OHP fresnel lens's FL for a 15" LCD
I have found somewhere a fresnel lens for OHP which has these specs:
dimensions: 310 mm x 310 mm x 5 mm
effective focal length: 125.5 mm
object distance: 182 mm
image distance: 406 mm
facet spacing: 0.508 mm
Is it good enough for a 15" LCD projector?
It is a really good deal for it in my country so I'm tempted to buy it.
Are these object/image distances the FL's of each split fresnel in the compound? If it is, is it too close to the lamp for a proper DIY projector?
Thanks in advance
I have found somewhere a fresnel lens for OHP which has these specs:
dimensions: 310 mm x 310 mm x 5 mm
effective focal length: 125.5 mm
object distance: 182 mm
image distance: 406 mm
facet spacing: 0.508 mm
Is it good enough for a 15" LCD projector?
It is a really good deal for it in my country so I'm tempted to buy it.
Are these object/image distances the FL's of each split fresnel in the compound? If it is, is it too close to the lamp for a proper DIY projector?
Thanks in advance
good one
Yes, that would be a good pair of fresnels for a non-split design. It is large enough to cover a 15" LCD, but no bigger. I would use it with a projection lens in the 300 - 320 mm fl range.
Yes, that would be a good pair of fresnels for a non-split design. It is large enough to cover a 15" LCD, but no bigger. I would use it with a projection lens in the 300 - 320 mm fl range.
Thanks Guy, I´ve seen a lot of your posts on optics and you seem to know your stuff about this.
I will close the deal, I hope it really is a good quality fresnel.
Just reinforcing the second question from the first post:
Are these object/image distances the FL's of each split fresnel in the compound? If it is, is it too close to the lamp for a proper DIY projector?
Thanks again
I will close the deal, I hope it really is a good quality fresnel.
Just reinforcing the second question from the first post:
Are these object/image distances the FL's of each split fresnel in the compound? If it is, is it too close to the lamp for a proper DIY projector?
Thanks again
object distance: 182 mm
image distance: 406 mm
Yes, these are the focal lengths of the individual fresnels. They may be glued or ultrasonically welded together, so you may have to use a non-split design. That is fine, as long as you don't need to do optical keystone correction.
The pair of fresnels act like a single thin lens, if they are close to each other. The effective focal length is calculated as:
1/efl = 1/fl1 + 1/fl2
The result is very close to the 125.5 mm figure they advertised.
What this means for a DIY projector, is that if you put the lamp arc 182 mm from the fresnels, then they will focus the arc image 406 mm away. But you are not stuck with those distances. If you move the lamp arc closer, then the arc image will be farther away, and vice versa.
You should use this kind of adjustment to focus the arc image right in the center of your projection lens. Let us say you go with a 320 mm fl projection lens and a throw distance of 2.4 meters. We can calculate the LCD to lens distance:
1/320 = 1/2400 + 1/x
x = 369 mm LCD to projection lens
If the fresnels are 20 mm before the LCD, then you need a fresnel arc image distance of 389 mm. That is very close to the focal length of 406 mm, but we can calculate where to put the lamp arc to get it exactly right:
1/125.5 = 1/389 + 1/y
y = 185.3 mm lamp arc to fresnel
I think this pair of fresnels will work very well for you. Since the condensor fresnel has a short focal length (compared to the standard 220 mm), it will be closer to the lamp arc. Applying the inverse square law to the lamp to fresnel distances, it looks like you will get 16% more light than you would with a standard 220/330 fresnel set. The 185 mm distance if fine for a DIY projector, but you should use IR and UV filters to protect your fresnels and LCD.
image distance: 406 mm
Yes, these are the focal lengths of the individual fresnels. They may be glued or ultrasonically welded together, so you may have to use a non-split design. That is fine, as long as you don't need to do optical keystone correction.
The pair of fresnels act like a single thin lens, if they are close to each other. The effective focal length is calculated as:
1/efl = 1/fl1 + 1/fl2
The result is very close to the 125.5 mm figure they advertised.
What this means for a DIY projector, is that if you put the lamp arc 182 mm from the fresnels, then they will focus the arc image 406 mm away. But you are not stuck with those distances. If you move the lamp arc closer, then the arc image will be farther away, and vice versa.
You should use this kind of adjustment to focus the arc image right in the center of your projection lens. Let us say you go with a 320 mm fl projection lens and a throw distance of 2.4 meters. We can calculate the LCD to lens distance:
1/320 = 1/2400 + 1/x
x = 369 mm LCD to projection lens
If the fresnels are 20 mm before the LCD, then you need a fresnel arc image distance of 389 mm. That is very close to the focal length of 406 mm, but we can calculate where to put the lamp arc to get it exactly right:
1/125.5 = 1/389 + 1/y
y = 185.3 mm lamp arc to fresnel
I think this pair of fresnels will work very well for you. Since the condensor fresnel has a short focal length (compared to the standard 220 mm), it will be closer to the lamp arc. Applying the inverse square law to the lamp to fresnel distances, it looks like you will get 16% more light than you would with a standard 220/330 fresnel set. The 185 mm distance if fine for a DIY projector, but you should use IR and UV filters to protect your fresnels and LCD.
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