Hey I just purchased an Elmo A305-SD OHP. The singlet lens definately can use some improvement by going to triplet lens.
I've seen posts about 135mm triplet. However, I don't know which focal length I should get. I also have another 3M 9700 projector that has a triplet, should I swap it out?
Then the problem is I don't know how to retrofit the lens since Elmo doesn't have a moveable arm.
I've seen posts about 135mm triplet. However, I don't know which focal length I should get. I also have another 3M 9700 projector that has a triplet, should I swap it out?
Then the problem is I don't know how to retrofit the lens since Elmo doesn't have a moveable arm.
Hey I got this same projector I end up taking all the internal out and making a wooden box. I am still waiting for my lens and I am going to get a wuxga of ebay.
OHP lens
Most OHPs use a projection lens with a focal length around 320 mm. If you change to a lens that is very different from your current lens, then you might have to change the fresnels to match it, to get a bright image. You would also have to change the height of the arm that holds the lens assembly.
You can figure out your current lens fl, if you want:
Focus an image of a transparency or LCD on your screen.
Measure distance1 from that object to the middle of the lens.
Measure distance2 from the middle of the lens to the screen.
1/lens fl = 1/distance1 + 1/distance2
(Use the same units for all.)
Most OHPs use a projection lens with a focal length around 320 mm. If you change to a lens that is very different from your current lens, then you might have to change the fresnels to match it, to get a bright image. You would also have to change the height of the arm that holds the lens assembly.
You can figure out your current lens fl, if you want:
Focus an image of a transparency or LCD on your screen.
Measure distance1 from that object to the middle of the lens.
Measure distance2 from the middle of the lens to the screen.
1/lens fl = 1/distance1 + 1/distance2
(Use the same units for all.)
What a nice hint!
I was trying the triplet from 3M 9700 projector and found although it focuses well, it's too far away from the LCD panel, making the image bright only in the center. Elmo's singlet is however much closer to the screen with a much brighter and even image.
Giving that I cannot adjust the distance between the fresnel and the light source in the OHP, I'll need to find a triplet that matches the focal length of the current one as you mentioned, right?
I was trying the triplet from 3M 9700 projector and found although it focuses well, it's too far away from the LCD panel, making the image bright only in the center. Elmo's singlet is however much closer to the screen with a much brighter and even image.
Giving that I cannot adjust the distance between the fresnel and the light source in the OHP, I'll need to find a triplet that matches the focal length of the current one as you mentioned, right?
getting a bright image
You have to get all of the light into the projection lens. The distance between the LCD and the projection lens is determined by the lens focal length and the throw distance. If you want a particular size image, you are stuck with one particular LCD to lens distance. So then you have to use the fresnels and the lamp to fresnel distance to adjust the position of the lamp arc image.
You could change the fresnels, or you could make the lamp to fresnel distance adjustable to accomidate a different projection lens.
It should not be difficult to find an OHP triplet you can use. Figure out the fl of your Elmo lens, and then look on eBay for OHP triplets. I have seen new aftermarket upgrade triplets there for pretty cheap.
You have to get all of the light into the projection lens. The distance between the LCD and the projection lens is determined by the lens focal length and the throw distance. If you want a particular size image, you are stuck with one particular LCD to lens distance. So then you have to use the fresnels and the lamp to fresnel distance to adjust the position of the lamp arc image.
You could change the fresnels, or you could make the lamp to fresnel distance adjustable to accomidate a different projection lens.
It should not be difficult to find an OHP triplet you can use. Figure out the fl of your Elmo lens, and then look on eBay for OHP triplets. I have seen new aftermarket upgrade triplets there for pretty cheap.
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