Talk:Retroreflector

Is retroreflection known to exist in any non-human artifacts, i.e. is it something that physical interactions and/or biological evolution could ever come up with? --Ryguasu 04:30 Feb 26, 2003 (UTC)

The eyes of, for example, a cat, I think. Not that this is an advantage for the cat, more a by-product of how the eye is built. - Patrick 10:05 Feb 26, 2003 (UTC)

Does this mean when the article says "Cat's eyes are retroreflectors in the road surface; although they stick out a bit you can drive over them", it's talking in a twisted way about feline eyes? I assumed "cat's eye" was maybe a britishism for those raised bumps they sometimes put on top of the middle line of a road. --Ryguasu 13:08 Feb 26, 2003 (UTC)

The ones in the road are named after the ones in the animal. - Patrick 13:14 Feb 26, 2003 (UTC)

However, retroreflective surfaces do not help pedestrians in the dark unless light is shining in the direction from which they are looking

This does not seem correct, I think this says light shining the pedestrian in the face, but it should be the opposite. - Patrick 18:26 Feb 26, 2003 (UTC)

I rewrote that sentence. I hope it makes more sense. -- Heron

I believe that in general, transparent spheres act as retroreflectors, and that when retroreflective material is used to make vests etc., it's made by putting large numbers of tiny plastic transparent spheres into the material. (I'm not 100% sure, so ^{[citation needed]}.) For example, a rainbow is an example of retroreflection from many spherical droplets of water in the air. Other shapes besides spheres can act as retroreflectors, I guess, but spheres may be more likely to occur in nature than a shape with flat sides that just happen to be at the correct angles. --Coppertwig (talk) 13:52, 16 February 2008 (UTC)[reply]

Invisibility and retroreflection

Is this "invisibility cloak" really worth counting as a retroreflector? All that thing is is a slide projector. I don't see how it involves retroreflection any more than watching a movie in a theater. If you look at the way a retroreflector works, you can't use a retroreflector for this purpose because the projected light would go right back into the projector. Projection systems like this often do use screens with directional scattering to enhance the image brightness in a particular range of directions (toward the audience), but that's certainly not necessary and it's not the same thing as retroreflection. Tarchon 23:25, 20 May 2006 (UTC)[reply]

On the contrary, the invisibility cloak itself is made of retroreflective material. Nothing but a retroreflective material will work as the invisibility cloak. The projection system too is far more advanced than a simple slide projector. Please read the entire article starting at http://science.howstuffworks.com/invisibility-cloak.htm before making any changes -- Wikicheng 06:25, 22 May 2006 (UTC)[reply]

Yeah, on the contrary, I did read it. The retroreflection just makes it more efficient. It's not an essential feature. All they use it for is a projection screen. And yes, it is just a projector. It's only an "invisibility" cloak in the minds of university PR hounds. It's really more like a virtual reality display system that operates though a "See-Through Head-mounted Display". It doesn't work unless the observer is cooperating. There's no reason to mention it separately from the conventional function in projection technology. Tarchon (talk) 19:06, 30 October 2014 (UTC)[reply]

How well do they work at an angle?

I did some diagrams trying to figure out if retroreflectors really work at any angle. Short answer yes, long answer is they drop off to zero.

Here's a single trap in our retroreflector \/ taken from the whole retroreflector: \/\/\/\/\/\/\/

Now let's label the sides: L \/ R

Obviously when the light is coming straight down, side L's beams are reflected sideways into R and back out again. And R->L works the same way. Perfect.

  | |
  | |
L \-/ R

Now slightly shift the source of light beams to the left. Two things happen. First, L starts reflecting at a shallower angle and compresses into R's trough. Likewise, the tip of R starts radiating into space instead of hitting L. Both ways, the tip of R becomes less useful.

In the extreme example of just-shy 45 degrees, L becomes near-parallel to the light rays (effectively useless), and R is likewise reflecting nearly everything into space instead of back into the trap. (Then at 45 you hit a singularity, L becomes parallel and R becomes a perfect mirror.) Thus, I think you can say that Retroreflectors drop off to zero as you traverse 45 degrees in either direction from the normal. Since we are talking about limits I'm guessing it's logarithmic. More important, their effectiveness at steep angles is recovered by having deeper traps.

Basically my point is that retroreflectors are optimally used in straight-on situations. They offer a slight fudge factor over standard mirrors, but since this fudge-factor drops off rather quickly, a more proper term for these devices may be "diffuse mirrors." Broodlinger 05:01, 5 June 2006 (UTC)[reply]

Interesting analysis! For most applications, the observation angle (see the article for a picture if you're not sure which one that is) is very very small, on the order of 0.1 to 1.0 degrees. Were you keeping your observation angle at 0 degrees or your entrance angle at 0 degrees? I think the reflectivity drops off significantly faster as the observation angle increases, whereas the entrance angle can vary greatly with a smaller change in retroreflectivity. For example, in specifying highway signs, the sheeting is measured for observation angles of 0.1, 0.2, 0.5, and 1.0 degrees. The 0.1 and 1.0 are special cases for long distance (0.1) and trucks (1.0). They also specify entrance angles of -4.0 and +30 degrees. This has to do with whether you are facing a sign head on, or if it is in a curve and at an angle to you. Special:Contributions/167.7.17.3|167.7.17.3]] (talk) 14:32, 2 July 2009 (UTC)[reply]

Slight fudge factor? Um, Yeah, the effective aperture of a corner cube array becomes smaller as the array is tilted off axis---it reflects less light back toward the source. But what happens with a flat mirror? The mirror will not reflect any light at all back to to source unless the source is exactly on the mirror's axis. When you want a retroreflector, a corner cube array is not slightly better than a flat mirror! It is infinitely better. Even a sheet of white copier paper reflects more off-axis light back to the source than a mirror will reflect. 173.75.33.51 (talk) 18:37, 3 March 2019 (UTC)[reply]

It depends on the type. Cat's eyes can work equally well at any angle. Corner cubes are obviously more limited. Diffuse mirrors do not work the same way. They follow the law of reflection but with the scattering spread out around the ideal angle of reflection. For 45 degree incidence, the retroreflector and the diffuse mirror have peak reflectance angles 90 degrees apart. Tarchon (talk) 22:00, 28 October 2014 (UTC)[reply]

Soft retroreflective material

"Retroreflective" redirects here. I didn't read every word of the article, but I got the impression that soft retroreflective material is not mentioned. I think it deserves a section of the article at least. For example, vests worn by construction crews; retroreflective tape stuck to bicycles; retroreflective strips on backpacks and other clothing and used by joggers etc. A picture of at least one of these things would also enhance the article. --Coppertwig (talk) 13:52, 16 February 2008 (UTC)[reply]

Those materials aren't necessarily "soft", just flexible. The sheeting used to make signs for example is relatively flexible, but will fail and crack after lots of bending. They make a special type of sheeting just for that application, which is much more flexible. It is similar to the tape you mention, and is used from signs that roll up to tape on traffic cones. The material that reflects is always going to be either glass beads or prismatic material (either metal or non-metallic). It is usually so small that it doesn't appear to be made of rigid materials (like glass). —Preceding unsigned comment added by 167.7.17.3 (talk) 14:49, 2 July 2009 (UTC)[reply]

Retroreflector?

Is this a retroreflector? --Una Smith (talk) 21:37, 2 June 2008 (UTC)[reply]

it could be. anything with a layer of spherical glass beads is retroreflective, but they do have to be spherical. —Preceding unsigned comment added by 167.7.17.3 (talk) 14:51, 2 July 2009 (UTC)[reply]

Conclusive proof?

The article says: "They are considered to conclusively prove that man-made equipment is present on the moon[2] and thus disprove some Moon landing hoax accusations. Additionally the Soviet Lunokhod 1 and Lunokhod 2 rovers carried smaller arrays."

OK, I'm not going to claim that the moon landings were a hoax, but how is the presence of an Apollo retroreflector proof of the authenticity of the Apollo moon landings, given that the very next sentence describes a similar device brought to the moon by a vehicle which did *not* carry humans to the moon? :-)

(The page it links to is dead.) —Preceding unsigned comment added by 204.16.40.113 (talk) 06:29, 7 October 2009 (UTC)[reply]

Retroreflective tape

There is no mention in the article about retroreflective tape. It is commonly used on semis to make it clear to other drivers in the dark where the boundaries of the vehicle are. The Department of Transportation approves certain tapes suitable for use for this purpose.

Plcaine (talk) 03:46, 20 January 2010 (UTC)[reply]

Natural Retroreflection

This article currently only pertains to manmade retroreflectors. What about natural retroreflection, such as animal's eyes, dewdrops (Heiligenschein), or Lunar regolith? 216.80.140.25 (talk) 07:09, 21 December 2010 (UTC)[reply]

The article does mention the tapetum lucidum which is the part of animals eyes that reflect light.--CMG (talk) 13:55, 21 December 2010 (UTC)[reply]

The photo of the balloon's shadow is a good example. The early-morning dewdrops act as retroreflectors, and the halo is centred on the basket, and the photographer. At high resolution, the glow appears to come from individual blades of grass. A photo taken a minute earlier (not published on Commons), when the shadow was on bare ground, shows no halo. A similar halo may be observed around other aircraft, but the balloon is such a stable platform that a useful photograph can be taken hand-held with relatively modest equipment. Doug butler (talk) 03:35, 14 September 2022 (UTC)[reply]

Observation angle

Concerning Figure 1 (Observation angle) is it possible with some sort of geometry to have the return beam be at a slight angle from the entrance beam? If so, this would be useful in automotive applications, where the eye is always higher than he headlights. --Guy Macon (talk) 18:27, 16 February 2012 (UTC)[reply]

Yes, otherwise retroreflectors used on roads would not be effective. However, the larger the observation angle, the less apparent brightness you will see. This is important for heavy vehicle drivers, since they sit so far above and behind their headlights compared to most passenger car drivers.

By the way, both Figures 1 and 2 could be improved. My favorite is Figure 2 from this page. It's a US government website, but I'm not 100% sure it's public domain. It could belong to the researcher's employer, which I believe was Texas A&M University.

--Triskele Jim 18:00, 19 March 2013 (UTC)[reply]

Metrics of reflectors

Going to add something about the metrics of various reflectors. The usage of "cross section" borrowed from radar terminology, =(π³ d⁴)/4λ² gives very large numbers in units of area. For instance the 100-cube Apollo lunar array has a cross section of 5 x 10⁸ m², and the 300-cube arrray 1.4 x 10⁹ m², thats billion square meters.^[1] Can a reference be found explaining the physical meaning of those values, either in optics or radar?165.121.80.238 (talk) 06:44, 19 March 2013 (UTC)[reply]

References

^ [ilrs.gsfc.nasa.gov/docs/apollo_arrays.pdf "Cross section of the APOLLO Lunar retroreflector arrays"] (PDF). {{cite web}}: Check |url= value (help)

BLITS collision

http://www.aviationweek.com/Blogs.aspx?plckController=Blog&plckScript=blogScript&plckElementId=blogDest&plckBlogPage=BlogViewPost&plckPostId=Blog%3a27ec4a53-dcc8-42d0-bd3a-01329aef79a7Post%3a46eb8e83-a92e-49d4-96c4-22bc76cecdae

did it ever happen? --83.84.137.22 (talk) 12:04, 12 April 2013 (UTC)[reply]

Cat's eye beam divergence

In the acticle it's written in the cat's eye section "For one, it is sometimes preferable to have an imperfect, slightly divergent retroreflection". I am quite sure that we have always a slightly divergent retroreflection at the cat's eye retroreflector as long as the refractive index of the sphere is constant. The optimal refractive index of 2 mentioned in the text is probably minimizing the beam divergence but doesn't set it to 0 i.e. perfect retroreflection. A perfect retroreflection is obtained only for the approximation that the rays are paraxial, but in most cases this is not satisfied in reality (see Figure with the ball retroreflector). A literaure reference would be helpful to understand the optimality of the mentioned refractive index.

Lead is too technical

This lead needs to be rewritten for the lay person.--That man from Nantucket (talk) 05:37, 13 October 2018 (UTC)[reply]

Somewhere Under the Rainbow

The sky inside a rainbow is brighter than the sky just outside. I've not seen the phenomenon mentioned in textbooks, but it's readily observable with any clear rainbow. Another example ? Doug butler (talk) 03:49, 14 September 2022 (UTC)[reply]

[1] [ilrs.gsfc.nasa.gov/docs/apollo_arrays.pdf "Cross section of the APOLLO Lunar retroreflector arrays"] (PDF). {{cite web}}: Check |url= value (help)

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