the mesh, the computer compares
tiny regions around each feature and adjusts the coordinates.  

        This program gives resarchers more flexibility in the field.  It
allows them to take pictures as best they can on-site and "warp" them later
to fit together.  In contrast, manual image composition requires that
pictures be taken under controlled conditions, from vantage points that may
be inaccessible.--Gene Ware, chief technical specialist, College of
Engineering and Technology, Brigham Young University, and Kirk Duffin,
Computer Vision Laboratory, Brigham Young University."

        The infrared images in the article were black and white. The letter
"u" is as close as my e-mail program can come to a symbol which looks like a
small backwards y, or the russian letter tse.  I presume its a greek letter,
but not one I'm familiar with.

I realize this is a bit long and off the beaten path, but looked
interesting, at least to me.

Jim Wood


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