Full Version: Hami Using 3 Fresnels.
I was researching hami projectors on the web and found this link to a Russian guy who built a really small folded hami projector using a 330/220 fresnel combo before the lcd and a 220 after to a 200fl triplet. Can someone tell me what exactly does this setup acomplish? On page 8 he shows a diagram of his build.
QUOTE (GLAPPE @ Jul 3 2006, 10:36 AM) 
I was researching hami projectors on the web and found this link to a Russian guy who built a really small folded hami projector using a 330/220 fresnel combo before the lcd and a 220 after to a 200fl triplet. Can someone tell me what exactly does this setup acomplish? On page 8 he shows a diagram of his build.
well, if he has the 330 and 220 together the focal length of the pair will be a combination of the 2 different fls, theres a formula somewhere, but a rough guess would be 250-275mm??(but I don't know what it actually would be) .....so its the same as having a single fresnel of that FL(or whatever it happens to be)
although I think with a small hami like that you can use a fresnel with <200mm Fl, I've seen people use 180mm FL fresnels with the hamis(gets more light than a 220mm Fl, plus with the smaller lcd the vignetting will be ok, with a larger lcd, the vignetting(corener to center light rtio, light eveness) would be worse with a 180 than a 220
Stack fresnels work with the following formula
1/a + 1/b = 1/c
1/220 + 1/330 = 0.0045 + 0.0030 = 0.0075 => 132mm
stacking reduces the focal length, this is possible but you move the bulb (heat source) closer to LCD and since we don't really have point source light this limits the ability of the fresnel to capture light from the steeper angles. This all can work but will take a fair amount of experiemantation to make all the trade offs.
1/a + 1/b = 1/c
1/220 + 1/330 = 0.0045 + 0.0030 = 0.0075 => 132mm
stacking reduces the focal length, this is possible but you move the bulb (heat source) closer to LCD and since we don't really have point source light this limits the ability of the fresnel to capture light from the steeper angles. This all can work but will take a fair amount of experiemantation to make all the trade offs.
QUOTE (pagercam @ Jul 4 2006, 03:08 AM)
Stack fresnels work with the following formula
1/a + 1/b = 1/c
1/220 + 1/330 = 0.0045 + 0.0030 = 0.0075 => 132mm
stacking reduces the focal length, this is possible but you move the bulb (heat source) closer to LCD and since we don't really have point source light this limits the ability of the fresnel to capture light from the steeper angles. This all can work but will take a fair amount of experiemantation to make all the trade offs.
1/a + 1/b = 1/c
1/220 + 1/330 = 0.0045 + 0.0030 = 0.0075 => 132mm
stacking reduces the focal length, this is possible but you move the bulb (heat source) closer to LCD and since we don't really have point source light this limits the ability of the fresnel to capture light from the steeper angles. This all can work but will take a fair amount of experiemantation to make all the trade offs.
well, obviously I was way off
and that explains why he is using that combo for the small screen....
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