How to live-project a large, high-visibility double slit interference pattern? I want to do a double-slit interference pattern for a lobby display.
I've done double slits many times in classrooms and labs.  They're small and not highly visible. I'd like to make the pattern large and easily visible from across the lobby, even during the day.  I want the pattern to be real and live, produced with EM or particles, a genuine QM effect, not faked with sound waves.  A phosphorescent or pixelled backscreen that reacts to the incoming particle/waves is acceptable.  The laser or particle beam needed be visible as long as the patter on the screen is highly visible.
The best possible demo would involve particles crossing one at a time constantly, each leaving a trace on the screen that lingers for a time, so that someone looking at the display can see BOTH the individual dots appearing live AND the accumulated interference pattern. But of course, in such a setup, the screen would have to somehow provide the energy to convert single photon/electron strike into a visible trace.
 A: Here are 2 basic documents on the double slit from Harvard and MIT. The first having a nice demo based on single photons but could be easily converted to a cheaper camera and many photons (same results).  And the second gives the "classical" interference math which gives a good approximation of the image geometries based on lamda and other variables that effect the pattern size.
https://dash.harvard.edu/bitstream/handle/1/27413728/single_photon_paper.pdf?sequence=4&isAllowed=y
http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/coursenotes/modules/guide14.pdf
It would be much simpler to forget about use of single photons, it has been proven that the interference pattern results (or is identical) in both situations. You can use a laser diode and have a bright visible image, just exercise some caution.  If you want to get into the QM nature of it you could use the polarizers as Harvard did to erase the pattern and then make it reappear. 
Also the Huygens principle is an historical (1700s?) explanation that does not stand up to the modern results of the single photon experiments.  A more modern explanation is based upon the "photon wave function" and Feynman path theory.  It says that photons must travel in multiples of there wavelength (similar to a laser cavity) and that certain paths are allowed (bright areas, many photons) and others are not (dark areas, no photons).
A: If you don't demand that you are doing a "two slit" experiment, then there are inexpensive diffraction gratings that will give you first maximum angles of ten degrees or more with visible light.
Which means projection distances less than a meter will work, and that in turn means that an inexpensive diode laser is more than good enough.
Such grating can be obtained from any supplier of classroom deomnstration equipment, and the usual limit on how cheap they go is that they want to sell them to you in packs rather than singlely.
A: Whatever people see in your demo setup will not be the photons themselves, but something representing the photons.  So, you might as well just use a TV screen, displaying the output of an imaging photomultiplier.   If you want the setup to be visible to onlookers while it's operating, you might be able to put a single-wavelength filter over the photomultiplier tube so it only sees light from your laser.  An attached computer will allow you to accumulate photon detections over any desired period of time and display the results, or show individual detections as they come in.
