# projection lens distance

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#### sharp81

I have a fresnel lens with fl=24 cm and the projection lens has fl=31.5 cm

Can you tell me at what distance shld the projection lens be placed in order to get a throw of 360 cm frm the projection lens to the screen. Also does the projection lens need a diameter of atleast that of the lcd panel.

sharp

#### Rox

sharp81 said:
I have a fresnel lens with fl=24 cm and the projection lens has fl=31.5 cm

Can you tell me at what distance shld the projection lens be placed in order to get a throw of 360 cm frm the projection lens to the screen. Also does the projection lens need a diameter of atleast that of the lcd panel.

sharp

if you have 24 focal projection lens and need 360cm throw, then;

1/24=1/360 + 1/X X=25,7cm is the separation form lcd to the projection lens. (tis is meassured from the optical center of the projection lens, lets say fisical center by now)

now you have 31,5cm focal lens fresnell (only one?, by now i would think it is the field fresnell) you need to do some trick so full light enters the triplet, for instance you can place it 20mm gap from the lcd, unsplit, and then move the lamp something farther so the light will be focused at 25,7cm from the lcd or 27,7 from the fresnell.

luck

#### sharp81

do i need to have fresnel lens. I thought just one fresnel lens will do the trick.I have placed the fresnel lens at its focal distance and the projection lens is placed at the distance calculated from the formula.

Btw what is meant by upsplit/split fresnel lens.I have placed a mirror above the fresnel lens. Can you tell me what angle it shld be placed at and what shld be the size of the mirror to be used.

regards
sharp

#### Rox

maybe you have a dual fresnell lens like this

An externally hosted image should be here but it was not working when we last tested it.

they are glued but there are 2 fresnells actually, first collects light from the bulb (located at focal point) and does it paralell rays, then the second fresnells collects this paralells rays and does it focus on the second focal point where the projection lens should be located.

when we split them (split=separate) we can use the lcd between them, so paralell light will go throw the lcd (this is a good idea because the lcd will be more uniformly iluminated). But take care about the groves (concenytric circles), if you place the fresnell too close to the lcd, then the circles will apear on the projected image as if they were in the lcd. You need to place it 20mm separated from the lcd, so when you focus to the lcd, the groves won´t be focused and you won´t notice them on the projected image.

Where did you take your fresnell from?

#### Lentroo

I didn't quite follow Rox's calculations. If the projection lense has FL= 31.5 cm, the distance beween the lcd and projections lense needs to be X=(31.5^-1 - 360^-1)^-1 = 34.5 cm
To make the light converge at 34.5 cm, you need move the frensel and the lamp closer to each other. If you hold a piece of paper in the light path, you can see where the light converges to a point. That point should be at the same point as the projection lense so that as much light as possible flows through the projection lense.

As a side note, I think you will get a too large projection at 3.6 meters. I think about 2 meters would be enough, but it depends on your projection lense.

#### sharp81

The following picture even though not great gives some idea abt my design.I have just used one fresnel lens and a projection lens.

I have one more question if i use 2 fresnel lenses then the distance from the 2nd fresnel lens to the projection lens must be the focal length of the second fresnel lens and it cant be the 34 or sth like that which has been calculated.

I dont know after seeing ur suggestions that just a single frenel lens is going to work.....though i hope it does

regards
sharp

#### Attachments

• projection.jpg
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#### Rox

sorry, i messed the fresnell lens focal and projection lens focal,
lentroo is right.

1 fresnell won´t work if you place the light at its focal point, then the output will be parallel and only the "shadow" of your projection lens will be projected on the wall (shadow=mean only the area that is under the projection lens).

a shomehow solution is to place the bulb somewere at 77cm from the fresnell, this should focus the light at the projection lens (this solution is a temporal one until you get another fresnell.)

#### sharp81

I have one more fresnel lens but that has a very short focal length of just 14 cm. and the problem is i have already made the box and have the distance made for 34.5 cm ..... will i have to redo the box or is the distance from the second fresnel to the projection lens can be kept at 34.5 cm

regards
sharp

#### Lentroo

Are you sure your fresnel lense isn't really two ones glued together?

As I said before. To get a focused image at 3.6 meters, the projection lense has to be 34.5 cm from the lcd.
To get a bright image, you have to make sure the light from the fresnel converges at the projection lense. Experiment with different distances from the lamp so see if it is possible.

#### sharp81

Does the grooves portion of the fresnel lens need to face the lcd or the light source in the configuration.

regards
sharp

#### Lentroo

It probably makes no differance since you only have one fresnel. When you have two, the groves should be facing each other.

#### sharp81

I have been caught up with one more problem.This is to do with the light source. I am using a series of white LED's obtained from Roithner.

I have to paste them on the heat sink and it isnt sticking in.I tried applying the heat sink paste but it hasnt helped much.

Thank you

sharp

#### sharp81

I am little worried right now that my dimensions might have gone wrong as well.

I have a mirror that is placed at an angle of 45 deg.The size of the mirror is 15 cm.

My fresnel lens is also about 15 cm in length.The edge of the mirror is abt 2 cm from the fresnel lens.

So with this arrangement at what height shld be my projection lens be placed from the fresnel lens so that maximum light is captured.

I will try to make a diagram and post it here...