A Drum Scanner or a Flatbed?
Making the choice
Often we hear people say that the quality achieved from scanners using CCD technology is comparable to that of the drum scanner's photomultiplier technology. This is true for general prepress work, but not the case for people who really do need the best in quality.
Listed below are some of the reasons why CCD technology is still not capable of providing the high level of reproduction currently being offered by drum scanners.
CCD's can produce an effect called flare. This manifests itself as a halo in sharply contrasting areas adjacent to each other, such as a white bar against a dark background. As a consequence sharp edge definition is lost.
Photomultipliers in drum scanners capture a wider density range than CCDs in flatbed devices, they therefore 'see' a greater depth of colour. To compete with this, flatbeds may use software to artificially extend their density range, allowing them to quote density 4.0 and above in their sales literature.
Another characteristic of the CCD is how it can go 'blind' towards the end of its density limit with the result that noise is created in the scanned image. This shortcoming does not occur with a photomultiplier due to its superior capacity to resolve high density values.
With CCD density range restrictions there is also the risk that the CCD will not 'see' all the shadow detail in contrasty transparencies, and as a consequence not reproduce these areas fully.
In the majority of cases CCD scanners are limited to around 5500 dpi for true optical resolution, higher figures quoted are usually interpolated values. With drum devices, customers can enjoy the benefits of 12000 dpi optical quality resolution. Some people question the need for such resolving power, but the real "quality gurus" utilise this capability. Large file sizes of 1-1.5 GB are not unusual, archiving for long term future use also being a consideration, photo lab work, high end image retouching, and of course high quality large format output. Niche markets that demand the 'no compromise' culture.
Dust is very hard to avoid in any work place and is a constant enemy for the scanner operator. However the threat posed to a drum scanner tends to be less than with a flatbed. A spec of dust lying on a CCD sensor may cause streaks across the image in the scan direction. Effectively the sensor has a blind spot. A similar spec of dust situated on the drum scanner sensor i.e. the photomultiplier will be proportionally so small in size to the rest of the sensing area that its affect will be insignificant.
Just opening the lid on a flatbed scanner causes air to be sucked into the scanner bed area. Dust is therefore drawn into the place where it's least wanted. This isn't a problem with a drum scanner.
A Closer Look
To understand the differences between Drum and Flatbed scanners there is a brief description of their respective designs outlined below.
A typical drum scanner has a perspex cylindrical drum, inside of which is a periscope like tube and a lens, which face the analysis head. A fine point of light is emitted internally or externally to the drum depending on original type (reflection or transparency). The light source and lens assembly move down the length of the drum as it spins around at high speed, capturing data, line by line.
The light bouncing off or transmitted through the original image then passes via the lens, into a prism, where the light is split and directed onto the photomultipliers. They precisely convert light into electrical values. Once this stage is reached the electronic signals are then passed through a device which changes them into digital data.
A flatbed scanner works in strips unlike a drum scanner which works in fine points. The light source and read head are wide strips, with the illuminating lamp being the full width of the scanner bed.
The read head is the part that contains a set of three CCD arrays. Light is reflected or transmitted onto a mirror(s), then passed through a lens across the width of the CCD.
CCDs are transistor like light sensitive devices, emitting tiny electrical signals when light hits them, which are then converted to digital information.
The light source and mirror move down the flatbed and allow the CCD read head to capture the stationary image, line by line.
To conclude, CCD-based scanners can produce acceptable results, if not very good colour reproduction, depending on your requirements. However, it is clear that when you need to create the highest quality scanned image from film originals, a drum scanner is still the best tool to use.