Strange optical phenomenon: lighting a candle in front of a TV screen

[mike2020]

I observed a strange optical phenomenon when lighting a candle in front of a tv screen, in the dark (attached picture). There seems to be two lines forming an 'X' figure with a colored pattern (of diffraction, I suppose). I think it must be related to the material structure of the TV screen (flat-screen TV). The candle is set in a recipient made of glass. Did anyone ever witnessed this phenomenon, and/or can explain it in terms of physics ?

 

[Ibix]

It's an interference effect - if you have any of that stone-age technology known as CDs or DVDs lying around you'll see a similar rainbow effect from the silvery side of them. It can come from multiple thin layers of mostly transparent material, or from materials with repeating patterns built into them. Either could be the case for a TV screen.

The basic mechanism is that light strikes the screen and reflects off two different things into your eye/camera. In layered structures one reflection comes off the front surface and another off the next surface. In periodic structures such as CDs the reflections come off adjacent dots. Either way, the two reflections involve light traveling very slightly different distances to reach you. If that distance difference is an integer multiple of the wavelength of a colour of light the two reflections add up and you see bright light of that colour. But if the difference is an integer-plus-a-half multiple of the wavelength then the two reflections cancel out and you don't see that colour at all. And there's all the betwixt-and-between cases where the colour is dimmed. The extra distance depends on the angle you view at, which varies across the field of view, and you get this kind of rainbow effect.

Thinking about it, if this were purely an effect of thin layers in the screen it would be a circular pattern. So it's presumably the pixel structure of the screen. I thought that pixels were more or less square, which ought to yield an x pattern at right angles rather than flattened as you see. Perhaps the individual pixels are actually slightly rectangular?

 

[Andy Resnick]

I think Ibix figured it out- a combination of back-reflection and pixel structure. Interesting point- one could estimate/calculate the pixel aspect ratio from your image.

 

[sophiecentaur]

The angle of the cross indicates (pretty strongly) that the vertical and horizontal spacings of the pixel structure are different. This link has pictures.
At a lower tech level, net curtains and umbrella fabric will produce similar 'star' patterns at night with small, bright sources. Also, the lines left by windscreen wipers will give similar patterns - one dimensional fringes, mostly.
Every keen photographer has tried starburst filters which use grids of thin lines at various angles and all the images you can seen from Hubble have cross shapes, visible on the brightest stars, due to the 'spider' which supports the secondary reflector.

 

[Ibix]

The angle of the cross indicates (pretty strongly) that the vertical and horizontal spacings of the pixel structure are different.

On reflection (groan), I think that the angle of the cross hints that there's some structure that isn't at right angles. A rectangular grid would give a right angled cross pattern with different pitches in the x and y directions (the Fourier transform of a rectangular grid is separable, with a resulting intensity distribution for a given wavelength something like $\sin^2(x/\lambda X)\sin^2(y/\lambda Y)$ where the capitals are the dot spacings). You can actually see non-rectangularity (if that's a word) in the images you linked - the red, green and blue pixels are slightly vertically offset.

I wonder if there's actually a vertical line in the OP's image, but short and dim enough that it can't be seen for the candle flame. That line would be due to the vertical spacing. The cross we see is due to horizontal spacing convolved with the diagonal pattern of the colour dots.

 

[hutchphd]

https://i.rtings.com/images/reviews/tv/vizio/p-series-2018/p-series-2018-pixels-large.jpg

I believe this rectilinear array of pixels, and the broad continuous emission range is sufficient to explain the pattern. The "diagonals" come from wavelength variation

.

 

[Vitina]

LCD color screens can vary but the most common configuration use to use a light distribution panel that consists of an edge lit panel with an etched gradient over a white or aluminized plastic panel. On top of this is one or two diffraction gradient panels with their gradient lines at an angle to each other. A mat surfaced polarized panel, conductive lines, thin glass, liquid crystal layer, thin glass, conductive lines (at right angle to the back lines, color pixel layer (RGB), top polarizer at angle to the back polarizer. The undefined angles are determined by panel thickness and type of liquid crystal core.
Different layer configurations can give varied results, as well as off angle viewing. OLEDs eliminate the polarizers, reflectors/diffusers, liquid crystal core, and sometimes, the glass.
I hang some gradient panels on the door wall to bring rainbows into the house for the grand kids entertainment.

 

[Baluncore]

@mike2020 Welcome to PF.

Interesting. It confirms that there is not much blue light available from a candle flame.

Are the colours in the diagonal lines a full spectrum or are they the pixel colours used in the TV screen? I wonder what effects the camera has on the pattern or the colour resolution.

If a black disk was used to prevent the direct candle light reaching the camera, the exposure might be different and so we might see other radial lines or structure in the pattern.

The resolution of the diagonal interference patterns may be determined by the size of the candle flame. Is it possible to use a smaller flame or aperture?

 

[mike2020]

It's an interference effect - if you have any of that stone-age technology known as CDs or DVDs lying around you'll see a similar rainbow effect from the silvery side of them. It can come from multiple thin layers of mostly transparent material, or from materials with repeating patterns built into them. Either could be the case for a TV screen.

The basic mechanism is that light strikes the screen and reflects off two different things into your eye/camera. In layered structures one reflection comes off the front surface and another off the next surface. In periodic structures such as CDs the reflections come off adjacent dots. Either way, the two reflections involve light traveling very slightly different distances to reach you. If that distance difference is an integer multiple of the wavelength of a colour of light the two reflections add up and you see bright light of that colour. But if the difference is an integer-plus-a-half multiple of the wavelength then the two reflections cancel out and you don't see that colour at all. And there's all the betwixt-and-between cases where the colour is dimmed. The extra distance depends on the angle you view at, which varies across the field of view, and you get this kind of rainbow effect.

Thinking about it, if this were purely an effect of thin layers in the screen it would be a circular pattern. So it's presumably the pixel structure of the screen. I thought that pixels were more or less square, which ought to yield an x pattern at right angles rather than flattened as you see. Perhaps the individual pixels are actually slightly rectangular?

Thank you for the explanation, so it does come from the TV.

 

[mike2020]

The angle of the cross indicates (pretty strongly) that the vertical and horizontal spacings of the pixel structure are different. This link has pictures.
At a lower tech level, net curtains and umbrella fabric will produce similar 'star' patterns at night with small, bright sources. Also, the lines left by windscreen wipers will give similar patterns - one dimensional fringes, mostly.
Every keen photographer has tried starburst filters which use grids of thin lines at various angles and all the images you can seen from Hubble have cross shapes, visible on the brightest stars, due to the 'spider' which supports the secondary reflector.

thank you for the link, very interesting this concept of 'sub-pixels' with rectangular shape. I wonder why all TVs have the sub-pixels stacked horizontally and not vertically? Probably just a convention

 

[mike2020]

@mike2020 Welcome to PF.

Interesting. It confirms that there is not much blue light available from a candle flame.

Are the colours in the diagonal lines a full spectrum or are they the pixel colours used in the TV screen? I wonder what effects the camera has on the pattern or the colour resolution.

If a black disk was used to prevent the direct candle light reaching the camera, the exposure might be different and so we might see other radial lines or structure in the pattern.

The resolution of the diagonal interference patterns may be determined by the size of the candle flame. Is it possible to use a smaller flame or aperture?

thank you, I am sorry, I don' know how to make a smaller flame

 

[Baluncore]

I don' know how to make a smaller flame

Cut the candle wick slightly shorter to reduce the flame.
Mask the direct light to the camera with a black mask, or a diagonal mirror, so stray reflected light from the mask does not illuminate the TV screen.

 

[sophiecentaur]

I hang some gradient panels on the door wall to bring rainbows into the house for the grand kids entertainment.

They love things like that. Grandkids verboten these days, though.

thank you, I am sorry, I don' know how to make a smaller flame

The pattern you see is dependent on the dispersion of the grating and the width of the source. Dispersion is there all the time and for every object. It's only visible when the source is small and the contrast against the background is high. A hole in a black sheet will make a smaller source angle - standard practice in spectroscopy.

 

[RonL]

This might be another example, to consider.