We usually print @ 55 lpi or 65 lpi depending on the artwork. As for losing the dots at the low end of the spectrum that is only one of the problems you can actually 'fill in' the dots at the higher end as well say from 90 percent to 100 percent where 90 percent fills in and become 100 percent even though it shouldn't be. A lot of how good you can hold the dots at both ends of the spectrums depends on many factors among them are: the accuracy of your output device and rip, the screen mesh that you are printing through, the type of light source you are using to burn your screens. Another factor that should be considered is how you have photoshop set up to deal with dot gain. If you have a densitometer you can do an pretty good test by doing the following: make up a set of squares that goes from a 1% dot all the way to 100% print this out on your rip (at any lpi that you want to) and then burn a screen and expose it. You will see where you are not holding the dots correctly at both ends of the spectrum just by doing this (ie 5% becomes totally washed out). You can then create a set of custom curves to apply to your channels seps in photoshop that will effectively adjust your 5% dot to 0% and say your 92% to 100%. Once this is done you can take this same file and re-output it with this custom curve. It should now allow you to have a percentage difference across the spectrum from 1 - 100%. To take it all a step further you will need a densitometer. Take the adjusted screen and put it on the press and print it. You can now take the densitometer and measure your print across the spectrum to further refine your custom curves. For instance if you read the 50% box and find that with dot gain from priniting it is now showing a 55% fill patern, you can pull back the curve 5% at the 50% part of the box (so that your rip is giving you a 45% dot at 50% percent and test again to remeasure. It can be a lot of work, but you really can refine this if you want to. Reality is that most sim process prints will look good if you are holding in the range from 5% up to 93% or so which you can figure pretty well by doing the above tests without a densitometer, if you happen to be lucky enough to have a meter though you can really tweak it all in. Once you have your curves set you can then use those curves to adjust your final seps prior to output or some rips will allow you to have them use a custom curve during the output (which will allow you to view the sep on screen the way you want to without having the custom curve affect the way it looks on screen, but yet have it applied when you do the actual file output).
Why do all of this? Why not use someone else custom curves? You will find that while using another's set of curves may work for you it may not too - the variables are too high - screen tension, mesh counts, point exposure light source, and even your press and off contact can all effect this so having your own set is the best route as it will be set to your conditions. Understand though that if you don't use newman's and keep your screen tensions the same and the variables under control then it's probably not worth the time to do all of the above testing - it only takes some slight variation and it could throw your curves off. Most printers find that they get acceptable results for most clients without ever doing all of the steps I mention here, but I thought you might be interested in what is attainable and how to get there though, so I explained it.
Hope this helps, let me know if you have any more questions or I can help further.
For more information on index printing - try google for index separations - you will get a bunch of stuff - it's really the separation method rather than printing it that makes it an index print. Index usually requires more colors (and screens) to get the same look you will with less in the channel separation using halftones.
Thanks,
Dave
