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Bi-directional PIR - Pyroelectric Infrared Motion Detector.

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I have been assigned a project to build an Infrared motion sensor that will have two beams in parallel, able to decipher whether a person has passed in/out of a room. Through my research I have found a single chip solution through LSI Computer Systems, Inc. I made a prototype with there sample chip, which worked beautifully, but is not the direction I want to go. Depending on one company for a single chip solution is a bit scary.
The direction I am pursuing now is to use a PIC microchip (cheap, powerful and have had lots of experience with coding). Your basic data aquisition system. From the pyroelectric sensor itself, comes a very small charge measured with a FET. From this point I need some filters, amps, then to the A/D converter on the chip. Code could then be written to tweak the system and send signal on to the rest of our system to trigger alarms or flags.
I have found there are many things to keep in mind when creating PIR, such as Fresnel Lens type, temp, light, etc, but still need more information on how these sensors are constructed.
What I need a hand with is finding out as much info, datasheets, products, schematics, and example usage of PIR's as I can possibly get. I have found some manufacturers such as www.mstmicro.com. This project is in the infant stage, but my goal is to have a substantial product within 6 months.
Thanks for all your help and let me know if I can post anymore details.
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