Advantages of a Drum Recorder, When Compared with a Capstan Imagesetter

Here with some comparisons of the drum recorder with a typical capstan imagesetter, used for the production of four color process film.

The alternative to the capstan imagesetter (which moves the film down a path, using rollers) is the drum recorder where the film remains stationary. It is a proven fact that the drum recorder is more accurate than the conventional recorder, however since we are only considering the width of a hair, the difference may not be significant for a particular application.

The concept behind a drum recorder can be visualized by giving an example. Imagine, if you were to take a 55 gallon drum, slice it in half, lay the media on the inside, (or the outside), of the barrel and expose the film (or plate) to a light source, you would have the first part of the basic concept of a drum recorder.

The question is: to do work, something has to move. In an internal drum recorder, what actually moves is the spinner or reflector mechanism. The dots that make up the artwork are produced by firing a laser beam against a mirror, as the mirror turns, it deflects the laser beam onto the imaging surface. The laser beam is then turned on or off as necessary to create the image.

The issues that you deal with in internal drum technology are really pretty interesting, because the basis of the technology has been around for the better part of fifteen to twenty  years. One of the major aspects of doing it this way has to do with accuracy. So, for this reason, we have had to wait until the last few years for the science and engineering to advance to the point where a sufficient level of technology increase became available that would permit manufacture with the precision necessary to do high quality graphics.

In designing a drum recorder, you have to decide how you will move the carriage back and forth and how you will spin the mirror with the least vibration. From a technology point of  view,  how accurately you can  move the carriage back and forth and how accurately you can spin the mirror.

For instance, if you take a motor with a mirror on it which is spinning very fast and you direct a laser beam onto it which  in turn is reflected onto a drum surface, you will find that, if the motor is spinning exactly right, you will write a nice straight even beam across the imaging paper. However if the motor, when turning has what is called a "wobble characteristic", causing the mirror to move around a little bit and therefore not spin exactly on it's center, the mirror gets deflected somewhat and you end up bouncing the beam, so that it does not hit on the drum where it is supposed to hit.

This  wobble  characteristic  of the motor is one  of  the  prime reasons influencing what we call "pixel placement accuracy". (How accurately you can place a specific dot on the actual surface of the drum).  If there is any combination of vibration or random wobble to the motor, you end up with a beam that writes inaccurately as you go across the area of the drum.

The technology to control this type of reflector is really dependent upon the quality of the bearings on the motor and the type of motor. You have to be able to measure the degree of precision of the motor so that you know how much deviation is present
when the laser beam is deflected.

For example we are talking such high technology that this spin motor had to be developed using aerospace technology! I know for a fact that the motor used inside the Agfa SelectSet was based on the  "Gyroscopic Ring Laser Navigational System" from the military. A system which had to meet some amazing requirements for accuracy of rotational capability!

In  past times, one of the drawbacks of internal drum recording used to be the wide variation of the laser beam caused by inaccurate 
motor technology. This could be reduced by not permitting the beam to travel too far. Manufacturers of some of  the older drum recorders reduced the possibility of error by making it necessary for the beam to cover a wider angle.

Compared with this, many modern drum recorders wrap less than 180 degrees. Some others have to wrap 270 to 300 degrees
to image the same  size.  One of the primary reasons for using a  platform  of more  than 180 degrees, has to do with motor wobble  
characteristics.  If you physically make the beam travel smaller  distances, you reduce the visual effect of a poor motor condition that will move the laser beam all over the place.

For  example  in some modern drum recorders, the  laser  beam  is stationary,  with  the imaging done through  mirrors,  which can move varying distances from the light source. Since the laser  is considered to be a very high energy coherent light source, changing the distance of the light source to the film, does not result in  a  loss of power, according to exponents of  this  particular configuration, but, which may be contrary to the opinion of  others.

The energy of a laser source will vary over time, but in the case of a helium-neon laser, the variations will be extremely small,
amounting to only fractions of a fraction of a milli-watt, over the course of a day. Deviations are so small, that they do not show up in a graphic arts application.

If you run a density check once a day, you may find slight variations, which you will find cannot be analyzed as caused by variations
of the laser. Film emulsions, chemistry, batch numbers will generally have a greater effect upon variations of the exposure.

Some manufacturers claim that locating a laser on the spinner motor can influence exposure levels. Not everyone agrees with this.
Most interesting, is another claim that mounting the laser light source on the traveling carriage will reduce vibration. An idea that violates one of the fundamental laws of physics, in as much as, "if something moves, it is next to impossible to move it without vibration".

According to some manufacturers, if you have an  imaging device translating in one direction, rotating in another, with a laser source on top of all this, you are guaranteed that things will be vibrating all over the place.

This is not to say that you cannot engineer it correctly, but to make the statement that by placing a laser source on the carriage reduces the impact of vibration more effectively than other methods is considered by some engineers to be a real fallacy.

Conclusion: Not all drum recorders are created equal, although most will execute a job very accurately. Some of the earlier drum models, which have a shorter laser path, may be unable to compete as effectively, in accurately laying down dots for four color work.

Fred Simper. May be reproduced in full at no charge, with credit.