Not so. Sonora is NEGATIVE working plate which means that laser writes the image area. Same as AGFA Azura or Fuji Brilia Pro-T(any version). In my experience, most plates have less than half of total surface covered in image (to remind: positive plates have to remove all non mage areas).
Energy required (and therefore imaging speed) is in range with most chemical plates. Azura TU and TS need processing (without expensive developer - processing is done in long lasting gum arabic) after exposure which allows a long time between exposure and printing. Processless plates (which "develop" on press) typicaly have 24 hour image latency (time that plate producer can guarantee a plate will work according to spec) in dark dry place. If you plan to make the plates long before printing (like CtP as a service and courier overnight transport in my case), chem free plates are way to go. Completely processless plates are for people that have CtP in house or short time from plate imaging to printing machine. There is a lot of others factors to be counted in, also.
In short, negative working plates use less laser time then positive plates and non-image areas rarely make problems since they are not bonded to substrate.
Well, the actual issue of the laser life is much more complex that a simple negative vs. positive explanation.
1. In CtP systems that use laser head (Kodak Magnus, Trendsetter, Screen or Agfa w. GLV heads) the laser is turned on at all time during the imaging process. It does not matter if the plate is positive or negative, as the “on/off” effect is generated by opening or shutting a light valve, while the actual laser is turned on. In this type of CtP it is NOT TRUE that the negative working plates use less laser time then positive plates.
In CtP systems that use individual laser diodes (i.e. Kodak Lotem, some Screen and Agfa models) the laser diodes are indeed turned off while an area on a plate is not being imaged. In this type of CtP it IS TRUE that the negative working plates use less laser time then positive plates.
2. The second, more important, aspect of the laser life is what is the main reason behind the wear process. Contrary to the common belief, the time that the laser is in ON position is not the main wear factor. Much more important is a current drawn by the laser. In short: low current drawn for a longer time to image a regular process plate is not as damaging as a high current drawn for a shorter time to image a chemistry-free plate. In general any chemistry-free plate requires to draw a relatively high current to attach the emulsion to the base of the plate. On the other hand, the current drawn when imaging a traditional thermal plates in much lower.
This explains why sometimes users of Sonora, Thermal Direct, Azura, Eco-Max, Brilia Pro-T have to replace their lasers much quicker that users of traditional thermal plates.
In the end let me say that I really believe that the chemistry free plates are the way to go for many printers. At the same time it is good to know how does the CtP imaging process really work.
