# Several Design Question

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#### mattcosturos

Ok, so I think i've got a pretty good handle on everything, but just wanted to ask a few questions to clarify.
The focal length of the field fresnel should be close to that of the projection lense correct?

The FL determines how far we put the projection lense from the LCD (D), and along with this, the distance from the lense to the screen (T) determines our focus.
if 1/FL = 1 / D + 1 / T we should get a focused image correct?

Is the D in that equation the distance from the LCD to projection lense, or from the field fresnel to the projection lense?

I made an excel sheet that will let you enter focal length of projection lense, desired throw distance in feet. and lcd size in inches.
It will calculate the distance the lense needs to be from the lcd screen, and also how large the final image will be.
Someone want to take a look at it and make sure i didn't make any errors.

I currently have a Dukane 4003 OHP and a 5" PSone LCD.
I wanted to make a smaller portable projector, so im thinking of squeezing a pico projector design into an old computer case.
Im going to go home and measure the FL of the fresnel and projection lense on my OHP and if they are suitable, i will start building.
Since i want to make this as small as possible a 220 fl projection lense would be ideal correct?

#### Attachments

• projsize.zip
2.8 KB · Views: 43

#### Rox

the excel is fine, the maths under it are correct.
just wonder, do you know where to get that 220mm focal projection lens?

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