Possible hole fix - Drip those holes away!

[jstrack2]

I am confused about why an aperture is being used. I know that it is for attenuation, but is it also to focus the beam? I am not sure if this is the way to go. How about using a colored plastic sheet to attenuate the beam? The material cost of this would be effectively zero and it would attenuate the beam without hurting the beam quality much at all or adding other significant complications (although this will reflect the laser so shielding from the dot that is reflected will be needed). Using an aperture creates multiple modes and increases beam divergence as Feign pointed out with the equation y=D((mλ)/d). Therefore using an aperture will always significantly diminish print quality.

Focusing the beam is a trickier matter. It is not possible to have a super long tight beam because of fundamental physics. The best way to get a tight long beam is to focus it then collimate it (have the light rays be parallel again). To do this we could just use two opposite facing identical convex lenses (or alternatively a convex and a concave lens). The focus on the laser could be adjusted to eliminate the need for the first lens so only one lens would be needed. I think that a convex or concave lens could be obtained for 25 cents pretty easily, and possibly even significantly cheaper. The laser would focus the beam, and then the external lens could collimate it so that you would have a tighter beam that has almost parallel rays.

However the beam will not stay super tight. If the beam is a perfect Gaussian (which it won't be so things will be worse so this is best case) then the beam will diverge by the equation θ = λ/(π*ω). In this equation θ is the divergence, λ is the laser's wavelength, π is pi and ω is the radius of the beam at its smallest point. So focusing the beam will cause it to diverge faster. For example, if λ equals 405 nm, and ω is focused down to 0.1 mm then θ = (405*10^-6)/(3.14*0.1) = 0.00129. The diameter a given distance (l) away from the smallest diameter is approximately d = 2*l*θ + ω. So it would be 2*300*.00129 + 0.1 = 0.874 mm. This is not that great. If the beam wasn't initially as tight then close to 0.5 mm beam diameter 300 mm away would be theoretically possible. Also there will be a big difference in detail between the top and bottom of a print which won’t look good. I think that this final diameter could be cut significantly further (close to in half) if the beam was slightly converging after leaving the second lens so that it had a minimum diameter half way to the max distance of 300mm. In this case then the middle of the object would be capable of the highest detail and the top and bottom would be the least. This could be pretty decent, especially for the basic Peachy.

Therefore if real small features are desired then either the beam must be focused closer to the target, or there must be adjustable focus. Focusing closer to the target could be done by either moving the laser optics with a platform that floats so as the resin rises the optics are always close to the resin (I believe this has been discussed some on the forum), or by just saying that only small prints can have very high detail (this may not be so bad, since prints will only need to be small in one of the three dimensions). Also multiple higher detail small parts could be printed and then glued together later.

Adjustable focus would allow for super high detail throughout the entire print. Perhaps a super cheap servo motor could control it. I don’t know if it could be included in the basic kit, but having it would be a big step towards ridiculously high quality prints.

So anyway does what I have just posted seem correct? It is definitely possible that I made one or more mistakes. Also sorry for this post being so darn long!