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In the night, when I look at a distant light source (for example a street lamp a few hundred meters away) I do not simply see a spot but rather a spot which is surrounded by 'light beams'.

So instead of seeing the image on the left-hand side of the picture below I see the one on the right-hand side.

enter image description here

I know a similar effect also comes up by using camera lenses, even though I am not sure if the reason is exactly the same. Below, for example, is a picture with four 'beams' (Source: https://astromallorca.org/astrofotografias/dsc_0015/).

enter image description here

So, why do I see with my own eyes three beams whereas with a camera there appear to be more?

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  • $\begingroup$ Related: physics.stackexchange.com/q/34222/2451 $\endgroup$
    – Qmechanic ♦
    Commented Jan 12, 2018 at 11:42
  • $\begingroup$ @Qmechanic The effect described in your link is in fact similar. Nevertheless, I somehow doubt that both effects really have the same reason. $\endgroup$
    – Tho Re
    Commented Jan 12, 2018 at 17:18
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    $\begingroup$ MinutePhysics has a good video on this topic. youtu.be/VVAKFJ8VVp4 $\endgroup$
    – agweber
    Commented Jan 12, 2018 at 17:56
  • $\begingroup$ Questions: Are the spikes as symmetrical as the drawing? Do you see them without glasses? Is it only a certain type of street-light? Does it have to be cold out (ice crystals in the air)? $\endgroup$
    – amI
    Commented Jan 12, 2018 at 19:12
  • $\begingroup$ @aml 1.) The symmetry is quite perfect with both eyes open, and somehow weaker with one eye closed.. 2.) Yes, without glasses. 3.) Every kind of light. (Maybe not laser, but I do not regularly look into a laser) 4.) No, it also can be 30°C or more if you wish $\endgroup$
    – Tho Re
    Commented Jan 12, 2018 at 20:42

3 Answers 3

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From physical point of view:

  • One reason is diffraction spike by the mounts (support vanes) of optical reflector of reflecting telescope. Four-fold mounts give four-fold diffraction pattern. However, three-fold mounts give six-fold diffraction pattern since we can see intensity but not the phase of the diffraction pattern. This can be reduced by other design.

  • Photographers can add star effect filters (which contains etched gridlines) on their cameras for shooting Christmas lights and so on.

    enter image description here

From medical point of view:

  • Another reason may be due to astigmatism, glare or starbursting of someone's eye.

    enter image description here

  • See the link for the image here and vision simulations here and the causes of starbursting appears in this site.

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  • $\begingroup$ Any idea about direct viewing (not through a camera?) $\endgroup$
    – Bort
    Commented Jan 12, 2018 at 14:55
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    $\begingroup$ Because I've short-sight and astigmatism, my vision (with spectacles) at night is also spiky. $\endgroup$
    – Ng Chung Tak
    Commented Jan 12, 2018 at 14:58
  • $\begingroup$ @NgChungTak Your answer and the wikipedia page you link are quite nice. But I cannot see where it is explained, that I - and I believe others too - see three spikes instead of four. Unless you wanna say that my eyeballs are supported by three mounts!? $\endgroup$
    – Tho Re
    Commented Jan 12, 2018 at 17:06
  • $\begingroup$ I'm not an optometrist or medical doctor to ensure whether is due to imperfection of your cornea or simply astigmatism. It's unlikely due to diffraction (by optical instruments). Perhaps, it's due to glare – light scattering due to dirty or impure optical materials in your eyes. Would you like to refer the picture in this link? $\endgroup$
    – Ng Chung Tak
    Commented Jan 12, 2018 at 17:22
  • $\begingroup$ @NgChungTak Have you actually seen the effect that I describe in the OP with your own eyes? $\endgroup$
    – Tho Re
    Commented Jan 12, 2018 at 17:31
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Thanks to agweber's comment which pointed me this Youtube video, I found the right solution: The cause of this effect are the so called suture lines.

A sketch of the lens and these lines can be found here.

Another image plus an additional video is on this webpage (Better watch it only if you can stand medical pictures / videos)

There one can really see the Y-shaped suture lines. On that site, they also state that:

... sutures form because of how the lens epithelial cells proliferate during formation.

So, the effect is absolutely normal, and no deseases or abnormal defects are responsible for it.

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It depends on someone's eyes. Astigmatic myopes, that is, people with both near-sightedness and astigmatism, have the highest "beaming" effect when they see lights in the distance, as the cornea is of the wrong orientation. In many cases, windshields and eyeglass lenses can cause this effect, or amplify it. Air can also cause light to be retracted in such a way as to make the lights appear to have pointed rays.

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