# how best to focus and collimate light for DIY projector?



## danieruz (Apr 1, 2013)

Hi,
Lately I've been interested in making a DIY projector. Poking around on the WWW and seeing what others have been doing, often it seems that the overall light output ends up being quite dim, even with say a 400w metal halide bulb. 

Most DIY setups I've seen are using a fresnel lens to collimate light directly from the mh bulb to pass on to the LCD screen. I see a parabolic dish sitting behind the bulb to redirect some light towards the fresnel, but it seems a fair amount of the bulb's light output that radiates up,down, and to the left and right is still lost.

Being a novice in optics, I would like to ask the question: Is there an effective way to use reflection and lenses to redirect most of the bulb's light to the fresnel for effective collimation?

The difficulty, as near as I can tell, is that if the fresnel lens will be used to effectively used to collimate light for the LCD, then the light it's collinating needs to come from a point source at it's focal point, so just putting a larger reflector that pushes all the light in the general direction of the fresnel won't work since the light on the other side will be coming out at different angles, i.e. not in parallel, right?

Just had a thought: Would it work to add a parabolic dish that reaches further around the bulb (the bulb sits inside the dish). At the focal point of this dish will then be an additional diverging concave lens, which will then pass on all the light to the fresnel (this additional lens would also sit at the fresnel's focal point)

Does anyone know where to find sellers of such a parabolic dish that a bulb could sit inside of and or suitable lenses for this task?

Also, a loosely related question: If I tried to use an LED, the difficulty seems it's not a point light but rather a given area radiati ng light in a general direction; any way by using optics to bring the light to a point to pass on to the fresnel?


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## citruspers (Apr 11, 2013)

Why reinvent the wheel? I highly suggest you look into the way ERS (ellipsoidal reflector system) lights are built, those are the lights that are generally used for music and theatre shows. They're being used for more than 40 years now so I guess it's still a great way to focus light. A side diagram should give you a clear idea on how to design the reflector and lensing system.
One thing you may want to look into is a dichroid mirror system for the reflector, that way you only reflect visible light towards the lensing system and ultimately the projection, and allow the IR to pass through to a cooling area.


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## MikeAusC (Apr 11, 2013)

Projector Lights use a spherical reflector behind the bulb to focus it back around the filament and then onto the front lens.

If you want to use a large area LED e.g. a 50 w array, then you need a large, very short focal length aspheric lens, which will focus the image of the array onto the LCD.


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## danieruz (Apr 14, 2013)

Good tips, thanks!


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