Condenser And Fresnel Focal Lenght

[chasse]

I have read a lot and try to understand how condenser and fresnel focal lenght distance has to be match from the lighting source to the LCD panel.
The light is reflected through a condenser.The condenser have a small focal lenght say about one inch as i understand and have to cover the entire frensel to be effective.
Does the frensel focal lenght has to match the condenser focal lenght to be effective.

 

[Hezz]

No, only if the diameter of the condenser lens is the same as the fresnel. What is important is that the focal points of the fresnel and the condeser are at the same or nearly the same point.

Visualize two light cones with thier points being at the same point and facing opposite each other. The light coming from the condenser needs to come at the fresnel at close to the ideal angle for the fresnel to be the most effective.

Because you are not focusing an image this does not have to be perfect but the closer that you get it the more effective the light will be transmitted.

Hezz

 

[chasse]

OK
If i have a frensel with 7 inchs focal and a condenser with 1 inch focal there must be about 8 inchs betwween the two.
Am i correct?

 

[Karnage]

Not quite true

Even if the focal points of the two lenses are at the same point the angle of light from the condenser needs to be the same as the angle of light to the fresnel for it to be 100% efficient. Finding a condenser and a fresnel with exactly the same (focal length/size) is mundo difficult, and you'd probably end up getting something "as close as possible" but not perfect.

http://www.diyaudio.com/forums/showthread.php?s=&postid=278816#post278816

This is my "proof".

 

[Hezz]

Chase,

what Karnage says is true however you are on the right track. The problem is that with a condenser with a 1 inch focal length the angle that the light is shining out is a much greater angle than is ideal. It will work but there will be some wasted light. If you take the diameter of your fresnel and then go out seven inches to the focal point then draw two straight lines from the edges through the focal point and beyond this is the angular amount that the condenser should be projecting. In otherwards the diameter of the condenser should somewhere between the crossing lines. And the ideal focal point of the condenser lens will depend on the diameter of the condenser lens. What you have will work but some of the angle of light from your condenser may be too step and may be rejected by the outer edges of the fresnel.

Hezz

 

[jcbklyny]

To make things easier... just focus your fresnel on the condenser and not the bulb. The bottom fresnel IS a condenser. Placing a condenser lens into the light engine creates a pre-condenser.

So if your fresnel has an 8 inch focal length... place it 8 inches away from the condenser.

Condenser lenses have 2 focal lengths... back and front. That 1 inch focal length sounds like the back... whats the front? And how big is it>

 

[ace3000_1]

what Karnage says is true however you are on the right track. The problem is that with a condenser with a 1 inch focal length the angle that the light is shining out is a much greater angle than is ideal. It will work but there will be some wasted light. If you take the diameter of your fresnel and then go out seven inches to the focal point then draw two straight lines from the edges through the focal point and beyond this is the angular amount that the condenser should be projecting. In otherwards the diameter of the condenser should somewhere between the crossing lines. And the ideal focal point of the condenser lens will depend on the diameter of the condenser lens. What you have will work but some of the angle of light from your condenser may be too step and may be rejected by the outer edges of the fresnel.

i wouldnt get too specific hezz and karnage as it realy depends on the frensels capabilites to collimate light too, some are of a higher quality then others, also ring count matters also. But the theroy is right. The best thing to do is make up a test bench in the items u are going to use and get it to work that way, place your light engine (asuming u have a spherical reflector and condenser setup) say 9 inches behind the frensel and if its not an even light move the light engine forward or backwards in small increcments, but realy if u have a frensel thats 9inch focal and it works at 9inches with an uneven light on the top of the frensel at the sides, then move the condenser to alter the beam angle, there is no such thing as having this angle perfect to the other, u wont get an even light on the outer edges, the frensels arent that good that we use but good enough, and the only way to make the beam wider is with the condenser, its easy to set up and realy we had a guy in here who had trouble with this not so long ago, i dont know what he did wrong but he couldnt get it to work properly, mine works with or without any reflector or condenser, it just alters my brightness and thats realy the way it should work, but as i say its got to do with the frensels quality and also i might add placing the frensel at the right focal length from the light to the frensel.

Trev

 

[jcbklyny]

With my big 400 watt setup... when I place my fresnels to focus on the bulb... it works great, without a condenser. With the condenser in place and the fresnels focusing through the condenser I get bright round image on the wall. This is because you have a Rectangular fresnel focusing through a round condenser. Makes sense. When moved back, focusing on the condenser... the image goes back to the way it should be. Rectangular.

This is one of the reasons Ace recommends a Rectangular condenser. You can get away with more as far as spacing goes.

 

[ace3000_1]

This is one of the reasons Ace recommends a Rectangular condenser. You can get away with more as far as spacing goes.

especially on a 16:9 ratio lcd, here is a pic of the shape my condenser is,(same shape different one), although not fully rectangular it is more rectangular then round and works great, i have about 1/2 inch of over lap all of the way around the back of the lcd of light so not much is wasted and what is wasted will go back through from the far ends of the pyramid with the mylar reflecting it.

Trev

P.S. this isnt the same lens but similar, the one i have is abit more rectangular

 

[chasse]

JCB
The condenser i have is a 60mm bi-convex.
I have done some experiment with it.I put a light at 1 cm from it and i have a 20 cm (8 inchs)circular beam at 16 cm(6 inchs) in front of it.I want to use it with the same LCD that ACE3000 is using,7 inchs wide.

 

[jcbklyny]

A fresnel lenses collects light from a point source. When using a pre-condenser... all your looking to do is collect more of the light from the bulb and make a larger point thats focusing more of the wasted light towards the bottom fresnel. Thats why, when setup right, your image will be much brighter with a pre-condenser in place. The fresnels are getting help. This is also why people use spherical reflectors with our setups. A spherical reflector helps brighten the point source of the bulb. So everythings working together. The reflector is directing more light towards the arc of the bulb, the pre-condenser is able to collect more light from the bulb and focus it towards the fresnel... which is collimating the light and then directing it towards your projection lens.

Place your fresnel at it's focal length from the pre-condenser... and fool around with the position of the pre-condenser like Ace said. You should see results.