# Harry Potter DIY - optical cloaking on a budgetSeptember 29, 2014 7:45 AM   Subscribe

John Howell and Joseph Choi at Rochester's Institute of Optics have built an optical cloaking device which uses just 4 readily available lenses. Eat your heart out Harry Potter.
posted by Chairboy (19 comments total) 4 users marked this as a favorite

Harry's fine with his literal cloaking device. No hearts will be eaten out.
posted by inturnaround at 7:51 AM on September 29, 2014

posted by smackfu at 7:53 AM on September 29, 2014

Scroll down, there's a diagram, but it's basically as simple as this:

1. Purchase 2 sets of 2 lenses with different focal lengths f1 and f2 (4 lenses total, 2 with f1 focal length, and 2 with f2 focal length)

2. Separate the first 2 lenses by the sum of their focal lengths (So f1 lens is the first lens, f2 is the 2nd lens, and they are separated by t1= f1+ f2).

3. Do the same in Step 2 for the other two lenses.

4. Separate the two sets by t2=2 f2 (f1+ f2) / (f1— f2) apart, so that the two f2 lenses are t2 apart.

For their demonstration cloak, the researchers used 50mm achromatic doublets with focal lengths f1 = 200mm and f2 = 75mm. Achromatic lenses provide best image quality. Fresnel lenses can be used to reduce the total length (2t1+t2) Smaller total length should reduce edge effects and increase the range of angles.
posted by Chairboy at 8:03 AM on September 29, 2014 [3 favorites]

I would never have thought of the surgery or driving/trucking applications. That's pretty neat, and way more societally beneficial than sneaking people around castles.
posted by NoraReed at 8:03 AM on September 29, 2014 [1 favorite]

Separate the two sets by t2=2 f2 (f1+ f2) / (f1— f2)

Does this mean that the f2 lens must be the one with the shorter focal length?
posted by solotoro at 8:11 AM on September 29, 2014

Step 1: Surround first lens with wall (ie. make it a window).
Step 2: Put webcam just after first lens, facing viewer.

Result: Invisible spy camera.
posted by Orange Pamplemousse at 8:12 AM on September 29, 2014

Result: Invisible spy camera.

Hm...if the webcam is in the area that light is being bent around in order to bypass, would it be able to see anything within the field of view it is being hidden from?
posted by solotoro at 8:15 AM on September 29, 2014

just make sure bad guy is always looking through your lens, then you'll never be seen!
posted by k5.user at 8:22 AM on September 29, 2014

Some real-life magicians probably just got very, very interested. The concept of hiding a prop directly in plain sight has amazing applications.
posted by itstheclamsname at 8:27 AM on September 29, 2014

More of a cloaking telescope.
posted by blue_beetle at 8:34 AM on September 29, 2014 [1 favorite]

Harry Potter, huh? That's what people think of when they think "invisibility cloak"?

Sheila the Thief weeps in her grave.
posted by gurple at 8:53 AM on September 29, 2014 [4 favorites]

solotoro: Separate the two sets by t2=2 f2 (f1+ f2) / (f1— f2)

Does this mean that the f2 lens must be the one with the shorter focal length?
Well, you could possibly have f2>f1 if your "object" was virtual and at a negative distance from you (an image created by other lenses, for instance), but IRL: yes, that's what it means.
posted by IAmBroom at 8:53 AM on September 29, 2014 [1 favorite]

"I've built this amazing optical device. You look through it and YOU CAN'T SEE STUFF!"

I don't get the use cases. Looking at the lens layout, it seems that this works by turning the light from the scene you're looking at into parallel rays by two lenses. which are returned to their normally diverging state by a second set. This only renders stuff invisible if it traverses the light path in the middle of the set-up. If you're doing surgery on something, you're going to want your hands right next to the thing you're hacking away at, not in the middle of the apparatus. There may be situations where you've got some surgical hardware between you and the patient, I suppose... but, y'know, mirrors, prisms, cameras...

Perhaps there are some smart things you can do with this in radio or optical data, but I keep thinking up easier ways to fix any problem this may apply to.

Neat trick, though, and who knows - it may have something mindblowing up its sleeve. Here's hoping. (You know, astronomers find some sort of gravitational lensing configuration that lets us directy observe the far side of the Milky Way...)
posted by Devonian at 8:54 AM on September 29, 2014

Orange Pamplemousse: Step 1: Surround first lens with wall (ie. make it a window).
Step 2: Put webcam just after first lens, facing viewer.

Result: Invisible spy camera.
As long as the viewer doesn't move.

The described arrangement has true depth perspective (move left, and the closer objects move leftwards against the farther objects/background).

Otherwise, a simpler arrangement is: Put hole in wall, paste picture over hole. (Although your variation handles moving backgrounds.)
posted by IAmBroom at 8:56 AM on September 29, 2014

This seems better suited for mugging people:

SCENE: A dark alley

ME: Hey buddy, check out my magic looking glass! Put your hand inside and look through!
SUCKER: Huh? Ok, I'll... WOW! I can't see my-
ME: (Clubs SUCKER over the head with a blunt object and takes his money)
posted by grumpybear69 at 9:02 AM on September 29, 2014 [3 favorites]

Harry Potter, huh? That's what people think of when they think "invisibility cloak"?

Sheila the Thief weeps in her grave.

And who put her there? Motoko Kusanagi, that's who.
posted by Strange Interlude at 9:06 AM on September 29, 2014 [2 favorites]

If you read the Arxiv PDF, it's clear that the cloaking provided is fairly modest - it's just some off axis areas in the lens relay where light rays don't pass through. In particular, anything centered on the optical axis is not cloaked and will block the light making it through the lenses.
posted by pombe at 10:05 AM on September 29, 2014 [1 favorite]

Inspired perhaps by Harry Potter’s invisibility cloak, scientists have recently developed several ways—some simple and some involving new technologies—to hide objects from view.

I'm sure that's it, it's not that optics has a history stretching back 3000 years, it's not that it's been studied by Euclid, Ptolemy, Hero of Alexandria and Newton, no it's because of Harry Potter that people are interested in this...

And now I'm really scared that they might be right.
posted by Ned G at 1:54 AM on September 30, 2014

It's just an attempt to get a hook on an article. Jesus. And it's not like knowing about Harry Potter suddenly erases the illustrious history of fictional invisibility and/or optics.
posted by NoraReed at 3:14 AM on September 30, 2014 [1 favorite]

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