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Most museums use tube cameras (usually called Vidicons) for infrared
imaging. Although they are relatively cheap, they are not very stable,
and they suffer from (sometimes quite severe) geometric distortions.
More modern solid-state cameras are still expensive, but are becoming
more widely used because of their greater stability. This
guide assumes you are using a tube camera, but almost all of it applies to
solid-state cameras as well.
Whatever your camera, there are three main sources of error which have to be
addressed in order to be able to make successful mosaics:
- Tube cameras suffer very badly from distortions in the image, usually either
`pin-cushioning' or `barrelling'. These distortions result in alignment
errors when sub-images are joined together.
- The sensitivity of the tube varies across its surface, causing some parts of
each sub-image to be brighter than others. This is made worse by unavoidable
variations in illumination. When a lot of such images are joined together, the
result is a `brick wall' effect.
- The sensitivity of the tube also varies between sub-images, partly as
the overall lightness in the field of view changes, and also because the
electronics in the camera change as the camera heats up. This leads to a
patchy, unbalanced mosaic.
The first two problems will be different in each Vidicon and will change
each time a tube is replaced. All three problems need to be addressed to
create successful infrared reflectogram mosaics.
Subsections
Next: 3.2.1 Setting up your
Up: 3. Assembling infrared mosaics
Previous: 3.1 Introduction
Contents
John Cupitt
2004-12-20