It looks like the bulb of the lamp beside it but how why what

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    $\begingroup$ That's funky indeed. Please provide the full make & model of the camera you used to take this - the details of the answer depend on the internals of the camera and those change from device to device. $\endgroup$ Jan 7 '17 at 20:20
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    $\begingroup$ Related: photo.stackexchange.com/questions/69043 $\endgroup$
    – Mostafa
    Jan 7 '17 at 23:11
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    $\begingroup$ An escapee from JJ Abrams’ workshop. $\endgroup$
    – JDługosz
    Jan 8 '17 at 8:39
  • $\begingroup$ I take it there is no frosted shade / diffuser around the lightbulb and that if the bulb is enclosed in anything, the enclosure is clear and not frosted. If so, then Martin's answer is good. If not, then we need another explanation! $\endgroup$ Jan 8 '17 at 10:42

It is the bright light from the bub reflecting off the image sensor in the camera, then reflecting off the back of one of the lenses and then hitting a different bit of the sensor where it is detected.

Even if the sensor is absorbing 95% of the light hitting it (ideally you want the sensor to absorb 100%) and the lenses reflect only 5% you still have 5% * 5% = 0.25% of the light from the bulb hitting the sensor for the second time - so the blue image is 400x fainter than the white bulb. That's why you only see the effect for bright sources like the light bulb or the sun.

The reason it is blue is likely that both the sensor and the lens have coatings to reduce the amount of reflection. These don't reduce all colors by the same amount, the sensor is also more sensitive at some colors and so might detect fainter blue than red.

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    $\begingroup$ The blueish color also makes me think effects from an anti-reflective coating. and reminds me of color shifted mirror images that can occur on glasses. Mine right now under a single LED lamp bulb: imgur.com/TegaCQm $\endgroup$
    – Squid
    Jan 7 '17 at 19:05
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    $\begingroup$ Is your explanation still consistent with the observed vertical inversion (/180° rotation?) of the image? $\endgroup$ Jan 7 '17 at 20:20
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    $\begingroup$ @EmilioPisanty you would expect it to be directly opposite the source relative to the center of the optical axis. At least if the lens axis and sensor are well aligned. $\endgroup$ Jan 7 '17 at 20:33
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    $\begingroup$ I don't mean the position of the image, I mean the orientation of the image, which appears to be inverted. That wouldn't happen with the double reflection you describe. $\endgroup$ Jan 7 '17 at 21:19
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    $\begingroup$ @EmilioPisanty See the last section here, and also here for more examples. These Ghost images are indeed mirror images of the original bright object. $\endgroup$
    – Mostafa
    Jan 7 '17 at 23:10

It's a lens flare of the spiral bulb in the socket.

I assume this is the full uncropped picture. Then the bulb and lens flare position are symmetric with respect to the center of the image (in other words, the optical axis).

Similar lens flares are typically seen in images of the sun, for example in Apollo footage; moon hoax conspiracy theorists get all worked up about them :-)

Here's a picture of such a bulb. If you examine the above picture carefully, you can see the spiral and the diagonal segment.

Spiral light bulb

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    $\begingroup$ Same thing happened when my friend took a picture of a "blue moon". He never shuts up about it, and since he wrote off SE as being, "a bunch of idiots on the internet," I'll never hear the end of it. $\endgroup$
    – Mazura
    Jan 7 '17 at 21:56
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    $\begingroup$ Isn't this more appropriately described as a filter flare? $\endgroup$ Jan 7 '17 at 23:22
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    $\begingroup$ @EmilioPisanty No: lens flare is the standard term, and lenses without filters flare too -- filters just add more air-glass surfaces which doesn't help any. $\endgroup$
    – user107153
    Jan 7 '17 at 23:35
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    $\begingroup$ @EmilioPisanty trust me, lens flare is a very standard term. But clearly you know best. $\endgroup$
    – user107153
    Jan 8 '17 at 1:42
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    $\begingroup$ @EmilioPisanty A filter flare uses exactly the same mechanics as a lens flare. The only difference is which surfaces get reflected off of. The only reason to distinguish them is that a photographer who is using a filter should be aware that some reflections that they see may be reflected off of a filter, and thus could be resolved by removing the filter, while some reflections are purely within the lens. Fundamentally, both are just secondary light paths reflecting off the optics. $\endgroup$
    – Cort Ammon
    Jan 8 '17 at 18:27

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