Laser Speckle Eye Test

[@SilverXTikal]

FINALLY SOMEONE EXPLAINS IT!!!! It’s like the textures in the Chowder cartoon. The characters would move but the textures on their clothes would stay still.

[@potatomeatlabs]

There's another unique property of laser speckle which I actually leveraged while working in a research lab. We were developing a camera system to visualize blood vessels underneath the skin. We used a near IR laser (which can penetrate a couple centimeters into tissue), and a camera sensor sensitive to NIR. As fluid (or blood) flows through a vessel, the speckle pattern changes more rapidly than in the surrounding tissues. So I wrote an application with CUDA and OpenGL to quantify the rate of change of the speckle pattern, and convert that into an 'intensity' image, which was overlaid on top of an RGB camera image, so you could see your blood vessels underneath the skin. It was even possible to differentiate between arterial flow and venus flow, based on how intense the rate of flow was. Ultimately we wanted to use it for liver surgery, where there are bunch of hidden and difficult to see blood vessels, which if damaged, can cause serious complications during surgery.

[@NickMoore]

That's an awesome application. Do you need a super stable mounting for the laser or can you account for global shifts in the speckle?

[@luminousfractal420]

they switched to green light, which i see as a health concern because they use green light to kill bacteria in blood. and were wearing these lights strapped to our arteries with wearables?

[@NickMoore]

Green light is not energenic enough to use for sterilization unless you are using it to heat surfaces to a few hundred degrees.

[@shamancredible8632]

very cool, but I still don't trust doctors. you all get paid when there's sick people, not healthy ones.

[@potatomeatlabs]

@@NickMoore No we hooked it up to a robot to help with an autonomous surgical platform we were working on. It uses a relatively short temporal window. The speckle pattern produced by flowing fluid is "blurrier" to the camera due to it changing rapidly within the exposure window for each frame, so the effective time window for quantifying flow rates is the exposure length used to capture each frame of the video. The CUDA kernel was basically quantifying the spatial 'blur' of the speckle pattern. So it was pretty robust to being moved. If the entire camera was being actively moved (and the entire scene had motion blur), the data produced would be garbage, but that was the only limitation. And that could be reduced by using a more intense NIR laser and a shorter exposure time.

[@hackleberrym]

I wondered for years why and how laser produces those speckles and now I finally know. Thanks for the video!

[@NickMoore]

Glad you liked it, I've wondered about laser speckles since the late 90s when laser pointers first started to be a thing. It was nice to finally have the 'ah ha' moment.

[@Muonium1]

I noticed this effect as a kid in the 90s, though I couldn't really explain what was happening at the time. I always wondered ever since why it hasn't been used to precisely diagnose the exact visual correction power needed in eye exams. Instead of an interminable and frustrating "does the 'E' look better...or worse" "uhhhhh" "how about now, better, or worse" "ummmmmm". All you'd have to do is make a continuously varying diopter corrector optic and tell the patient "tell me when the speckles start moving in the opposite direction". It seems to me this would be vastly easier, more objective, and accurate an assessment of the visual correction a patient needs for their next glasses prescription. Higher order aberrations would still need to be done separately, but even so, it'd be worthwhile.

[@NickMoore]

The fact that speckles move when I move has also bothered me for decades, literally nothing else does this. When I finally read the name "Subjective Speckle" all of my failed attempts to use it made sense. The pattern does not exist except on my retina. I noticed while setting this up that the pattern moves very slightly in a diagonal direction for one eye and almost not at all for the other (astigmatism maybe?) It might just be that lasers are still quite new compared to the "better or worse" setup that has been around for (probably) hundreds of years. Or maybe it's just too hard to get people sitting in the chair to understand the exercise.

[@exponentialnegative1]

​@@NickMoore you could save so many people from so many literal headaches! Optometrists and poor sighted people alike!

[@torhgrim]

There is actually an objective measure of your eye lenses done when you do an eye exam, it's the part with the machine where you look at the tiny house or hot air balloon on the horizon. This measurement is then confirmed by the subjective test where you the try different lenses. It's normal to not be able to tell the lenses appart btw as the doctor will try lenses above and below your actual correction. If both lenses are equally blurry then it means your actual value is in the middle and they can narrow it down from there.

[@Muonium1]

@torhgrim hmmm I have never done this!

[@StephanBuchin]

When I made the test to determine the correction values for my new contact lenses, I simply had to rest my face in front of what looked like a glass lens and in less than a minute or so they had all the informations needed including astigmatism. When I got my lenses a week later, they were spot on.

[@kevinjbakertribe]

My physics lecturer in the early 1980s was into lasers, and told us about this. Apparently a problem opticians can have with people that are too nice is when the optician says "Is that better or worse" they don't want to offend so always say "Better" - the idea was by asking "Which way do the spots move" would not get that bias.

[@eveleynce]

this is your regular reminder that you should always be honest with your optician, their job is to make sure you can see properly, you can be polite after the exam is over

[@5roundsrapid263]

That’s the difference between good opticians and great ones. The great ones can tell you’re not focusing right.

[@dorgodorato]

I have to align blue lasers a lot and always saw this speckly pattern. I'd always assumed my eyes were just shit.

[@Muonium1]

Well they probably are, the laser is just trying to tell you precisely what KIND of shit they are 🤣

[@NickMoore]

You're eyes have WAY less cones that are sensitive to blue and thanks to chromatic aberration blue light doesn't ever focus directly on your retina. So yeah, everyone kinda has shit eyes for blue light.

[@LexanPanda]

@@NickMoore This explains why I can't stand looking at blue neon signs at night!

[@zachreyhelmberger894]

@@NickMoore I've noticed gas stations with bright red prices were much easier to read than stations with bright green prices.

[@amarpersaud2950]

Avoid cheap green lasers. They use an infrared laser and a crystal to upconvert the infrared photons into green photons. This means that a good amount of infrared light can leak through, and can damage your retina. Good green lasers use a filter to block this infrared light, but many cheap chinese lasers omit this filter.

[@NickMoore]

I have a couple of near IR cameras, I should check this green laser.

[@luminousfractal420]

green laser light is used to kill bacteria and sterilize blood. id stick with ir tbh.

[@bpark10001]

If the laser is diffused, there is no worry, even for the infrared. You want to AVOID the newer "direct diode" lasers! Some of them don't make "clean" enough light to get good speckle pattern.

[@frogz]

well you just earned a sub, no one has explained this as clearly why the pattern doesnt move!

[@NickMoore]

You must have good vision! It drifts ever so slightly in the near-sighted direction for me.

[@Zolbat]

Thank you! I noticed this as a child with a red laser pointer. I noticed the 'dots' move with my vision, so they had to be created by my eyes, not the laser. Thanks for the explanation. I also noticed another, perhaps similar effect: when at a beach, I looked very closely (~5cm distance) at a water droplet - especially the reflection of the sun (so a very small defocused full spectrum source of light). I saw colorful concentric circles with 'smudges' on them. I assume these are diffraction patterns of sorts, but I'd love to know whether to 'read' them (where do the smudges come from? A misshaped pupil? Retina?)

[@NickMoore]

I think I know the pattern you're describing. It might be some 'point spread function' witchcraft along with chromatic aberration?

[@Zolbat]

@@NickMoore yeah that could be it

[@MadScientist512]

Seeing the same pattern no matter how close your eye gets to the surface is a cool effect too.

[@NickMoore]

@@MadScientist512 You are directly probing a light-field for its phase information!

[@pizzablender]

Interesting look at this phenomenon. As for eye tests, rather fancy automated devices (autorefractors) exist. The retina is illuminated and a camera looks at the retina via the eye lens. Fully automated, you just keep your eyes open and look at an image that appears to be at infinity (actually its is moving to get the eye to focus the farthest that it can).

[@oisiaa]

This video is a hidden gem! I didn't expect to learn so much so quickly!

[@NickMoore]

I try to keep things as short as possible. I find a lot of 10min explainers have too much filler.

[@BallinKermit]

Hey, have you tried this? Grab a magnifying glass and step away from the speckle. Hold the magnifying glass up and between yourself and the speckle until you see a very unique and distinct pattern while looking through the magnifying glass. You may have to play with distances until you get it to focus. Very strangely, GE makes an oven/stove that has this exact same pattern on the stove top. The model name? Spectra.

[@BestSpatula]

This explanation is excellent. Well done!

[@StevenDanschutter]

My compliments to how well this is explained

[@NickMoore]

Thanks, glad you liked it.

[@gsxr1kmatt]

This is something I’ve always wanted to understand. Thank you!

[@NickMoore]

Glad you liked it, I have wondered about this for >25 years, since cheap diode lasers became available.

[@DanteTomaino]

The cat at the end of the video was absolutely necessary for my comprehension of laser speckle.

[@EngineerNick]

omg thankyou that was a much better explanation that i got when googling it in the past

[@NickMoore]

I only read about subjective-speckle and the interference of scattered/reflected laser light last weekend, before that I didn't even know the right search term to figure out why lasers look so different. Glad you liked it!

[@animehair05silently88]

THAT'S WHY LASER DOTS LOOK WEIRD?? I thought it was just like. a low quality laser thing or something, idk. or the laser crystal making a staticy output but i didn't know why

[@NickMoore]

Apparently more expensive lasers cause less speckle but I've never actually seen a "good" laser, only cheap diodes.

[@Featinwe]

This is was quite interesting and very well explained - I always wandered why is the reflected light from laser having those granules. Now i know, thanks!

[@Lew114]

I’ve always wondered why this happens! Thank you!

[@saveddijon]

Back in 1997 the science museum in London had a speckle vision machine. It featured a rotating drum illuminated by a red laser. You observed this through a window, and indeed the motion of the speckle varied with the focus of your eyes. I am not sure if the machine is still on display today.

[@Chaisz3r0]

That was a great explanation for a phenomenon I've always wondered about. Thanks for showing up on my feed. Also, give that pretty kitty some scritches from me, please.

[@NickMoore]

So curiously I have been getting

[@CXT14GamerMouse]

This solves a question my physics teacher posed me around 15 years ago and I never stopped thinking about. It was always in the back of my mind. I have to rewatch it, to think about if we could use it to calibrate cameras to be exactly focused on a given surface.

[@NickMoore]

Yeah, if you had a high contrast pattern on the surface you could track that vs the speckle to see if they move together (and that your camera is within a wavelength of focus >_< )

[@JonathanGeier]

Nick, love the videos! I always wondered what that laser pointer fuzz was!

[@NickMoore]

@@JonathanGeier Glad you liked it! Cheap lasers have been around for so long that I had taken it for granted that 'lasers break my eyes,' it was nice to finally get not only an answer but also a party trick!

[@IonRoux]

I see this a lot on cars with laser based headlights. You can see the pattern rolling across your windshield as if the car coming at you is driving behind bushes or something.

[@typograf62]

Thanks, I've wondered about this for more than 40 years.

[@NickMoore]

Heh, you have a couple decades of wondering on me but I'm glad you liked it!

[@jeffk204]

Wow, I had no idea. Thanks for the info!

[@amiralozse1781]

extremely well explained, Thank You!

[@NickMoore]

Glad you liked it!

[@coscostan3334]

When I was 20 years old, I had a green laser, and I noticed that pointing it at the wall created those incredibly bright patterns, but I didn’t understand their origin. I thought it was some sort of “laser microscope effect,” but now I’ve realized that those patterns are actually caused by light interference. Moreover, if the room is very dark, a very complex pattern is projected across the entire wall. Here too, I thought it was the “laser microscope effect.” To observe it, it’s very important not to move the laser during the experiment.

[@SkyChaserCom]

I remember in college physics our professor told us how this works. Very interesting.

[@NickMoore]

I wish I had gone, I find all of this stuff by chance. Check my 'Ballance Balls' video if you want a good "ah ha" moment!

[@MickeyD2012]

I noticed this first when I shined a laser into my eyeball as a kid. It looked cool.

[@mattilindstrom]

I remember some 35 years ago some optometrist's shops used to have in their window a device for this test, for people to view from outside. It was a marketing thing, notice specle movement and come in for an eye test.

[@NickMoore]

I'll have to see if I can find a photo of one. Sounds cool!

[@CoolDudeClem]

I just thought that was something weird going on with my eyes, like the lazer light isn't making all the cones or rods (or whatever they're called) respond.

[@NickMoore]

It kinda is, light arriving at your eyes cancels out to make dark, that sounds pretty weird to me.

[@wva5089]

great vid. how's the whooping going?

[@NickMoore]

Thanks, I grabbed an Air65 for the indoor season and have been beating up my walls with it pretty good.

[@GregConquest]

Thanks for the explanation of laser speckle. I had been wondering why it had that appearance. I don't think your explanation of near-sighted and far-sighted is correct, though. Near-sighted people don't always focus closer than the object being viewed, as you suggest. Instead, they focus exactly on the object. It's just that they can't focus on distant objects. It's like taking the focus ring on a camera and blocking off distant to infinity for near-sighted people -- and blocking off 2 to 25cm for far-sighted people. Between those extremes, both focus the same.

[@luminousfractal420]

is this why i could never get my monitor tuned in 😂 that "slide until the colour matches" thing.

[@jwm6314]

I bet this video had to be compressed juuuuust right.

[@NickMoore]

Yup, it took a few renderings and uploads to retain the spackle. Even as it is a lot of it got lost compared to the original recordings.

[@Unlikelyjellyfish]

Woah always wanted to know what that effect was

[@NickMoore]

Glad you liked it, it took me 25 years to find the answer >_

[@BryanBortz]

People are currently going around claiming these speckles are the code behind our simulated universe.

[@solvated_photon]

5:03 seems cat fur may not be the best material for light diffusion.

[@MrBrax]

I thought there was some huge invisible thing inside the laser emitter on those cheap lasers when i was a kid. Very strange effect

[@eveleynce]

so that's why the room always seems 'fuzzy' when I point lasers around at night

[@NickMoore]

Depending on what surface you shine it on you might be getting a mix of this as well as speckle that is projected onto your walls from the surface. You can tell if the pattern holds still when you move your head.

[@boomchacle]

I wonder if you could use this pattern to make screens or displays that work for people with bad vision from far away

[@NickMoore]

The shape of the pattern can go out of focus but the pattern does not. It might be possible to do something clever with the out of focus bokeh but I'm not sure.

[@boomchacle]

@ ah, OK.

[@neilg322]

Interesting, I wrote my degree thesis on the use of the laser speckle technique in the determination of astigmatism in 1995.

[@NickMoore]

Awesome, is there anywhere I can read about it?

[@neilg322]

@NickMoore well I guess there is a copy in the university library and one in my cellar, but unfortunately it was before the days of Pdf's and the Internet as we know it today.

[@moriopl6176]

lol never knew what caused my lasers to have this sparkle. Thanks a lot, now i know

[@bobafettjr85]

I'm confused by the diagram in the second half. Which side is the origin side and is the circle the sensor that's seeing?

[@NickMoore]

The origin is on the left. The circle on the far right is the image that is recorded by the sensor.

[@bobafettjr85]

@NickMoore Okay. Thank you. I get it now.

[@johnpekkala6941]

I have also wondered about this thing forever why laser light is "dotty".

[@NickMoore]

Glad you liked it, I wondered about this for >25 years before I read a good description last weekend.

[@amiralozse1781]

5:05 wait, was that a cat blocking part of the screen and casting shadows? 5:16 yes, definitely a cat!

[@NickMoore]

My lab assistant.

[@amiralozse1781]

@@NickMoore Perfect! thats why your experiments and videos are so well done!

[@TehPwnerer]

The laser speckles are fixed it has nothing to do with the return source but everything to do with the source. Speckled pattern does not change depending on what you point the laser at. If you don't believe me buy a green laser and try it for yourself point it at anything and the speckles are exactly the same as they are anywhere else

[@NickMoore]

There are objective speckle patterns that are 'fixed' to the beam but this video does not show that type of speckle.

[@marcmarc172]

I've always wondered about this.

[@NickMoore]

Me too, nothing else in nature looks like a laser speckle.

[@hellmine]

nice video bro

[@JustinKoenigSilica]

Great explanation! What a fun little thing only possible thanks to SCIENCE! (I don't think there's a natural source of coherent light?)

[@NickMoore]

Lasers are totally unique to earth (or other planets with intelligent life), so of course we use them for Pink Floyd concerts and blinding pilots.

[@hamjudo]

​@@NickMoore A recent paper described the conditions necessary for an interstellar cloud of hydrogen to form a huge natural laser. Once astronomers knew what to look for, they were able to find a few examples. Admittedly, it's a far longer wavelength than visible light.

[@mattcintosh2]

My eyes are like -4.25. I found it weird that the speckle looks the same with or without my glasses on

[@NickMoore]

A few years ago I tried to use laser speckle for filming and couldn't figure out why I could not get the setup into focus (it looked the same regardless of how I adjusted the lens). Turns out I was looking at it totally backwards, the spot wasn't always out of focus it was always IN focus, at the time I didn't think such a thing was even possible so I got it wrong.

[@PaulG.x]

I saw a cat! I must be Meowpic

[@1800Supreme]

So looking at the speckle pattern of a 5mw red laser with my eye. The pattern is densely packed in the center and magnified around the circumference. Any idea why its like that, instead of the pattern density having a even distribution throughout.

[@NickMoore]

I'm not sure, it might have something to do with the intensity close to the center just saturating your retina. It's hard to see a black dot superimposed on a field of light. When I was filming this the center of the spot looked like it had no speckle at all until I increased the shutter speed and turned the sensitivity. Also, curiously, KZbin gave me a pair of "suggested replies" to your comment. One of them was "that's just how lasers work" but the other incorrectly suggested that the airy disk was the reason. But airy disks have more to do with focusing optics and the speckle pattern is independent of the laser optics. Lazy AI maybe?

[@1495978707]

The speckle comes from wavefront phase noise. It's actually something that many lasers don't have (much of). It's common in diode lasers which are what most of the smaller consumer grade lasers are. The speckle "spacing" varies like this for the same reason that interference fringes do on a flat surface. If you have a spherical surface centered on the focus of the laser, I would bet that the speckles would be evenly spaced, as with any far field interference pattern

[@efrandsen72]

5:06 cta

[@Chriva]

That's a cute shitten :D

[@NickMoore]

My lab assistant.

[@1495978707]

4:10 This is an oversimplification. There's actually a whole beam composed of rays emanating from all over the surfsce in different directions (a wave) that all come together and interfere in a certsin way on each part of the retina. This is way more complex to visualize and explain though, so I get why you explained it this way

[@NickMoore]

Yeah, the single ray was the most simple case I could explain. Trying to illustrate that light from all points on the surface come together to interfere at all angles at all places in space in front of the surface was a not an elephant I wanted to eat. At 0:35 maybe I should have said that the subjective speckle that is always in focus does occur on the surface?

[@1495978707]

0:35 incorrect. The speckle pattern is also really there. You can stick a camera without lens in there, or a photographic film, and there will still be speckle. And not all lasers have significant wavefront noise like this. The pulsed lasers in my lab look smooth when illuminating paper or on a camera sensor 1:00 correct. The wavefront phase noise is there throughout the beam. This doesn't mean it isn't there on the paper, it is both speckled on the paper and everywhere else in the beam. I think you just misstated your case at the beginning

[@videblu]

Do you know what the cause is? Such as if it's due to imperfections in the laser or film/sensor surface? And why some lasers don't have much speckling?