[RobAndJonK]

Right i was reading in some other thread about pre condensor lenses, and how you place them between the bulb and the first fresnel to allow a greater arc of your lamp to reach the fresnel lens, therefore giving you more light output.

This got me thinking.... And i want all your opinions..... .......

What would happen if we placed to 220mm rear fresnels on top of each other?

After all lens are additive are they not? So placing two on top of each other should reduce the focal length, and allow you to get your lamp ALOT closer to the fresnels, increasing your light output.....

Now i know about the heat issues, but forget these for a minute, would this idea work?

[mikyd1954]

RobAndJonK, on Nov 8 2005, 01:29 PM, said:

Right i was reading in some other thread about pre condensor lenses, and how you place them between the bulb and the first fresnel to allow a greater arc of your lamp to reach the fresnel lens, therefore giving you more light output.

This got me thinking.... And i want all your opinions..... .......

What would happen if we placed to 220mm rear fresnels on top of each other?

After all lens are additive are they not? So placing two on top of each other should reduce the focal length, and allow you to get your lamp ALOT closer to the fresnels, increasing your light output.....

Now i know about the heat issues, but forget these for a minute, would this idea work?

I did this with my psone(smaller screen it will be much better for) and yes it does work..not sure how optimal it would be for a 15" though, I believe it was rox who showed themath saying that yes it would be more light, however the differences between the corners and the center(as far as brightness) would be even more exxagerated than with a normal set up and on a secondary but similar note, placing a 330mm behind the lcd would even out the brightness but lower the total lumens gathered...so 220 is a good tradeoff point... however, if you used 16:9 exclusively it might be worth trying anyway since you wouldn't be concerned with the corners anyway ... maybe a couple of 330mm might be a better option

This post has been edited by mikyd1954: 08 November 2005 - 07:36 PM

[RobAndJonK]

Cheers for the quick reply!!

Just for everyone else....

A simple picture


 untitled.JPG (22.35K)
Number of downloads: 52



Glad some one has actually tried it!! And yes there would an issue at the edges of the image, but only if your fresnels are close to the size of LCD. If they are larger, and your triplet is a large diameter then you should be able to squeeze more light through

[mikyd1954]

RobAndJonK, on Nov 8 2005, 01:40 PM, said:

Cheers for the quick reply!!

Just for everyone else....

A simple picture
attachment
Glad some one has actually tried it!! And yes there would an issue at the edges of the image, but only if your fresnels are close to the size of LCD. If they are larger, and your triplet is a large diameter then you should be able to squeeze more light through

well, yes, those things would help but I think its a direct consequence of the inverse square law, not much you can do about it, no matter the size of the fresnels the corners are still going to get much less light(and its a relative differnce only, yes, the corners would be brighter compared to the corners of a straight 220mm lens, but its the relative brightness between the corners and the center that matters), but like I said, if you do 16:9 most of the time its definitley worth a shot, most of the stuff I watch is 16:9 so I don't really notice the 4:3 corners at all, you will also lose at least 10% of the total light by adding a second fresnel...

[SupraGuy]

Okay...

From what I understand, here's the way to get the effective focal length of 2 lenses stuck together.

1/(1/F1 + 1/F2 + 1/F3...)

In this case: 1/(1/220 + 1/220) = 110mm

Now, there are 2 problems....

1. Fresnel lenses are really intended to have collimated light coming into or out of the grooved side. Non-collimated light ends up casthign shadows because of the grooves. This will signifigantly decrease the transmission of the lens. Most likely to the point of negating any gain you get from it.

2. Dim corners. [James Doohan voice]Ye canna change the laws of physics![/James Doohan voice] This is a natural and unavoidable consequence. What matters is the ratio between the distance from the lamp to the center of hte lens, and the distance from the lamp to the corner of the lens. Basic Pytahorean math: At 220mm, the center of the lens is 220mm away. (Duh!) but the corner is sqrt((220*220) + (190.5*190.5)) or 291mm away. The center is about 76% of the distance to the corner. This results in a decrease in brightness of roughly 42%. (76% squared leaves 58%) Now halve the distance to the center at 110mm. Distance to center = 110mm. Distance to corner = sqrt((110 * 110) + (190.5*190.5)) = 220mm. This results in a ratio of 50% and a decrease of brightness in the corners of 75%! This means that even if the rest of your projector is PERFECT, the corner will only be receiving 1/4 of the lux that the center of the image will. I cannot see a way that this could be acceptable.

[phutton]

Just place your lamp closer to the rear fresnal. This will give the benefit of greater arc and also more focused light at the triplet. Many of us have found out that a distance of around 200mm provides a much brighter image. I don't know what the ray tracing diagrams show, so if anyone can do that it would be greatly appreciated.

[DAZZZLA]

phutton, on Nov 8 2005, 10:46 PM, said:

Just place your lamp closer to the rear fresnal. This will give the benefit of greater arc and also more focused light at the triplet. Many of us have found out that a distance of around 200mm provides a much brighter image. I don't know what the ray tracing diagrams show, so if anyone can do that it would be greatly appreciated.

Basically moving the arc closer to the rear fresnel will collect more light from the edges of the arc but it is at the price of creating a larger arc image at the triplet. The 20mm or so that is reported is probably the limit any more and the arc image will be clipped by the triplet housing. It can be due to different distances to the screen as well.
If you want a quick ray trace done tell me your FL’s and distances from the LCD to the triplet and the gap between the LCD and fresnels. And if it is split or un-split.

DJ

[RobAndJonK]

Hey cheers guys, those are some really cool answers :-)

I hadnt actually thought about the inverse square fall off of light, and it convinces me that this isnt such a hot idea!!

Many thanks

Rob