How to project a gobo onto the sky(clouds)

[timsabre]

Can be done, have a look at this page in the Waaaaayback machine.

 

Unfortunately, the mob who did this locally to me no longer appear to do so.

 

http://www.firework.co.nz/searchlights/eagle600.jpg

 

Umm, this is only recognisable because the photo is taken from right next to the fixture projecting the image.

From anywhere else you would not see anything.

[paulears]

There's also the odds against having exactly the right weather conditions to guarantee a suitable formation of cloud at the right height, in the right position at the time you want it.

 

The point about shining searchlights into the sky and needing permission, is quite important. In America the penalty is up to 20 years in prison and $250,000 fine!

[kerry davies]

Do check with local authority and air traffic but I would go with Cedds rotating dots. We used a Solar 250 projecting from an inflatable spaceship with a blue gel and smoke which blew peoples minds and gave the "beam me up Scotty" feel to things. KISS, as usual, seems optimal. No image is probably better than a blurred one that only a few can identify.

 

Oh and I have used a Super Trooper in the past as a searchlight but that was overkill 'cos we could! Don't forget that the more powerful the beam the more power supply you will require.

 

I also scrounged several weather balloons off the Met Office in Cardiff once, might be worth a shot.

[dbuckley]

Umm, this is only recognisable because the photo is taken from right next to the fixture projecting the image. From anywhere else you would not see anything.

 

Hmmm.

 

Don't understand why this is the case, please explain.

 

In Christchurch there's some beams scanning the sky whenever it is dark now as a memorial to those who lost their lives in the quake, and they can be seen for miles around. Now OK, its just a round blob on the sky, but it's still visible.

[Ashley R]

But what I think he is getting to is that if you looked at the coat of arms from any other direction, other than where its being projected from, you would just see a blob of light, and not said coat of arms.

[Brian]

... please explain.

 

Clouds don't give you a flat projection surface, as the light travels into the cloud it illuminate individual droplets through the cloud.

 

When you stand next to the light source these droplets stack up one behind the other making an image (there will be some defocussing due to beam divergence). However, when you stand off-axis to the beam you will see a side-on view of the volume of illuminated water droplets so the 'wrong' droplets line up behind each other giving a defocussed image.

[timsabre]

Yes, exactly what Brian and Ashley said. From other directions it would just be a blob of light.

[ramdram]

Of interest?

 

http://skeptics.stackexchange.com/questions/7639/is-a-spotlight-like-the-bat-signal-possible

 

The image as seen, if I understand the way light works (ha!), is from the inside of the water droplets. I recalled something about TIR and critical angles, hence found this:

 

http://en.wikipedia.org/wiki/Total_internal_reflection

 

The images are all in the eye of the beholder, ie the relative position of image to eye to light source. I believe that the "blob" of light as seen from a long way from the source of light (different angle) is something to do with the beam scattering in the droplets as a cloud and the reflection from the outside of the droplets as well.

 

I gather the angle of light reference viewing position is all important, eg rainbows seen from the air are complete circles.

 

Slightly OT, I presume too that "a" beam of light behaves this way when a lantern is shone through haze for effect. Some of the light bounces off the haze particles and some must carry on through to the "target" so to speak. And when you attempt to drive through fog with main beam on.

 

Funny thing, light.

[IRW]

Slightly OT, I presume too that "a" beam of light behaves this way when a lantern is shone through haze for effect. Some of the light bounces off the haze particles and some must carry on through to the "target" so to speak. And when you attempt to drive through fog with main beam on.

 

Funny thing, light.

 

Completely OT, but a few weeks ago I was riding through extremely thick fog on my motorbike, when a car came up behind with his high beam on. For a few seconds, I could see a perfect silhouette of me out in front of me! Although bringing it back on topic, as others have said, if I had been off to one side in an out-of-body experience, all I would have seen was a straight shadow coming out forwards.