Along these lines, I recall posts where folks were grinding their own GG's. Just curious, how do folks decide which grit to use? It would 'seem' the finer the grit the brighter/sharper the image, but there must be some practical limit. Anyone know what that is?
The way Sinars and Satin Snows grinds their glass is a trade secret. But the finer the grit the smother the focusing surface.
Richard T Ritter
www.lg4mat.net
I can't remember but there's a current thread on this and the guy is offering to make a few GG for free. I believe he "starts" with 60 grit and moves finer in steps. But I'm looking for something with no scratches nor pits of any kind.Originally Posted by bobwysiwyg
I previously posted about using acid etched glass which I may still do.
Hmm, seems almost counterintuitive to take a smooth piece of glass and have at it with a course grit, which 60 seems, and then work down to fine. Why not go straight to the fine even though it could take a while??
Re: making your own ground glass: http://www.dokasphotos.com/techniques/ground_glass/
Very interesting, thanks. I think I have another thing to try some time.
There is a tradeoff between the fineness of the grit used, the brightness of the screen and the resolution of the screen. The principle here is that you need a light scattering surface to generate a real image. Anything that will scatter light will do the job hence the many suggestions. In emergency situations I've used strips of frosted Scotch tape - pretty good actually - and you can leave a bit of space between each strip to focus precisely on the aerial image there.
There is not any particular grit size commonly used but the range from coarse to fine screens is made from about 180 grit (76 um average) for a coarse screen to perhaps 800 grit (12 um average) for a fine screen. During the grinding operation the grit, flying freely in water is crushed against the glass causing fractures in the surface of the glass. These fractures are what scatter the light. The scattering from each fracture is at different and random angles as a function of the lapping conditions, such as force applied, lapping speed and the material of the lapping block. These conditions, as well as grit size, have an effect on the brightness of the screen. In general as the grit gets smaller the screen is less bright.
When the grit size is too small say less than 2 to 5 um the damage to the glass is reduced to the point where the grit acts as a polish, with light enough pressure, and no scattering occurs and no real image is formed.
As Richard implies above, all screen producers have their own little tricks for maximizing brightness and resolution at the same time, but there are definite limits to what can be done, so we employ Fresnel screens and a few other tricks to improve brightness.
Nate Potter, Austin TX.
I'll add my .02 here, you're going to have a huge problem using hot glue, it's formulated to be a certain viscosity
at it's melting temperature, the only way to change that is to start messing with flow modifiers
and adding waxes, I know that because I worked on an animated commercial that required a translucent
hand prop, we wound up using 3M Jet Melt hot glue and we added wax and flow modifiers to get it to cast
properly. On top of that depending on the glass thickness and type I would worry about thermal shock
putting a room temp piece of glass on top of hot glue that may be past 200ºF may crack it or the glue
will form flow lines from contact with the cooler glass as it spreads out.
An oven would be better, heat both pieces of glass at the same time as well as your mixture.
I'm curious as to what makes the Boss <sp?> focusing screens so great, any pictorial examples comparing it
to fresnel/GG and plain GG ?
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