Volume 5, Number 2, June 1983, p.5
Two future NASA projects will depend upon a very special light detecting sensor to provide information never before available. "Space Telescope" will peer farther out into space than Man has ever seen before, and "Galileo" will examine the surface of Jupiter with a microscopic precision.
These two multi-million dollar projects depend upon a small sensor less than one half inch in total surface area called a Charged Coupled Device, or simply CCD.
What makes this sensor so remarkable is that in the space of 1.48 square centimeters some 640,000 individual light detecting units (called 'pixels') have been built with the capability of detecting light levels as low as 50 photons or as high as 100,000. This makes the CCD sensor tens of times more sensitive than the most sensitive Vidicon camera ever built and hundreds of times more sensitive than panchromatic photography. In other words, if a black and white photograph has about 40 grey levels to display at maximum, the CCD image "holds" thousands.
Obviously there is no way to display 5,000 grey levels, but by taking the data delivered from the CCD and giving it a numerical value (converting the electronic signal to a digital format, a process called Analog-to-Digital Conversion), virtually any 40 grey level 'window' of the total signal can be individually scrutinized. This could have a staggering potential for conservation.
This potential has recently been reviewed by the Research Laboratory of the Conservation Center at the Los Angeles County Museum of Art as part of a larger study sponsored by the Armand Hammer Foundation. In the initial proposal, the mounting of the Codex Hammer by Leonardo Da Vinci was to be carefully examined for potential problems that would not be expected to be evident for many years after the mounting. We decided to use normal conservation photography for the documentation of an experimental mock-up which would be environmentally stressed. These images would then be computer enhanced to display changes not normally evident to the human eye. As a secondary effort, it was decided to use the CCD's enormous capabilities to try to detect changes of an even more subtle nature. Unfortunately, the technology was so sophisticated we experienced many problems: all of which have been isolated and defined for the benefit of future user. Yet, the results we produced represent a qualitative examination of this technology.
One of the most important notions of CCD imagery is the fact that any image can be processed to display textural information in areas of dark shadows and white highlights with the same resolution as a perfectly exposed mid-grey region. By sliding our 'grey level window' down the whole dynamic range of the CCD image it is possible to produce literally dozens of unique pictures. Each picture can then be enhanced differently to display differing sets of data.
Our study indicates that with advanced CCD camera technology, it should be possible to produce data that can be later reviewed and analyzed to answer questions not conceived during the initial photography. For example, a raking light photograph can be produced from bidirectional lighting.
This study was supported under a grant from the Armand Hammer Foundation in conjunction with the National Aeronautic and Space Administration (NASA) facility at the Jet Propulsion Laboratory. CCD sensors were supplied under a JPL contract with Texas Instruments and implemented at JPL.James Druzik