# Batman spotlight in the sky

I have noticed that obstructing a spotlight typically results in a blurred shadow unlike the crisp batman symbol in the comics of batman. Is there a way to create a spotlight with a crisp batman symbol?

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I would be interested in knowing why you asked the question. It seems as if you are replacing your old spotlight, batman :) – Gordon Jul 5 '11 at 3:47
Actually, the police use the spotlight to signal Batman, so it's not really Batman's spotlight. You would think a guy called "Gordon" would know this... – Mark Eichenlaub Jul 5 '11 at 5:21

You need to have a focusing lens built into your projector. After that, it's just like any other movie projector sort of thing, you can focus it provided you have an object to project it onto (like a flat cloud). Physically, the clarity of the image is limited by two or three things.

(1) The size of your optics (the large diameter lens) gives a limitation in that it's impossible for a lens of, say, 2 meters, to focus light in a direction more accurately than the wavelength of light divided by that diameter. For the 550 nm that the human eye adapts to at night, this is about 550nm/2m = 0.000000275 radians = 0.00002 degrees. This is probably not going to be a problem.

(2) There's also a limit based on the size of your light source (the bright thing from which the light emits, for example, the filament of a clear glass light bulb). This is a thermodynamic limit; you can't focus your light source so as to produce a temperature hotter than the light source. In practice, this means that if your light source has a size of 1cm, then you can't focus the beam from it to a size smaller than 1cm. By the way, this restriction is the limiting presence in the number of pixels displayed by a projection system. High intensity (small size high light output) sources are expensive and so projector bulbs are sometimes almost as expensive as the rest of the projector.

(3) If the image is viewed off-axis, a non-flat projection screen (or a translucent screen such as a cloud) will make the image blurred. But this won't be such a big deal from near where the image is projected.

Neither of these restrictions will significantly degrade your image. Good hunting!

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I am not so sure that thermodynamics says that "you can't focus your light source so as to produce a temperature hotter than the light source"... – BarsMonster Jul 5 '11 at 8:07
@BarsMonster Think what perfect focus of an image means? you get the exact size of the light source. If the image were hotter than the light source you would violate conservation of energy. Maybe you are confusing it with the way the parallel rays from the sun can be focused and set fire to paper? But the sun is very much hotter than that. – anna v Jul 5 '11 at 8:33
@BarsMonster; For the solar furnace that can't heat to a temperature hotter than the sun. See: en.wikipedia.org/wiki/Concentrated_solar_power#Efficiency and Carnot's principle (i.e. 2nd law of thermodynamics). Also see page 4 of the August 2008 OSRAM application note "Projection with LED Light Sources" which you can find by google. It explains why "etendue" is important to projectors. And there's the wikipedia article: en.wikipedia.org/wiki/Etendue for the subject in general. – Carl Brannen Jul 5 '11 at 8:56
I had the same question as Bars. Thanks @anna and @Carl for clarifying! – Willie Wong Jul 5 '11 at 9:39
@Georg not if object being heated is smaller than the spot size ) – BarsMonster Jul 5 '11 at 11:21

You can use a Laser. Just track it whit moving mirrors like the beam of a TV tube. There are computers that do an outline but its totally possible to track horizontal lines that appear and disrepair at the proper moments and the smoke from fire woks can make a perfect screen. O ya and a BIG laser.

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Here they have green lasers with 1.2W for \$1600 lasever.com/DPSS_Green_Lasers.htm. – whoplisp Jul 7 '11 at 0:07

If you use a huge laser (or maybe several smaller ones) you would keep the light from spreading too much. Also, it only work with clouds to reflect it! and the smoother the better.

I don't know if there is another solution.

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No no, lasers are unidirectional. The criteria for a sharp Batman signal requires that lines of movement of the light do not cross, but the magnification from a laser would be 1. You need a point source of light the right distance behind it in order to create a LARGE signal that is sharp. – Alan Rominger Jul 5 '11 at 3:43
@ Zassounotsukushi This comment is strange. I wonder if that is due to wrong english expressions or lack of knowledge in optics. – Georg Jul 5 '11 at 9:49