DIY Projection Lens Triplet - diyAudio

long throw projector

Dee:

First of all, I would not recommend a 122" image. I think that will be too large to view comfortably. (Your neck gets tired after a while from moving your head back and forth ???? ) But I will show you the math, so you can work it out if you really do want 122".

Here is the optical geometry for a 100" image, assuming a 16' throw distance and a 15" LCD:

M is magnification
A is the LCD to projection lens distance
B is the projection lens to screen distance
FL is the focal length of the projection lens

M = 100" / 15" = 6.67

FL = B / (M + 1) = 16' / (6.67 + 1) = 2.086' = 636 mm

A = B / M = 16' / 6.67 = 2.40' = 731 mm

So you would need a 636 mm fl projection lens. You can try making that from a pair of positive meniscus lenses using this:

EFL is the effective focal length of a multi-element projection lens
FL1 is the focal length of the first simple lens
FL2 is the focal length of the second simple lens
D is the distance between the lenses

FL = (FL1 * FL2) / (FL1 + FL2 - D)

If you space them too far apart, then you limit the field angle too much. Eyeglass lens blanks come in 1/4 Diopter increments.

Diopters = / focal length in mm

So you can get +1.0 Diopter ( mm fl) and +0.75 Diopter ( mm fl) lenses. The large steps make it difficult to make specific long-throw projection lenses from eyeglass blanks. So let's see what happens with a mm and a mm lens with 30 mm of space:

EFL = ( * ) / ( + - 30) = 578.81 mm

Feeding this back into your room and LCD size constants:

M = (B - FL) / FL = 7.43
Image size = 7.43 * 15" = 111"

Now we can recalculate A for this lens:

A = B / M = 16' / 7.43 = 2.15' = 656 mm

So that tells us you need about 656 mm from the LCD to the projection lens. If you use a split design, then you would put a (656 - 20) = 636 mm fl fresnel 20 mm after the LCD. If you use a non-split design, then you would put a (656 + 20) = 676 mm fl fresnel 20 mm before the LCD. These long throw fresnels may be very difficult to find! Lumenlab.com used to sell a "790 mm" fl fresnel (that I measured to actually be about 770 mm fl). You could ask them if they have any left. They were changing to a shorter focal length (650 mm?, 600 mm?), for use with a 450 mm fl triplet, so they may have a "790" they could sell you. Even if they don't, you could still use a 650 or 600 mm fl fresnel by adjusting the distance between the lamp arc and your condensor fresnel.

So the short answer is: Keep the 220 mm fl fresnel. Find a 770 mm field fresnel. Buy +1.0 and +0.75 Diopter eyeglass lenses and put them in a tube. The Rolyn lenses that tgreenwood mentioned only go down to +1.0 D in the acrylic. You will have to buy a glass +0.75 D. This will give you an image that is good enough for watching video, but a bit blurry around the edges for running windows. If you want a sharp image from corner to corver, you will need to find a triplet or tessar lens designed for somewhere in the 550 to 650 mm fl range.