# The optics of the "Bat Signal" **URL:** \ **Category:** Factual Questions\ **Created:** [June 28, 2003, 9:41am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951 "2003-06-28T09:41:13Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Ranchoth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranchoth/32/1066_2.png) [@Ranchoth](https://boards.straightdope.com/u/Ranchoth)\ **Post date:** [June 28, 2003, 9:41am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/1 "2003-06-28T09:41:13Z")
…It’s a question on applied optics, this time. Namely; Is it possible to build a working [Bat Signal](http://www.destinationhollywood.com/movies/batman/feature_batsignal.shtml) in real life? And by “working,” I mean “can actually project a legible silhouette onto low-hanging clouds.” Initial experiments performed during my childhood, using a flashlight and pieces of construction paper, produced negligible results. However, I’m willing to concede that I _may_ been going about my work the wrong way. Well, thanks for your patience, Ranchoth ---
**Author:** ![raygirvan](https://avatars.discourse-cdn.com/v4/letter/r/ac91a4/32.png) [@raygirvan](https://boards.straightdope.com/u/raygirvan)\ **Post date:** [June 28, 2003, 10:21am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/2 "2003-06-28T10:21:35Z")
I think it would be feasible with a laser. I have a laser keyring (see [here](http://www.morsecode.co.uk/lda.php)) that projects logos on any flat surface using a laser LED shining through a diffractive/holographic - not sure of the optics - disc. It should be possible to scale it up. Or maybe [projection mask](http://www.roctronics.com/patnmask.htm) technology might work. ---
**Author:** ![Kamandi](https://avatars.discourse-cdn.com/v4/letter/k/5daacb/32.png) [@Kamandi](https://boards.straightdope.com/u/Kamandi)\ **Post date:** [June 28, 2003, 10:24am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/3 "2003-06-28T10:24:40Z")
Well, the screen you’re using (the cloud) would have to be reasonably flat, wouldn’t it? In that case, as long as you’re projecting the image more-or-less straight up at the flat bottom of a cloud layer (or individual cumulus clous), I’d say it would probably work. You’d probably have to use a pretty powerful floodlight of course, with some sort of lens or reflector to focus the light in a parallel beam. ---
**Author:** ![Bosda\_Di\_Chi\_of\_Tricor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bosda_di_chi_of_tricor/32/16845_2.png) [@Bosda\_Di\_Chi\_of\_Tricor](https://boards.straightdope.com/u/Bosda_Di_Chi_of_Tricor)\ **Post date:** [June 28, 2003, 10:25am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/4 "2003-06-28T10:25:38Z")
The modern Bat-Signal uses a laser projection system, similar to those used in light shows. The _old_ system used a modified WW2-style searchlight, many thousands of times more powerful than your childhood flashlight. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [June 29, 2003, 3:34am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/5 "2003-06-29T03:34:30Z")
Actually it will work just fine with a large spotlight… particularly the giant kind they have outside nightclubs and theme parks. My college fraternity projected the greek letter Phi onto low hanging clouds a few years ago with one we, er, borrow from a local attraction… ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [June 29, 2003, 6:34am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/6 "2003-06-29T06:34:43Z")
The “searchlight with a cut-out bat” will work. The key is having a source of reasonably parallel light. A searchlight or an overhead projector achieves that quite nicely by combining a small intense light source with a good quality lens and/or a parabolic mirror. Flashlights unfortunately are not built to the same standards, and your bat silhouette will be partially filled in by the diffuse light coming directly off the bulb. A rapidly-scanning laser spot is really the best way to go - if you use it as a rangefinder as well, you can map the relief of the the bottom of the cloud with each scan and compensate for it not being flat. Very cool! ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [June 29, 2003, 7:02am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/7 "2003-06-29T07:02:56Z")
Take a look at this laser projection photo: [http://www.ctalasers.com/la4.htm](http://www.ctalasers.com/la4.htm) ---
**Author:** ![Moo\_the\_Magic\_Cow](https://avatars.discourse-cdn.com/v4/letter/m/ebca7d/32.png) [@Moo\_the\_Magic\_Cow](https://boards.straightdope.com/u/Moo_the_Magic_Cow)\ **Post date:** [June 29, 2003, 7:09am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/8 "2003-06-29T07:09:22Z")
This sounds relatively simple, apart from getting your hands on a searchlight… has this ever been done (apart from the movies) just for kicks? By the way you can buy [your very own 8 inch tall bat signal.](http://www.monstersinmotion.com/anim/batsignalpro.html) If only it were 8 _feet_ tall. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [June 29, 2003, 7:52am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/9 "2003-06-29T07:52:15Z")
Thanks **Desmostylus** , most impressive! On reflection, my mentioning of an overhead projector was a bad example - they don’t use parallel light and they have a focal plane. Plus “parallel” is an oversimplification - the light needs to be divergent or your bat logo on the cloud will be no bigger than the cut-out on the spotlight. I’ll shut up now. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [June 29, 2003, 6:13pm UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/10 "2003-06-29T18:13:32Z")
Next you’re going to tell me the Thundercat sword signal wouldn’t work, either. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [June 29, 2003, 7:28pm UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/11 "2003-06-29T19:28:13Z")
Moo the Magic Cow… see my post- we did it rather easily. Finding a large enough generator to wire the light to was the hardest part- it wasn’t exactly a case of “run an extension cord from the kitchen”… ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 29, 2003, 8:44pm UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/12 "2003-06-29T20:44:00Z")
I’m having a really hard time believing that image linked by **Desmostylus** is genuine. ---
**Author:** ![Epimetheus](https://avatars.discourse-cdn.com/v4/letter/e/c4cdca/32.png) [@Epimetheus](https://boards.straightdope.com/u/Epimetheus)\ **Post date:** [June 29, 2003, 10:28pm UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/13 "2003-06-29T22:28:17Z")
> [@](#): > > \*Originally posted by Mangetout \* > \*\*I’m having a really hard time believing that image linked by **Desmostylus** is genuine. \*\* I am hedging my bet it is: A search on laser projection photos turns up many like it. There is even a company that advertises for laser billboards- [http://www.laservision.com.au/frameset.htm?html/macro/p\_LaserBillBoards.htm](http://www.laservision.com.au/frameset.htm?html/macro/p_LaserBillBoards.htm) Here are some that project on buildings: [http://www.laserforce.net/LF2001/pani\_boris.htm](http://www.laserforce.net/LF2001/pani_boris.htm) Of course projecting them on buildings is childsplay, the laser billboard seems fascinating if it IS indeed true. ---
**Author:** ![slipster](https://avatars.discourse-cdn.com/v4/letter/s/df705f/32.png) [@slipster](https://boards.straightdope.com/u/slipster)\ **Post date:** [June 29, 2003, 10:44pm UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/14 "2003-06-29T22:44:03Z")
University City is one of the inner ring of suburbs to St. Louis–that is, it borders the city directly. The city hall, erected in the first decade of the 20th Century, resembles a huge fireplug. Atop it is what the city has long claimed is the largest spotlight in the world, which is employed from time-to-time, generally to mark holidays. Back when the Batman television series with Adam West was in its initial run, one or more pranksters placed a bat-shaped silhoette on the lens of the spotlight, and made an effective (if amateurish-looking) Bat Signal. To my eternal regret, I missed seeing it, but the event was well attested-to at the time. The whole city center of U. City is a bit bizarre; a block west of the city hall is a building modelled after a Babylonian temple. An “Egyptian” building, with sphinxes lining the stairway to the front door and hieroglyphs painted in the hallways, formerly stood across from city hall. Nearby are the towering lion gates, which inspired the name of the subdivision where Michael J. Fox lived in the Back to the Future movies. (The author of the films is a native). ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [June 30, 2003, 12:32am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/15 "2003-06-30T00:32:38Z")
> [@](#): > > \*Originally posted by Mangetout \* > \*\*I’m having a really hard time believing that image linked by **Desmostylus** is genuine. \*\* Why wouldn’t it be real? The website belongs to a company that does laser light shows. Presumably they’d have some difficulties over false and misleading advertising if the photo was a fake. ---
**Author:** ![Master\_Wang-Ka](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/master_wang-ka/32/3325_2.png) [@Master\_Wang-Ka](https://boards.straightdope.com/u/Master_Wang-Ka)\ **Post date:** [June 30, 2003, 1:16am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/16 "2003-06-30T01:16:44Z")
You don’t eve need CLOUDS to project a pretty coherent laser image in a heavily populated area. Most major cities have enough atmospheric “haze” (part pollutant, part elevated CO2 levels) to make it feasible to put a bat up there. And yes, laser imaging is that good, I’ve seen it. I dunno about the WWII searchlight idea, though. Never seen that sort of thing used the way Commissioner Gordon did… at least, not in reality, with me own two eyes ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [June 30, 2003, 2:15am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/17 "2003-06-30T02:15:29Z")
I can’t quite imagine how haze would provide a clear image. Sure it would reflect plenty of laser light, but where would the focus be? So, how did the Commissioner call on Bat Man on clear nights??? ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [June 30, 2003, 2:41am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/18 "2003-06-30T02:41:47Z")
Here’s another shot of the American Airlines logo: [http://www.laserfx.com/Gallery/GalleryImage4.html](http://www.laserfx.com/Gallery/GalleryImage4.html) ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [June 30, 2003, 3:02am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/19 "2003-06-30T03:02:51Z")
> [@](#): > > \*Originally posted by rsa \* > \*\*So, how did the Commissioner call on Bat Man on clear nights??? \*\* Clear nights? In Gotham City? Surely you jest! ---
**Author:** ![Bill\_H](https://avatars.discourse-cdn.com/v4/letter/b/a5b964/32.png) [@Bill\_H](https://boards.straightdope.com/u/Bill_H)\ **Post date:** [June 30, 2003, 3:08am UTC](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951/20 "2003-06-30T03:08:11Z")
I just want to comment that having searched the link, I think it’s a travesty that I can’t seem to get one of **raygirvan** ’s laser keyrings here in the US, supposedly the most powerful nation in the world. [Next page](https://boards.straightdope.com/t/the-optics-of-the-bat-signal/184951.md?page=2)