# Talk:Speckle pattern

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## Expand

I talked with Laura Scudder at Wikimania yesterday. We hypothesized about this effect including what would cause bigger and smaller speckles and why the image of the speckles would appear to move differently for near- and far-sighted people. But that's original research, so should be avoided. Here's a fragment, though. Maybe someone can find a citation:
Consider an atomically-smooth surface with a step on it of λ/4, like so:
```_______
|
--------
```
Now consider a plane wave aligned with the surface bouncing off it. The reflected surface will have peaks 180° out of faze on opposite sides of the step. If you had perfect optics, an image of the step would give you full intensity. However, if you were slightly out of focus, the circle of confusion sorrounding the step would contain light from both sides of the step which would destructively interfere creating a black spot in the image.
Another factor would be that different rays will follow different paths through an optical system, so the image of a point might interfere with itself.
Does anyone know a reference for this. —Ben FrantzDale 13:54, 7 August 2006 (UTC)
The idea that perfect vision eliminates speckle is mentioned in this article, which even talks about sunlight speckle. — Laura Scudder 17:56, 10 April 2007 (UTC)

I've always understood that the speckle interference happens *on your retina* (or imaging plane for film or optical sensors). This explains why speckle changes when you move your head around a stationary illuminated object, as well as differences due to optics in the path (including near- or far-sightedness). I like to point out that even if you are extremely nearsighted, the speckle pattern will be crisp (in "focus") even if the object is blurry without correction. Once again, I can't find a reference. -- Jonathan Foote, rotorbrain.com

Yes, it's certainly interference at the imaging plane of your retina that creates the speckle. Unfortunately my library doesn't have the Goodman book, but I did stumble on this interesting article. — Laura Scudder 17:56, 10 April 2007 (UTC)

Hi I am new, so this might not be the way that things are done. But if you need a reference I recomend the new book by Goodman - Speckle Phenomena in Optics.

There is a article called eye testing using speckle which gives a reference explaining in detail how eyesight can be tested using speckle. Epzcaw (talk) 13:59, 11 May 2008 (UTC)

Re: speckle movement when you move your head, the person that made the Goodman "Speckle Phenomena in Optics" (ISBN is 0974707791) reference was right on target- c.f. the explanation in Chapter 6.1, "Speckle in the eye", with further references. The issues at hand include the difference between the image plane and the sensor plane (ie, for an optical system, the image plane is where the object is properly focused, while the sensor plane is where the retina or camera film/sensor is; ideally these are the same, but not for out-of-focus systems, e.g. near- or far-sighted), and the fact that when you move your head the eye moves to track a given point in the object if you are paying attention to that point. While the explanation isn't that hard, & is interesting, it would add a lot of bulk to the article confusing to the novice reader... I suggest leaving it out. Eifn (talk) 08:57, 30 November 2008 (UTC)

## Correction

The recent alteration by an anonymous contributor is incorrect;

"If light of low coherence (i.e. made up of many wavelengths) is used, a speckle pattern will not normally be observed, since the speckle pattern varies over time and for light with a large bandwidth the speckle pattern will shift fast enough for the eye or detector to average out to give a uniform field of view"

The lack of speckle in non-coherent light has got nothing to do with speckle patterns shifting in time, but is due to the fact that each wavelength produces its own stationary speckle pattern; the dimensions of the speckle patterns vary with wavelength, and the individual speckle patterns will therefore normally cancel one another out. Speckles may still be observed if the bandwidth is narrow enough, or the the surface is only slightly rough (speckle in your fingernail in sunlight).Epzcaw (talk) 07:54, 5 June 2008 (UTC)

## Needs a clear and simple lead

I have a science PhD, and have done a fair bit of work with laser optics. I know very well what causes speckle, but it was hard work for me to get through the basic "how speckle occurs" in this article. As it stands, like many Wikipedia articles, much of this article is near useless to anyone who doesn't already know the subject (as well as a lot of relevant background at a high level). Anyone care to have a try at making it accessible to a general audience? TheBendster (talk) 11 March 2009, 20:37 (UTC)

I've added an analogy with water waves in an attempt to do this. Comments welcome. Epzcaw (talk) 09:04, 28 March 2011 (UTC)

## random?

To my understanding speckle is not a random phenomenon but a deterministic one. While the cause of the specific speckle pattern may be random (like the distribution of the scattering material or in case of light random changes of the light source) the speckle pattern itself is a deterministic interference of waves. In case of acoustics or acoustic imaging you will find the same speckle pattern without random changes in case you send the same signals from the exactly same direction. Therefore I would like to propose to rmove the "random" in the first sentence in the definition of the Wiki page.

Looking forward to your comments. — Preceding unsigned comment added by 187.13.53.119 (talk) 17:41, 25 April 2012 (UTC)