Re: Projection Optics

From: Steve Eckhardt (skeckhardt_at_mmm.com)
Date: 03/24/05 

Date: 24 Mar 2005 15:47:25 GMT

In article <d1uj0u$1n9$1@singer.cent.gla.ac.uk>, magnatomdb@netscape.net
says...
>I have an application where I want to project (probably using an LCD
>projector) an image on a small screen (approx 10cm x 10cm) where the
>projector is approximately 6 metres away. To complicate matters the
>image will be projected on a screen at 90 degrees to the normal location
>of the screen (i.e. the projection must be reflected by 90 degrees). Is
>this possible (within a reasonable cost?) What type of lenses and
>mirrors would I require?
>
>This is for stimulus presentation in an MRI scanner, so my options are a
>little limited (i.e. no metal in the scanner room!)
>
>Thanks for your help
>Dave Brennan

I'm not sure what you mean by "a screen at 90 degrees". Do you mean that you
want the image to be in portrait orientation (longer dimension vertical)
rather than the usual landscape (longer dimension horizontal)? Or do you mean
that the image must travel 6 metres horizontally and then bounce off a mirror
onto a screen that is parallel to the floor (visible to a patient lying on
their back and looking up)? To do this without metal, you will need a mirror
with an all-dichroic coating. These are available, but plan to pay US$1000
for a piece 10 x 10 cm. A potential vendor would be Unaxis.

Projecting a 10cm image at 6m is easy. The magnification is about 4X, so
you'll need a lens 1.2m from the projector and 4.8 m from the screen. Its
focal length should be about 1.0m. Most projectors operate at about F/2.8, so
you should be able to get away with a lens (achromatic doublet) about 350 mm
diameter. A SWAG at the cost of a lens this large is US$25,000, but I've never
bought such a lens. (You'll also need to remove the normal projection lens.)

The final trick is that you need to relay the pupil of the illumination system
to the projection lens. If you're into delicate operations, just remove the
lenses from the LCD projector that are next to the imagers. Otherwise, buy an
LCOS projector. Most of the designs I've seen don't have the lenses by the
imagers. Removing the projection lens will still be necessary.

HTH 

-- 
Best regards,
Steve Eckhardt
skeckhardt at mmm dot com

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