Top-down or bottom-up?
In stereolithography you project light onto a light-sensitive resin which solidifies when light hits it.
There are 2 possible directions to project light onto a resin bath.
In a top-down configuration, you project light from above the resin bath onto the surface of the level. This has a few advantages, because it requires not as many components as a bottom-up system. In a laser galvo system you need the same lasersetup of course, but the buildplate in a top-down setup is simply a metal plate which can be lowered (and raised) in a reservoir with resin. So the only thing needed is a reservoir with resin and then you lower the buildplate to just below the liquid level. At that moment you can project light onto the resin which solidifies. Printing another layer is done by lowering the buildplate a little more, waiting a little for the resin to settle and form a new layer of liquid resin above what you allready printed and then print the new layer. (sometimes it is handy to lower the buildplate more then the layer height and then raise it again, because of the viscosity of some resins. you do want to get a very nice even new layer of fresh uncured resin for your new layer). One of the big disadvantages of this system is that you need a lot more resin than in the bottom-up system. On the other hand this resin is not lost. You just need
In a bottom-up configuration, you have to project light onto the resin too, but you project through a vat floot. The vat has a transparent bottom and in the vat you poor some UV curable resin. A buildplate is lowered to form a very small layer between the vat bottom and the buildplate and light is used to cure uv resin. 1 Big advantage of this way of working is that you require a lot less resin to get started printing. In the early days of resin sla printing, resins cost easily about €200 /liter. So it was very expensive. Nowadays resin can be found for around €50/liter which is allready a lot more acceptable but still 2 to 2.5 times more then plastic filament. On the other hand you normally don't print huge objects using this technique.
A big disadvantage of bottom up design is that you get a problem with the resin sticking to the build platform. You cure resin between a vat bottom and a buildplate, but to which of these 2 objects will the resin adhere. It SHOULD stick to the buildplate and not to the resin tank floor. People nowadays use several tricks to make sure that resin doesn't stick to the vat floor. On glass of acrylic floors you can put a layer of PDMS (silicone like stuff used in the production of solat panels) which has non stick properties. Some people even found a way to create a vat without floor and then they mount a FEPsheet (some kind of no sticky polyester like material) onto the bottom under tention. While these tricks do work (and if done right they provide excellent quality), they do require a lot of extra efforts. With bottom-up projection you also get a problem with resin spilling. Due to gravity, resin which is spilled falls down..... but all your projectin stuff is under the vat, so you actually spill onto your dlp projector of other projection system making it messy and very difficult to clean. This can partly be solved by using a front face mirror and not direcly projecting, but then again these mirrors are hard to get and not exactly cheap (more about this in another blogpost)
So, all things considered, it looks best for me to begin trying with the most spectacular setup. A laser galvo system projecting a focussed beam of light downward onto a bath of liquid resin. If that would fail (and it can of course) then there is a lot of other stuff to be tried, but at least the most impressive version would have been tested.
NOTE: At this moment you can actually buy a $600 projector and a $600 kit and make a complete bottom up projection system which is proven to be capable of good results, but where is the real maker spirit in that, huh ?