Each issue of Library Journal carries an ad for people counters at $249 each.
The company name is Laser Electronics and the phone number is 1-800-441-2665.

The 1996 Radio Shack catalog has advertised on page 150 an invisible beam
entry alert for $89.99 which includes a built-in chime and counter. The ad
says 50' wide coverage.

I've had no experience with either product. If the museum has the same
entrance and exit, one needs to divide the "count" in half assuming all of
the visitors leave !! Odd numbers mean someone is still in there !!!     :.)

Susan Riggs
Swem Library
College of William and Mary