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I have an experiment with He-Ne laser in my practical syllabus. In curiosity I took a photo of the incoming radiation directly from the laser with my mobile camera. Some strange pattern is observed. The photographs are attached below. Is this just an interference pattern or something beyond that?

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  • $\begingroup$ Your camera pixels do form a grating, and so do the 3 color filter pixels in front of them. There is also reflection on multiple surfaces of the lenses in front of the image sensor. Wether that is enough to explain the effect... that's a good question, one would really have to look at the details of how the image forms. You could get an old, cheap webcam and take it apart. I wonder what happens to the beam when it goes just trough the lens and what one would see just illuminating the sensor, without the lens in front of it. $\endgroup$
    – CuriousOne
    Commented May 11, 2016 at 6:52
  • $\begingroup$ @CuriousOne. Thank you for your attention. The lobes appearing can be attributed to reflection from multiple surfaces, but the circular pattern on each lobes due to the grating effect alone? $\endgroup$
    – UKH
    Commented May 11, 2016 at 6:57
  • $\begingroup$ I suspect it's the grating effect from the highly regular patterns of the sensors... but I could be wrong. Since the pixels are pretty small, one could almost suspect that the large scale pattern is caused by them, while the tighter spaced patterns are some reflection on multiple surfaces that are further from each other... just guessing. $\endgroup$
    – CuriousOne
    Commented May 11, 2016 at 7:00
  • $\begingroup$ OK, I can confirm that it's probably not the structure of the sensor (alone). I just took a couple shots of a laser directly onto the sensor of my DSLR and there is little sign of a pattern. It looks like a Gaussian speckle cloud. When I point the same laser pointer (cheap home improvement laser level) at the camera with the lens, I get a little bit of a pattern, when I do it into my computer webcam, it looks similar to what you are getting, just not as pretty. I suspect now that the reflections in the optics are important. $\endgroup$
    – CuriousOne
    Commented May 11, 2016 at 7:16
  • $\begingroup$ Another experiment: when I shine the laser onto the sensor, I can see a distinct square diffraction pattern, just as one would expect. If that reflects back into the lens (as it has to), the secondary reflection could cause the coarse patter that you are seeing. Overlay that with some Newton rings and you probably got the first two orders of the effect. $\endgroup$
    – CuriousOne
    Commented May 11, 2016 at 7:20

2 Answers 2

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I did a couple of experiments with a DSLR and a cheap red home improvement laser level. The most revealing factor is that a direct reflection of the laser on the CCD sensor gives a distinct square diffraction pattern. This is pretty much what one would expect from a camera that has square pixels. The reflections have sharp peaks that are spaced fairly far apart (the sensor has 4.7um pixel size and forms a very precise grating). When this pattern is reflected back from the surfaces of the lens, it could form the large scale structure seen in the images. I would agree with Jaswin that the ring structures seen on these reflections are probably Newton rings.

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It seems like some kind of Newton rings formed. Does your mobile camera has a flat thin sheet covering the lense. If so, the newton rings formed is captured in the sensor. Having said that, I can't explain why there are multiple of them, that too in a lattice formation. May be if the distance between the thin sheet and lense increases we have various newton rings. Also the stretching on the edges is because the laser is of finite width.

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  • $\begingroup$ The sheet covering the lens is flat. I thought that case too. But the arrangement looks like as atoms in a solid:) $\endgroup$
    – UKH
    Commented May 11, 2016 at 6:53

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