Curious optical effect related to light passing through a mesh

[mikenw]

I hope someone here can explain this curious light effect!

Some background: I have a table with a laptop on it, and a chair sitting at this table that has a mesh back on it that has a pattern of ~2mm holes cut into black material in a regular pattern. I happened to notice one day that I could see the dotted pattern from a distance of about 7-8 ft, as I looked through the chair back without my glasses on with my more nearsighted right eye (-2.50 diopter). Everything begins to blur at about 15" without glasses with that eye.

The effect: I am trying to understand what is causing my eye to still be able to see (some relic of) a dotted light pattern from across the room (~ 8 ft +). I can still see a very clear pattern of multiple dots of light, as though my vision was okay at that distance. Anything else at that distance is just a big blur so how come the light passing through this mesh is still perceived by my nearsighted eye with a pattern? Is something happening to the light that is causing some kind of interference pattern that my eye can see? How is this pattern surviving the trip through my nearsighted eye, showing kind of detailed pattern where everything else is just a blur?

I have attached some photos of the view to try and explain what is going on, although through a camera I can't make it see what my eye is seeing (maybe if I had a better camera), alone with a sketch of the setup. I hope someone can explain this very curious effect.

As a side note, I have begun to examine other cases where there is some tiny pinhole sources of light that also seem to yield some sort of detailed "version" of the source as seen by my uncorrected eye. For example, I have noticed that if I look through trees with foliage where the sun is behind the tree, I can also perceive very small "rings" of light that appear at each pinhole source of light. Again, this is as seem through my nearsighted uncorrected eye (-2.50 diopters). Thanks for any thoughts, I also posted this question on physicshelpform.com and received a few answers but I am still confused as to how the eye can image detail of light sources when the lens of the eye is focusing ini front of the retina.

 

[Drakkith]

My guess is that the holes are functioning like pin-hole cameras and cutting off part of the cone of light from each point on your laptop's screen. This would 'correct' your nearsightedness since aberrations are generally a function of how large the cone of light entering your pupil is.

In other words, when your eye focuses a cone of light down to a point, the rays at the outer edge of the cone are not focused down to the same spot as the rays near the center of the cone. This is why your vision gets blurry. Cutting off part of this cone of light means that the final pattern of focused light rays are much more concentrated and the image is less blurry.

 

[mikenw]

Thank you Drakkith for your reply.
If your guess is correct, wouldn't that mean that I should see the stars in the night sky as points of light? They are also point sources, like a pinhole. But the stars look very blurry to me.

 

[tech99]

Thank you Drakkith for your reply.
If your guess is correct, wouldn't that mean that I should see the stars in the night sky as points of light? They are also point sources, like a pinhole. But the stars look very blurry to me.

Maybe the array of holes forms an interference pattern on the retina.

 

[Drakkith]

If your guess is correct, wouldn't that mean that I should see the stars in the night sky as points of light? They are also point sources, like a pinhole. But the stars look very blurry to me.

We treat rays coming from an object as coming from every point on the object, meaning each object is composed of multiple point sources. Light from a point source is emitted in the form of a wide angle cone, and your eye or a lens picks up a portion of this emitted cone. When I'm talking about cutting off parts of the cone of light, I'm talking about this smaller cone of light formed by the small portion of rays that your eye actually intercepts.

Some of the rays of this smaller cone of light are what are getting blocked by the mesh of the chair. Thus the diameter of the incoming cone of light is truncated, the rays have less spread when focused onto the retina, and the image looks less blurry. This comes at a cost of reduced image brightness (and of course the mesh blocking parts of the laptop's screen), but the brain compensates and you probably don't notice the drop in brightness.

 

[Merlin3189]

To clarify the stars:

 

[Drakkith]

@Merlin3189 Yes, exactly like that!
@mikenw Now imagine that instead of a single star, every point on an object emits a similar cone of light, and the mesh of your chair acts like the small aperture in the picture, truncating the diameter of the cone and reducing blur.

 

[sophiecentaur]

@mikenw What sort of camera are you using? There is a big difference between what can happen with a DSLR and what a Smart Phone camera will produce. It's hard to 'fight' against the auto in some cameras to make them do what you want. Putting a pinhole right on the front of a standard DSLR lens will allow you to stop it down much more than the usual aperture limit and to simulate near sight by manually focussing close up. Also, do you have a manual focus setting? From the rest of the picture content, the camera seems to be focussing on the wall behind the display
I can't see how it can be a simple interference pattern because the source is wide and totally uncollimated. Point sources (street lamps) can give strange patterns through fabric (umbrella) sometimes and they are definitely interference patterns.
Looking more closely at the images, you seem to be getting some kind of Moirée pattern because the pitch of the actual grid on the chair back is the same throughout but the visible pattern pitch seems to change between shots. It could help if you gave labels for your images and described the set up for each.
Also, try putting the chair at an angle so that the vertical and horizontal pitches are different.

 

[mikenw]

To clarify the stars:
View attachment 242943

Thank you Merlin3189 for that! I will have to do more research on how interference works.

 

[mikenw]

@mikenw What sort of camera are you using? There is a big difference between what can happen with a DSLR and what a Smart Phone camera will produce. It's hard to 'fight' against the auto in some cameras to make them do what you want. Putting a pinhole right on the front of a standard DSLR lens will allow you to stop it down much more than the usual aperture limit and to simulate near sight by manually focussing close up. Also, do you have a manual focus setting? From the rest of the picture content, the camera seems to be focussing on the wall behind the display
I can't see how it can be a simple interference pattern because the source is wide and totally uncollimated. Point sources (street lamps) can give strange patterns through fabric (umbrella) sometimes and they are definitely interference patterns.
Looking more closely at the images, you seem to be getting some kind of Moirée pattern because the pitch of the actual grid on the chair back is the same throughout but the visible pattern pitch seems to change between shots. It could help if you gave labels for your images and described the set up for each.
Also, try putting the chair at an angle so that the vertical and horizontal pitches are different.

I was just using a small cheap camera, with not much exposure control.
As for the interference explanation, I am still trying to wrap my head around how that works.
I did try as you suggested to turn the chair at an angle and look through that and I still see the same effect although it seems fainter and looks a little skewed... at some point it does become just a blur to my eye when it is turned that way.
Thanks for your reply!

 

[Drakkith]

Thank you Merlin3189 for that! I will have to do more research on how interference works.

Note that Merlin's diagram is not showing interference. It's just showing how blocking the outer parts of the light cone results in a smaller spot on the retina when you have aberrations.

 

[sophiecentaur]

I did try as you suggested to turn the chair at an angle and look through that and I still see the same effect although it seems fainter and looks a little skewed

If you want any real sense in the thread, you would need to describe more about those photos. What are the distances involved, the pixel pitch of the camera (or some clue about the sensor format), and the pitch of the grid on the chair back. The photos suggest that the apparent pitch of the chair back changes. Is this true? Your eyes do not have a regular pixel pattern so that couldn't be relevant to the visual image.

Ignoring the photos, which could indicate all sorts of things going on, I do know that it's possible to look at grille patterns (with both eyes) and, by crossing the eyes slightly, change the apparent pitch. It's a similar process to how you see autostereograms, which were all the rage a few years ago. I remember 'playing' with a wallpaper pattern in our house when I was around ten years old and getting some weird effects where the eyes can lock onto a false stereoscopic image. Just an idea that may provide some explanation.

 

[Lasand]

Pinhole Glasses:

https://en.wikipedia.org/wiki/Pinhole_glasses

 

[mikenw]

I am aware of the pinhole effect, I just never considered that it would work at some great distance. Pinhole glasses are one thing, seeing point sources of some distant pinhole is another. The grid pattern is evenly spaced although since this is the back of a chair, it does have a slight bow to it, but I am looking at it straight on. It is quite unusual for me to be seeing any details at all with my myopic eyes of distant point sources appearing to be in focus. Why wouldn't I see stars in focus then? But stars... are not in a grid pattern?

I have been studying how the sunlight looks when seen through trees, and one thing I am noticing is that even though I am looking with a nearsighted eye, and life looks blurred in general, when it is looking at light sources like this, it becomes apparent that nothing is really totally blurred. Light is broken up into pieces, and there is some detail to those parts! That is what is very curious to me, why would light be able to express some detail within out of focus systems, like a myopic eye. Which is an out of focus camera.

For example, one consistent observation is that the point sources of sunlight, as seen through the little slivers and cracks between leaves, seems to yield, not some fuzzy spot of no definition, but what is actually observed is a kind of miniature halo, a ring of light, not a dot!
***

Here is a study I just made outside in pencil and paper of what my eye is actually, literally seeing with these more distant point sources, sunlight between tiny holes-slits-cracks between leaves.

That is not a sketch of the sun, that is a sketch of ONE point source of light (from cracks between leaves in a tree), that my out of focus eye is actually seeing. And these point sources of light are not actually points at all they are little halos of light around a darker center, (and in some cases I can detect some tiniest hint of light in the very center of the darker area, as noted here):

(1= the brighter halo of light; 2 = the darker center area; 3 = possible lighter center at the heart of this pattern)

So, it seems to me that point sources are not point sources at all! Not once it is in actual perception within an out of focus system. Then we can see some structure to light when looking at the tiniest points of light. I am not sure if a camera would ever see this structure though? What do point sources of light look like under the greatest magnification possible? Do they exhibit structure?

So... back to this perceived pattern of detail light points that my myopic eye perceives of this light-grid pattern in the living room here... I wonder that what is happening is some sort of reinforcing of these ringed patterns?In other words, I am not seeing the actual dots themselves, I am seeing what their overlapping rings are doing, and at those intersections it could be picked up as a point of light as perceived by my eye. As illustrated here:

And so, my present hypothesis is that within out of focus systems, the actual structure of point sources of light begins to be discerned. And that within grid patterns of these point sources, that they can overlap these structures and generates a secondary version of the pattern is perceived as a detailed pattern, even though the system is out of focus.

You would think a camera would see this phenomenon also, but I don't own a decent camera so can't test that out. Thanks to all of your suggestions and thoughts on this. If any of you are nearsighted, go outside and look up at these dots of light coming through between tree leaves, and see if you can perceive this same effect for yourself. If someone asks what you are looking at, just tell them you are looking for squirrel homes :).

 

[Drakkith]

It is quite unusual for me to be seeing any details at all with my myopic eyes of distant point sources appearing to be in focus. Why wouldn't I see stars in focus then? But stars... are not in a grid pattern?

You don't see them in focus because the incoming cone of light from the stars is not truncated to a smaller size by a small aperture like a pinhole (smaller than your pupil).

For example, one consistent observation is that the point sources of sunlight, as seen through the little slivers and cracks between leaves, seems to yield, not some fuzzy spot of no definition, but what is actually observed is a kind of miniature halo, a ring of light, not a dot!

There are no point sources of sunlight. The Sun will always appear either as a well defined disk, or an aberrated version of a disk. When viewed through leaves, you'll commonly be seeing a partial disk with some of it completely blocked off by leavers higher up in the tree. Hence how you can get rings and halos.

The image of the Sun through the trees in your image above is extremely aberrated, with a great deal of scattered light and other issues. The exact shape, size, and brightness of the resulting image of the Sun depends on how exactly the light is being scattered, reflected, and refracted between all of the (many) different objects shining or reflecting light into the camera's aperture, along with the details of the lens itself, the internal layout of the camera, and even the sensor.

And so, my present hypothesis is that within out of focus systems, the actual structure of point sources of light begins to be discerned. And that within grid patterns of these point sources, that they can overlap these structures and generates a secondary version of the pattern is perceived as a detailed pattern, even though the system is out of focus.

There's no need to come up with a hypothesis. This is just basic optics. Cutting down the size of the light cone entering your eye reduces the effects of imperfections since these imperfections are generally more pronounced the larger the light cone is.

 

[mikenw]

I can follow what you are saying up to a point, but that doesn't explain the ring effect which appears to be the case with all of the these small light sources when viewed out of focus. Why does a disc become a ring, with a darker center? For example, back in the last total solar eclipse for NA, I looked at all the images of the sun on the pavement and they were all little pinhole camera images of a crescent sun. So why didn't those images have a darker center region? I just find it curious that suddenly I am noticing that points of light when out of focus have some sort of structure to them, I never noticed that before my entire life.

So you have been noticing this effect too? For yourself? Are you nearsighted like me?

Thanks for your reply, I am just trying to understand what is going on here, and no one has offered any explanation yet that really explains it for me.

 

[Drakkith]

I can follow what you are saying up to a point, but that doesn't explain the ring effect which appears to be the case with all of the these small light sources when viewed out of focus.

What ring effect are you referring to exactly? If you're referring to the how the Sun looks in your image above, then it has nothing to do with it. That's a totally different effect

What do point sources of light look like under the greatest magnification possible? Do they exhibit structure?

No, you'll just see an airy disk (which is not the same thing as the disk of the Sun). Any finite size aperture can only reach a certain level of magnification since light isn't focused to a perfect point. Instead the light will be slightly spread out due to its wave properties. When you magnify the image of something you are also magnifying this spread-out smudge of light. At a certain point increasing the zoom does you know good because further details are smaller than the size of the smudge and cannot be resolved.

Here's a zoomed in image of a star:

You'll notice that the light falls into a center region containing most of the light (about 80% if I remember correctly), while the rest of the light falls into different rings around the core. Each ring has less and less light in it. This 'smudge' is called an airy disk and zooming in further only increases the size of the airy disk. It doesn't give you any better resolution.

 

[Bruzote]

One factor in blurry vision is the pupil size. If you notice, you are looking right into a light source. Your pupil could be very contracted. This means refraction is taking place. That could lead to a more precise location of the image plane plus that plane might be closer to or even on your retina. Try experimenting with a larger pupil size. Ideally, you would simply apply a drug to expand your pupil, maybe even taking a painkiller with an opiod, like Tylenol 3 with codeine. I would not do that just for this experiment, but people have done more in the name of science. You could also try lowering the screen brightness and measuring the distance at which you cannot distinguish the mesh. Or , use a white wall, as the backdrop for the mesh. Again, check if the mesh still looks in focus when your pupils are more dilated.

 

[bland]

This is the same effect that I noticed years ago, but much more dramatically. I first saw it in a yard that a fence topped with sheet metal that had a hole pattern. Each small hole was surrounded by six holes all perfectly symmetrical. Very similar to the pattern on the back of the chair, however the chair pattern is not quite symmetrical it looks more like three above and three below each hole.

The fence had another fence with the same pattern behind it about 6 foot away. The effect is the same as a Moir pattern. When you look at the two fences superimposed you can see the hexagonal hole pattern. As you walk backwards it gets larger and larger till a single pattern of six holes fills the entire fence. As you walk closer and closer, it gets smaller and smaller.

It seems to me that it's caused by the difference in size of the two superimposed patterns which are at different sizes due to their different distances from the observer. One section of the fence had the sheet metal with holes placed at 90 degrees, and there was no effect. Try turning your chair sideways.

 

[bland]

If you want any real sense in the thread, you would need to describe more about those photos. What are the distances involved, the pixel pitch of the camera (or some clue about the sensor format), and the pitch of the grid on the chair back. The photos suggest that the apparent pitch of the chair back changes. Is this true? Your eyes do not have a regular pixel pattern so that couldn't be relevant to the visual image.

Ignoring the photos, which could indicate all sorts of things going on, I do know that it's possible to look at grille patterns (with both eyes) and, by crossing the eyes slightly, change the apparent pitch. It's a similar process to how you see autostereograms, which were all the rage a few years ago. I remember 'playing' with a wallpaper pattern in our house when I was around ten years old and getting some weird effects where the eyes can lock onto a false stereoscopic image. Just an idea that may provide some explanation.

Oh yeah once you can easily do the stereogram thing with your eyes, you can do weird things with virtually any repeating pattern anyway. Like fence palings or any grid, just defocus them enough to shift the whole thing one pattern repeat across and wa la instantly the patterned object is shifted in the Z direction. But what is also very strange is that one appears to have greater clarity of the surface as it it's slightly magnified.

I can even do it with the keyboard keys in front of me in spite of their different letters. I can ignore the letters and when the keys get close enough your brain locks them in position and you see all the keys in a new layer closer to you. And the letters all appear superimposed on the floaty keys.