I have an application where I need two computer projectors to project
images on facing walls.  To get the size and position right, the
projectors may need to shoot through one another's projection -- the
walls are 35 feet apart and the projectors each need to be 20 feet off
the walls.  (They'll be angled down from the ceiling so the projectors
themselves won't block each other, but the beams would pass thru).

Will this cause interference of some sort or will the projected images
look the same as they would if they were projected on their own?

(If this is more complex physics than I'm thinking please let me know
and I'll raise the price.)

keith...

Well naturally, the first thing I did was get two strong
flashlights and fit one with a color filter, go to a 
windowless room and see what happened when I crossed the
beams of the two flashlights. Nothing that I could detect.

Then I went digging through the internet. Though I didn't
find an explanation (if there is one) the consensus is
that two beams won't interfere with one another.

In this discussion of the Physics of Light and Color -
Light: Particle or a Wave?, it says:

"Some argued that if light consisted of particles, then
 when two beams are crossed, some of the particles would
 collide with each other to produce a deviation in the
 light beams. Obviously, this is not the case, so they
 concluded that light must not be composed of individual
 particles."
http://www.mic-d.com/curriculum/lightandcolor/particleorwave.html

Obviously...  ; )


In this scholarly discussion about passing matter through
walls, on the Hypography Science Forums, Fishteacher73 notes:

"I suppose if one could remove the charge from the atoms in
 the wall one could just slide through, just as two light
 beams can pass through each other."
http://www.sciencelists.com/scienceforums/showthread.php?goto=newpost&t=1769


Nanogasm, on the Interesting Nonetheless forum posed your
question precisely:

"I was thinking... if you took two movie projectors, each
 with their own movie screen, but you positioned the
 projectors so that the light beam of each projector's
 movie intersects each other.....what would be displayed
 on each screen?"

Despite conflicts about the nature of light, it was generally
agreed that, as with my experiment:

"if you take two flashlights and make them face a similar
 direction so that thier paths cross, they will still have
 the same pattern of light as they did without the other
 flashlight."

and

"neither projection or beam of light would affect the other"
http://forums.interestingnonetheless.net/display.php?tid=1469&postid=182246


Experientially, this is borne out by the fact that radio
waves, or other frequencies of the electromagnetic spectrum,
don't interfere with each other. In a city with 20 radio
stations, this would be a major problem, if it were the case.


Other citations:

"Two light beams pass through each other without affecting
 each other at all."
http://www.exo.net/~pauld/summer_institute/summer_day8interference/Interference_laser.html

"If, for example, two light beams of flashlights (bosons)
 are aimed at each other the two beams pass right through
 each other, nothing at all happens."
http://www.hyperhistory.com/online_n2/people_n2/science_n2/atomic_theory.html

"The fact that two light beams can pass through each other
 makes possible much of the microwave and optical communication
 of today."
http://www.physics.montana.edu/faculty/babbitt/Physics353/holoQandA.htm

"'At the most fundamental level,' Leonberger explains, 'electrons
 like to interact with each other, and photons don't. So you can
 send two light beams through each other, and they both keep going.'"
http://www.zinkle.com/p/articles/mi_m1511/is_n2_v14/ai_13381537


Light, then, does not interfere with itself. Interference
only occurs when the medium through which it passes affects
it, as when it is subject to refraction when passing through
water.


Please do not rate this answer until you are satisfied that  
the answer cannot be improved upon by way of a dialog  
established through the "Request for Clarification" process. 
 
A user's guide on this topic is on skermit-ga's site, here: 
http://www.christopherwu.net/google_answers/answer_guide.html#how_clarify 
 
sublime1-ga


Additional information may be found from an exploration of
the links resulting from the Google searches outlined below.

Searches done, via Google:

"two projectors" beams passing through interference
://www.google.com/search?q=%22two+projectors%22+beams+passing+through+interference

"two beams" passing through interference
://www.google.com/search?q=%22two+beams%22+passing+through+interference

"two light beams" "passing through each other"
://www.google.com/search?q=%22two+light+beams%22+%22passing+through+each+other%22

"two light beams" "pass through each other"
://www.google.com/search?q=%22two+light+beams%22+%22pass+through+each+other%22

Wow!  That's a fast and excellent answer!  6-stars!

keith...

Thanks very much for the rating and the tip!

sublime1-ga

Hey,
I liked the question, and answer, very much.  I agree with sublime1 on
that the final effect, as seen on the two projector screens, is that
the two projectors won't interfere.  And I think that answers keithc
question.
Now, I don't agree with the general statement that the two beams of
light don't interfere at all.  If one were to look at the portion of
space where the two beams cross each other, there are situations were
the beams will definitively interfere.  This will mean, that at an
specific point in space, the two beams could add to zero light!!
Still, the beams continue marching on, and the image on the two
screens will not be affected.  Cool, ah?  The trick is that light is a
time-dependent vector field, that can be added at any point in space.
Light is NOT particles bouncing.
Of course, to produce any interference at all one needs some
technicalities to be in effect, such as the two light sources (the
projectors) be "coherent" meaning that the light vectors oscillate
together.  I doubt that keithc went into that expense for his
projector. This little fact pretty much seals the fate of the
experiment with two flashlights
More interesting, and accesible, stuff of this sort can be achieved
with another time-dependent vector: sound. There is a lot of cool
stuff out there, people using the interference of several speakers to
create spots with zero sound, while focusing it on a paser by, etc. 
The thing with soud is that because of its longer wavelenght, it is
easier to "feel" the interference spots
Cheers

For all practical purposes, electromagnetic radiation does not
interact with itself, which implies it is totally linear, meaning that
the field at any point is simply the sum of the fields from each
source. Where the light from two sources pass each other, you have
interference effects, since the fields can add constructively or
destructively depending on the relative phase, but in regions where
the light comes from only one light source, it makes no difference
whether it passed through a region with light from other sources.

The reason for this is that electromagnetic fields only interact with
things that are electrically charged, and the fields themselves have
no charge. Thus, Maxwell's equations, that describe electromagnetism
are linear. However, at very high field strength, elecromagnetic
fields can cause the vacuum to ionize, creating electron-positron
pairs, which are charged and thus, interact with the electromagnetic
field. Thus, very high intensity light does interact with other light.
Fortunately this would only happen for impractically high intensities
and is not a concern in any practical situation.

Thanks juanmoreira and kottekoe for the added explanation!