Simulating an Eclipse

[tonypace]

I'm a science teacher, and to demonstrate things like the phases of the moon and eclipses I need a strong light that casts very crisp shadows.

Ideally such a light would be battery powered, reasonably portable and priced at less than $100. Naturally it would need to be a purely throw type of light. I have a 500 lumen China LED flashlight and the shadows are not crisp enough for my needs. Most other functions aren't that important for my needs.

I would be using this in small classrooms at a distance of maybe 20 feet, using a 1.5 inch diameter moon and a 6 inch diameter Earth. Classrooms can't be in total darkness, So any light would need to dominate a room with a small amount of ambient light.

Eclpises are a little more difficult. To really show umbra and penumbra as well as have the range needed for eclipses, I would need a very wide unidirectional light, which might be difficult - I have no idea.

I could probably build a reasonable reflector, but it would likely have to be made from stripts of mylar backed with flexible plastic, and I'm worried that any imperfections would make the shadows terrible.

Does anyone have any suggestions?

 

[N8N]

I don't know if it would work, but I have a diffuser cap for one of my lights (Fenix TK41) that is intended to make it usable as an emergency lantern... might be what you are looking for. Or just use a naked light bulb in a drop light with the reflector removed?

 

[riccardo]

The fact is that the light projected from the reflector cannot produce sharp shadows unless the obstacle is quite far away from the reflector. The light coming from a flashlight is not like the light coming from a point-type source but is coming from a circle with the reflector's diameter (with higher intensity in the outer region of the circle).

The best for your application is a naked LED without reflector but with a shield (like black cardboard pipe) to allow light to escape only on wanted direction.
If you have a P60 dropin you can try to create a black paper cone that fit perfectly inside the dropin reflector with the tip cut enough to leave the LED unmasked, such light will produce very sharp shadows.

 

[JacobJones]

I find it intriguing that a science teacher couldn't figure this out. Anyway, clearly a bright omni directional light source will be required to replicate the sun. The sun most certainly is not a point source. The size of the light source will be very important, I'll edit this later after I've done some calculations to work it out unless someone else beats me to it.

Edit: Sharp crisp shadows are not possible if you want to demonstrate penumbra, crisp shadows would require a point source or parralel light rays, both of which have no penumbra.

The size of the light source isnt as critical as I first thought, I think anywhere between 2.5" to 12" will be acceptable, the larger the light source is though the more realistic the representation can be. The light source must be very diffuse. A frosted 100 watt bulb (the bigger the better) or a large diameter torch (atleast 2-3 inches) with a diffuser (very diffuse) should be adequate for your demonstration, I would say 1000 lumens minimum and make the room as dark as you can.

Hopefully you intend to swat up on some science before teaching this stuff to the kids.

 

[TEEJ]

I'd use an aspherical light, as its beam is very much like a movie projector's, etc, and we all know how easy it is to make sharp hand shadow figures, etc, using a slide or movie projector beam.

The Crelant with the Aspherical head from MD-Lightsource casts a pretty fat (Square) beam for an aspheric flashlight, and within budget. That would be my choice at least.

:D

You could also simply hold a loupe, etc, in front of ANY decent flashlight, and make the same type of sharp beam.

:D

 

[AnAppleSnail]

I suggest a bare light bulb and a dark classroom.

 

[inetdog]

I suggest a bare light bulb and a dark classroom.

I agree with tonypace's original inclination that for phases of the moon in a fairly bright classroom a thrower would be best suited and the beam source size would not really matter except for sharp edges, which will not be visible in high ambient light anyway. By covering the outer edges of the lens you could trade sharpness against brightness.

But for umbra and penumbra a wide circular source of uniform luminosity would be best. A simple large reflector thrower, maybe with a small dot in the middle of the lens to block direct light, will do the best job. A diffuser cap will just cost you output and will not be any better for eclipse purposes than a large reflector.

Now, to really push the limits, how can you duplicate the red color of the eclipsed moon by providing atmospheric refraction around your simulated earth? I suppose you could fake it by putting a donut shaped red filter over the edges of the lens of a wide reflector thrower. producing a red penumbra in the same location as the white(dark) umbra? :) But that would probably require you to get to a really low light level in the classroom anyway.
Keep those ideas coming in!

PS: An aspheric lens will have the same problems as a reflector of equivalent diameter. Go for the smallest brightest light you can find. You can get 200 lumens or more from an EDC light of pretty small size.

How big is the reflector of your 500 Lumen light?

 

[whiteoakjoe]

I think you would get the best shadows from a small point of light. you could do that with any flashlight by taking out the reflector and running a bare bulb or using paint to blacken the reflector. Just be aware of heat buildup. An LED will be a smaller point of light than a bulb in a traditional incandescent light so a cheep Walmart LED that you could pull apart would work fine.

 

[tonypace]

Thanks for the great replies everyone.

I strongly suspect my no-name 500 lumens light is nowhere near that output in real life, but it has about a 3/4 inch wide deep parabolic reflector. Its basic shape looks similar to the Crelant that TEEJ posted, minus the front lens. Generally speaking, I'm not all that happy with it.

The loupe idea is a good one TEEJ - just trying it out in my computer room it made a great improvement in shadow clarity. And I hadn't heard of aspherical lenses which seem perfect for my sharp shadow needs.

Reading further about eclipses, I read suggestions that an array of lights could do the job. I suppose you could make a collar of red cellophane around the earth and that would diffuse light to the moon, but you would need some very strong output for that to be visible.

Jacob, an elementary science teacher has many irons on the fire and can use all the help from experts he can get. I never claimed to be an expert in optics or astronomy. In my experience many science teachers are happy to copy diagrams from books on the board, but I am going to go the extra few inches to make a good demonstration.

Thanks again to everone who replied. I'm going to go buy a few cheap high power led lights to build into an array, and go shopping for something with an aspherical lens.

 

[jabe1]

Take a look at the Dereelight CL1H-NOHS. It throws a perfect circular spot of light.

 

[JacobJones]

Jacob, an elementary science teacher has many irons on the fire and can use all the help from experts he can get. I never claimed to be an expert in optics or astronomy. In my experience many science teachers are happy to copy diagrams from books on the board, but I am going to go the extra few inches to make a good demonstration.

Thanks again to everone who replied. I'm going to go buy a few cheap high power led lights to build into an array, and go shopping for something with an aspherical lens.

I appologise if my comment was unnecessarily harsh, I didn't realise that you teach at an elementary school. I assumed secondary education. Going further than required to make your lessons as enjoyable and educational as possible is admirable, it was thanks to teachers like that that I aspired to become a physicist, my plans changed but I still have a keen interest in all areas of science.

An LED array should work fine, just make sure the light is as even and diffuse as possible or you could end up with funky multiple shadows, white plastic bags and thin paper can be used as improvised diffusers if necessary.