Choosing the right image for Photo CD
Conventional drum scanners, those made by firms like Crosfield
and Linotype-Hell, have image sensors called photo-multiplier
tubes (PMTs) These devices have extraordinary dynamic range,
meaning they allow the operator to optimize the exposure range
selected during scanning. These PMT sensors can reveal excellent
detail in the darkest part of transparencies, while maintaining
subtle tonal information in the highlights.
As a result of this ability, the photographic and advertising
industries have learned over the past two decades to scan transparencies
that exhibit tonal gradations in the highlight end of the image
that are sometimes called 'slightly underexposed." Photographers
often underexpose transparencies slightly for more image saturation.
Photo CD scanners use a different technology for image sensing.
The sensor is a charge-coupled device (CCD), a solid-state phototransistor
array with three parallel rows of sensors. Each row is coated
with a color filter (red, green, and blue) so that is "sees"
only the red, green or blue component of the film being scanned.
The dynamic range of the electronics and sensors in the Photo
CD scanners is more limited than their photo-multiplier tube
counterparts.
Recognizing the dynamic range limitation of the Photo CD scanner
requires an acknowledgment that there is not a perfect scanning
device for all images. One of the key factors in limiting the
number of images that do not scan correctly is to reverse the
"conventional wisdom" and scan transparencies that
are exposed precisely correct. Choosing that "half-stop
under" transparency does no justice to the Photo CD image;
it is far better to give the scanner the best image to scan.
Then the population of images that exceed the Photo CD scanner's
capabilities becomes minute.
Remember that the dynamic range limitation is one of the scanner,
not of the Kodak YCC color space. Images from photomultiplier
tube scanners that are converted and stored in YCC format do
not have a dynamic range limitation.
