so welcome to my garage here we have the shelves of much junk we've got the table with the project on it that I haven't touched in 2 years we've got the giant Alpha Phoenix logo on the wall but today I want to talk to you about the laser that I keep by the water heater this laser is capable of turning on in just a couple NCS like ramping from off to full power in the span of maybe 5 NS today I want to take a video of the light leaving this laser traveling across this room and hitting the garage door 18 ft away so that distance is about 18 ft about 5 1/2 M but I actually like using feet for this project sort of out of character because one foot is almost exactly one light nanc this is how far light travels in a nanc one billionth of a second that means that if I'm fast enough if I can take a picture of this wall every billionth of a second I will be able to record 18 frames of video in between the light leaving this laser and the light hitting the garage door at the other end of the room in order to record such a video I'm going to use this camera that I built now I have wanted one of these things for 12 years ever since this group at the MIT media lab published an Incredible video of light passing through a bottle of water at a trillion frames per second if you haven't seen that whole video you should go watch it cuz it's amazing I was fascinated but somewhat put off by the quar million doll price tag I realized about a year ago that the oscilloscope based technique that I was using for a totally different project trying to measure waves of electricity on wires could be repurposed as a poor man's Street camera like those researchers were using clearly this sort of thing has been done before and done better but my goal was to keep it cheap I wanted to build the minimum camera capable of observing light move in a cave with a bunch of scraps and the final bill of materials for this whole setup on the T here is less than $1,000 so in this video I'm going to tell you how this camera works I'm going to talk about how I built it what sorts of problems you might encounter if you try to recreate this work and of course there are going to be some really cool videos of light shooting across the garage at the fastest speed that anything ever ever moves [Music] here's a picture of the laser ~ Introduction Segment Removed ~ Illuminating the wall captured on my phone and here is a video of the same scene captured by my custom light speeed [Music] camera so yeah this is a billion frames per second it starts totally dark because there's no light in the room then the laser turns on and you can see light Rush towards the garage door Illuminating all the shovels and rakes and whatnot on the wall eventually reflected light has enough time to bounce around and the whole scene fills up with light every frame of this footage lasts one nond that's a billionth of a second and I'm playing the footage back for you now at just five frames per second if you wanted to watch 1 second of real time at this frame rate it would take more than 6 years that's how fast light is I couldn't resist the anti- clickbait of just putting one of the hero shots here at the start of the video but I am going to make you wait until the end for the laser hitting a disco ball at a billion frames per second honestly I haven't filmed it yet but I think it's going to be really cool so how does this camera actually work well to start explaining that I'm going to watch a car drive past through a very small hole in the wall bear with me first thing we're going to need is a hole in the wall [Applause] there we go n I'm just kidding I don't have enough subscribers to go drilling holes in my house for every video we'll just lift the garage okay so now instead of drilling a hole through my garage door I'm just going to put a hole through this piece of black poster board now let me find it in here normally drops to the bottom shoot got it now if I put this in my my piece of black poster board we have our hole in the wall now if you look through this hole in the wall you can see the outside world like there's light that passes through this hole in the wall and the important thing is that if you change your perspective if you look through that hole in the wall from a different direction you see a different part of the outside world we can move again we can see a different part we can move again we can see a different part no matter where you are you're getting a different part of the scene if I'm standing here in the garage looking through that aperture all of the light coming my way has to be coming from a very specific Direction I mean obviously there's not going to be light that comes in and like makes a bend and then heads my way right now looking through that hole all I see is the gray of the asphalt road but if I wait just for a split second I see white and then it turns back to Asphalt gray maybe a car went by if I were the scientific type maybe I'd want to record some data if I was plotting the color I see through this hole over time it might look something like this for it's dark Road color then suddenly it's bright presumably a car then it's dark again that lets us establish that at this particular time the car was at this location but what does it really by us I mean not very much by itself clearly we need more data If instead I was looking through the same hole in the wall but I was standing over here I could have still seen the car but I'd only be able to see the car when it was in line with my new vantage point if I make the same plot starting at the same time I would see the car later it would look something like this likewise if I was standing over here I would have seen the car earlier if we add a lot more observers at a lot more positions behind the hole in the wall we could track the car the entire time it was moving even discern its shape this is exactly what a camera sensor does it just only saves the average color it sees in each Direction not a mini picture like I'm showing in this animation the instant this laser turns on it sends a signal back to the oscilloscope over there through this cable that basically says start recording and at that point the oscilloscope is recording everything it sees from the light sensor inside this camera but the light sensor doesn't see any light because light hasn't had time to get from the laser to the detector eventually light is emitted from this laser in many different directions and let's say that some small fraction of that light hits this exact spot on the wall when the light from the laser hits this spot on the wall it reflects again in many different directions but some of that reflected light is going to find its way over to this mirror which is going to reflect it down into this lens which is going to focus that reflected reflected light into this aperture and into my light sensor that tells me how bright the light is at any given nanc just like with the example with the car driving by this generates a graph like this where we see the brightness of light observed by this sensor as a function of time here we can see it was dark then it got really bright when the laser was on then it got dark again once the laser was turned off now you may be saying you've only got one sensor in there how can you possibly record an entire video's worth of pixels and I cheated I'm not recording a single event with this camera I'm recording thousands of identical events and every time that that laser fires I've got this mirror pointed in a slightly different location looking at a different part of the wall it's impossible to tell in real time but this laser is actually blinking on and off about 10 th000 times per second that means that whenever I have the mirror pointed at a different part of the wall I can take the most recent Trace off the scope and it will have the data that I want if we aim this mirror at a different part of the wall we can work through the same exercise again light leaves the laser hits maybe this spot on the wall and then travels over to the mirror bounces off the mirror goes down Focus aperture light sensor whatever but because the speed of light is a thing and this second route is longer than the first route when the mirror is pointing at this part of the wall we're going to detect that signal later these two traces become pixels in our final image you can see the first one gets bright then the other one gets bright and if we fast forward to the end you can see the left pixel also detects the light turning off before the right pixel now a lot of viewers may have noticed another sticky Point here when the camera is pointed at the second location why can't light bouncing off the first location on the wall still get to the sensor early and mess up the result and that's why I added the aperture in this funky Optics train any light that doesn't come from the exact location I'm pointing at with the mirror will either bounce off the mirror and miss the camera entirely or it will enter the lens at an angle and get focused onto a piece of black plastic instead of the light sensor down below this is exactly what the aperture is for in a real camera lens and it's functionally equivalent to viewing a scene through a hole in the wall this setup does need work even if I hadn't snapped part of it while filming a piece to camera a few minutes ago I I need to replace this whole set up with like a brushless gimbal of some sort these hobby servos are just not giving me the resolution that I want if anybody knows an easy way to do that I'd be all ears but honestly this whole pointing mechanism with the mirror and the lens and the whatnot was the easy part of this project I threw this mechanism together in like a week of coming home every night from work and doing Cad and printing Parts until I had this prototype the difficult part was the light and the light sensor think about this Frame I wanted to get a video of light moving from one side of frame to the other which takes a handful of nanoseconds in order to properly light this scene I need a light that turns on in significantly less time than that let me show you what I mean this is an idealized room that's one pixel tall and I'm simulating a light turning on at the left if this light source takes one nond to ramp from off to on we would get a video that looks like this the light turns on and appears to propagate across the screen if that light takes a few frames to ramp from off to on we would get something that looks like this the partial brightness light would start propagating across the room before the light was at full brightness which sort of smears the boundary but we can still see a propagation this is actually very close to what I was able to capture in real life but if the light was significantly slower say 50 NCS which is still absurdly fast for any reasonable light source we don't see anything useful yeah technically there's light propagating across the room but the width of this front is wider than our Field view so it would look like the whole room was just slowly fading into view which doesn't make for a very compelling video about the propagation of light so I am going to try to turn on the regular LED strip lights in the garage as a comparison yep got it all right so my lights in the garage turn on in six milliseconds Millie that's awful that is six or ERS of magnitude slower than what the laser does wild basically I needed an insanely fast camera flash to take my pictures now this significantly limits our choices for both lights and sensors you need a light that can go from off to full brightness in just a few NCS and you need a sensor that can go from detecting no light to detecting all of the light in just a few nanocs only if you have both of these things will you be able to observe propagating light and not just a room slowly getting brighter my first choice for a light that could turn on very quickly was actually an electric spark I'm not sure how I would have repeatedly triggered thousands of Sparks to create uniform illumination for the scene but I did expend some effort trying to time the light emission from a spark and deemed it way too slow two like you know something notable in 1.2 micros but I want like a thousand times less than that it's got to be like there knowing what I know now my sensor was actually way too slow when I was doing these experiments so I wasn't giving the arc a fair shot it's certainly better that I measured but I'm sure it also pales in comparison to the laser driver I ended up using for the electricity wave of experiments I'd been playing with these extremely fast rise time Schmid trigger chips and I was doing things like ganging a bunch of these circuits in parallel and pumping that turn on right now signal into random LEDs that I found in my old box of mystery LEDs needless to say this didn't work very well a little research turned up that LEDs and lasers should both be able to turn on pretty fast in the realm of tens of nanoseconds if you cram enough current through them but there's no way the circuits I was building could move electrons fast enough to overcome the capacitance of the device in a reasonable time frame and by reasonable time frame I mean a nanc which is kind of hilarious through all of this I was becoming increasingly frustrated that ligh works I mean detecting nond pulses of light in broad daylight is a commercial thing like it's done that's how a lot of self-driving cars do it and for some reason I couldn't recreate this for any reasonable sum of money most of the lar drivers I could buy online were absurd but somebody recommended this miniature driver circuit that was pre-made it claimed to be able to push 10 amps into a laser diode and turn on in a few NS it was still a really huge fraction of my target budget but exasperated by the math telling me that this was a really really hard problem to solve I went for it I also got some cheap red laser diodes under the assumption that any old diode could get really bright for 100 nond if you crammed enough current through it with this driver you get a readout pin and I could see that yes indeed I was shoving current through my laser diode really really fast but my sensor wasn't detecting it really fast if your sensor is slow you might have a situation where your light turns on really fast but the recorded light intensity lags behind there's this great bit in a corridor crew video where they talk about motion blur never stretching out longer than a frame you can turn on motion blur if you don't set it up completely correctly the size of the motion blur can be too long for how far the thing should be moving so like let's say in one frame an object is here in the next frame it's here if there was to be motion blur that motion blur streak could not be any bigger than the streak from this point to this point but if your light sensor's output can't ramp as fast as your recording data frame from it that's actually exactly what happens it's multiframe motion blur that happens over a couple NCS if this same type of temporal Distortion was in a regular video of regular things like me talking right now this is what that might look like all of the motion is sort of smeared out over time even the best videos I've captured with this camera so far have probably 5 to 10 frames of meaningful motion blur and that's because it takes 5 to 10 NS to record a single Photon using this light sensor this is what the signal from a single Photon actually looks like in this setup the signal rises from off to on in 3 to 4 NCS which is okay but then the next bunch of frames still get signal from that same Photon in the jargon of video recording I guess this would be like a somewhat blurry 3600 degree shutter angle it's just a very bizarre limitation of the specific sensor I chose to work with unfortunately for me I did not in fact have a lot of photons coming in let's do some back of the napkin math if we have a light source being driven at 10 amps which is enormous in a best case scenario we get one Photon out of the laser for each electron we pump in so 10 amps divided by the electron charge says we're creating 6 * 10 19 photons per second if those photons are leaving the laser and spreading out over a wall that's 20 ft long and 10 ft tall then we have about 300 trillion photons hitting each Square cenm of the wall each second if we assume we've got our mirror and lens set to record a spot on the wall that's pretty small maybe 4 Square in and those photons are spreading out through the room again before they reach the camera then we're looking at a few hundred billion photons entering the camera every second from our laser now that sounds like an awful lot but remember we're trying to record video at a billion frames per second and even at what would classically be considered a 360° shutter angle that means we only get to collect light for a billionth of a second leaving us with just a few hundred photons per pixel per frame best case in reality I was expecting to be dealing with just tens of photons per pixel per frame and when a light is so dim that measuring the brightness of that light is basically just counting photons you're going to need an absurd level of amplification the first tool that I used to convert this Optical signal to an electronic signal was a woefully insufficient silicon photo diode silicon photo diodes allow a single electron to pass through the device into a wire for every Photon that enters the window if you look at my oscilloscope specs to see the minimum detectable signal that this oscilloscope could ever possibly hope to measure we would need to push something like a 100,000 electrons into or out of this probe wire that's plugged into the scope clearly if the light sensor is only able to get tens or maybe hundreds of electron R that's not going to happen the light's going to turn on the light sensor is going to start chugging electrons through and this wire is going to slowly fill up with electrons until that scope can measure it personally I saw 1 nond rise time on the data sheet and got excited so excited that I forgot to do that math I just showed you so until I tried it I didn't realize this sensor was going to get me nowhere fast I think commercial lar systems mostly use Avalanche photo diodes but I skipped all the way to the most sensitive light sensor I could get my hands on a photo multiplier tube wow never really looked at one of these foldo multiplier tubes or pmts are vacuum tubes I love the Elegance of tubes and a ridiculous amount of modern physics was first figured out by just shooting electrons around in vacuum tubes and like looking it stuff glow really carefully but pmts specifically are amplifying sensors a PMT can take in a single Photon of light and release millions of electrons into a wire of real electronic signal that can be recorded by other equipment if all of my hobby experiments have taught me anything about radio frequencies it's that attaching a a 47 ohm terminating resistor with blue tape is a great idea I set up the simplest test I could a laser shining straight into the detector across the table and you turn off all lights again yes look at that that is so fast that's so fast can I block it oh my [Music] God this is going to work this is the first time that I've been convinced that this is going to work because I was emitting light from one device and detecting that light with another device and the rising signal on the detection went from low to high in less than 10 NS that was a lot of work to get there but that's all I needed at that point I knew it was going to work the realization that I finally had a fast enough detector circuit set off a week of playing with lenses fiddling with servos doing CAD well into the morning and giving my seldom used printer one heck of a sudden workout by the next weekend I was ready for testing after assembling the pivot setup and making sure I could run the motor I disabled the motors and tried to detect light bouncing off a white reflective board sitting directly in front of the camera not seeing anything there's anything blinking out there this should see it output control voltage 0 to five okay how did I make it work the other night this was just about the easiest test task that you could imagine plan A does always go up in Flames at this point I could tell that I was getting photons from the laser but there was just so much light in the room that my signal was being drown out so I went on a kick to darken the room every window with a Street Lamp behind it every battery charger every random LED that makes a bit of light in the garage covered up all of the electronics I needed to run this system went into a big black box with a curtain behind it and this reverse dark room cubby became my mission control for actual measurements I would remote control my laptop top from upstairs so I never even had to open the door and let light in from the kitchen and let me tell you if you walked around that room it was dark by the virtue of much averaging and noise subtraction I was able to get this clearly there was light moving from one side of frame to the other but the resolution was awful I couldn't actually tell what I was looking at I needed more light so that I could spend less time Gathering each pixel one real application for technology like this is lar and because people don't want their self Drive having car going down the street to look like a rave on Wheels with blinking lights all over it most lar lasers are infrared which makes high power visible lasers especially ones designed for pul operation very difficult to come by I also wanted a visible light because I wanted to be able to see it with my own two eyeballs and like set up an interesting shot on the wall also the PMT has a sensitivity that totally falls off by like three orders of magnitude between 8 and 900 NM so I can't see infrared you can't see infrared this camera can't see infrared I needed to find a high power visible laser okay this is really cool it's so bright that I can actually guide the beam now so I know some of it's going to hit this wall and then some of it's going to keep going and it's going to hit this area so I'm going to get this table out of the way that I will undoubtedly walk into in the dark [Music] later signal look at this I was like fighting for scraps and now I have to turn it down so I was I was like maxed out I was like here the Signal's like you know 3 miles below me this is going to work and it's going to work fast this is going to work as fast as I can move the motor why is one note using 22% of my [Music] CPU yes yep oh my God that's the circle on the wall this is the ceiling I've been looking at the ceiling that's really cool and then it goes away and then the ceiling goes away and then the sink keeps glowing for a long time unfortunately after this when I tried to record the hero shot for this video of the light traveling the long way across the wall I had a mishap the laser went dark that is nowhere near as bright as it should be now the failure mode here is pretty interesting the outer edges of the beam are still kind of visible but the center is gone I've got a whole video about how stimulated emission from lasers works but I don't talk that much about the mirrors in a diode like this the facets that reflect the light back and forth inside actually absorb a tiny bit of the emitted light and warm up if those facets get too hot they can melt or burn or otherwise stop being a window and then they absorb much more light which causes them to get even hotter which causes them to degrade faster you get the picture your laser diod is going to have a bad time the laser got hot enough to melt the solder attached to its leads and it almost fell off the driver so I had to wait a couple weeks to get new diode and get everything set up again but then I got to work figuring out how to Tastefully illuminate my entire garage with a single laser diode running at the rated current this time and I got some really cool shots [Music] and now the moment we've all been waiting for it's like covered with glass dust that's a ball covered with mirrors all right that's not A4 inch what kind of thread is it nope it's got to be some funky metric [Music] something oh I don't see the laser I guess the glasses really [Music] work here somewhere yep there it [Music] is oh you do get a bunch of them and then all the stars show up that's that's really cool that's really cool yeah the Stars show up fast cuz they're coming this [Music] way last one goes [Music] out that's really neat I need to get like a lot of mirrors if you're still watching this video right right now there is almost certainly an image of something in your head that you think would look really cool filmed at a billion frames per second so let me know what that thing is in the comments down below towards the ~ Interactions Reminder (Subscribe, Like, Comment) Removed ~ top of my personal list is to look at reflection and refraction it's going to be difficult to get like a giant Basin of water with a clear wall that fills up most of this area but I would love to be able to recreate a demo like this one from today's sponsor brilliant.org it would take hours of work in real life on this wall to set up different conditions of angled rays of light but if you really want to get an intuition for how refraction works at least I would need to see a lot of different conditions and sort of get a feel for it it turns out that when I was playing through this particular unit I didn't actually spend enough time with the demo before answering wrong twice but this actually turned out to be a great thing because if you get an incorrect answer brilliant's courses explain to you exactly what went wrong and then this unit even tied the correct answer back to real technology in the form of fiber optic cables honestly this one question on this one puzzle like changed how I think about total internal reflection as a concept it's fantastic brilliant has courses on loads of topics extremely relevant to the sorts of projects that I build and show off on this channel physics math computer science lots of programming and AI courses it's really great and honestly the puzzles and lessons are fun to play through if you made it this far through this video you clearly like learning stuff you should go check out brilliant.org I've left a link in the description so thank you to brilliant for sponsoring this video thanks to you for watching this video ~ Sponsored Segment Removed ~ and I'll see you next time remember to comment with things that would look better at a billion frames per second ~ Interactions Reminder (Subscribe, Like, Comment) Removed ~ [Music] ~ Outro Segment Removed ~ Back To Top