Breaking Taps - 2021-04-21
CONSIDER SUBSCRIBING 🥰 ☕Buy me a coffee? https://www.buymeacoffee.com/Breakingtaps 🔬Or Patreon if that's your jam: https://www.patreon.com/breakingtaps 📢Twitter: https://twitter.com/BreakingTaps 💻Discord: https://discord.gg/R45uCXcEv4 I built a 3D printed, scanning laser confocal microscope so I could collect 3D surface topology and profilometry data! The microscope stage is based on an OpenFlexure "Delta" stage, and the confocal optics are constructed with a 3D printed body, some lenses, a 405nm laser, pinhole and photodiode. The microscope is controlled by a Raspberry Pi (which controls an arduino to move the steppers). It's not beautiful, it takes forever to scan, and the images are of dubious quality... but it works! OpenFlexure: https://openflexure.org/ 0:00 Intro 1:56 Confocal vs Widefield Microscopy 3:25 OpenFlexure Motion Platform 4:01 Confocal optical breakdown 8:00 Delta motion stage 8:43 Photodiode amplifier 9:14 Confocal Pinhole demonstration 10:31 Camera vs Photodiode 11:31 Data processing considerations 14:05 Images and results! 18:22 Optimizations 21:37 Discord! Come hang out with us!
I have absolutely zero need for a "confocal laser microscope" but your channel is so incredibly well done I can't help but watch.
I'd love to have confocal laser microscope but I need it so little that I cannot justify even the cost of DIY project to get one. Just like I would want to have accurate spectrophotometer, too.
Dude your videos look soooo good. Also, sweet microscope :)
Well, your channel is bad.
Yea hes doing doubly good work
its the lighting
@Owen Davies who are you talking to?
@Jcewazhere uh... seems like something an amateur could make tbh, not the kind of thing Shane specializes in
Very cool. In the 1990s I worked with a startup developing a commercial confocal laser review station in the semiconductor industry. We scanned the laser in x-y and had a piezo stage for the z-axis. It also required a proprietary frame grabber board that was synced to the laser scanner and z-stage. My work was on the microcontrollers, so I only knew the general optics design. We experienced what we called the pit-particle issue where what we knew to be a solid above the surface (a sub micron calibrated latex sphere) could sometimes show up in the 3D image as a hole.That seems similar to what you were sing on the one image. Our optics engineering team spent a lot of time resolving that, but I don’t recall how they attacked it. Great video. Thanks.
That seems like a problem that could be "solved" (mitigated maybe) by analyzing the curve. If the peak is out of reach there should be a steady climb before, which should be detectable with a proper numerical analysis. Then I would replace this value with a max value. It would at least make it obvious in the image where the 'z cropping' happens, instead of random valleys.
I think one of the reasons why you're getting artifacts is because of how metals reflect light : it's mostly specular reflection(it's directional, in your case parasitic) and not diffusive (unidirectional, what you're detecting). metal oxides, however, tend to build up in tiny crevices of metal surfaces and they are dielectrics, so they create tiny diffusive areas in otherwise specular metal which might look like noisy height changes.
I’m sure you’re right because I didn’t understand anything.😁
I think you are right about reflections. And to add something else. I think some reflections are courses by lights other than the laser. Like the image with the "EC". The left, white part of the image could have a lot of room light. Then the lights were turned of for a little while. And then a small light in the room is turned on what causes the reflections on top of the characters.
But I could be wrong because the scans took multiple days and then I would expect to see more day night effects.
@Daniel What type of temperature fluctuation is seen in this workspace over 24 hours? The PCB has many components, each with a different CTE. Also conformal coatings can confuse the sense of sharp focus.
Egads! Sorry, this is not where I should have put the question!
I was thinking about reflectivity, too, after a recent experience trying to use photographs to create 3D models using Meshroom. Reflections make it basically impossible for the algorithms to figure out what's going on in the images. Anything reflective has to be dusted with a matte powder to get it to work.
If I may suggest - please consider putting the transimpedance amplifier very close to the photodiode, preferably mount the diode directly piggybacked on the opamp IC (directly soldered, no sockets) onto the DIP8 of the opamp. The capacitance of the long coax between the PD and the TIA limits the bandwidth, in your current setup.
Yep. And also using a smaller diode will give better speed (thx lower capacitance) and lower noise.
Cheers for the tips! This project really taught me that my EE skills are terrible and I need to spend some time learning the fundamentals. Just barely got the amp working and it really is awful in every way :) Will keep that in mind for the next one, I forsee myself needing a decent photodiode amplifier in the future :)
You could try to have the beamsplitter at the Brewster angle of you laser source and mirror material, could avoid the laser dump, when the optical path is geometrically redesigned.
@Breaking Taps brewster's angle is the kind of thing when you see it happen, the coffee goes out your nose
even if you're not drinking coffee
@George Tsiros This comment caused coffee to come out my nose. And judging by my neighbor's shouts, I know from where the coffee was teleported.
@Breaking Taps There are two more reasons for the poor SNR: (1) the laser beam is highly aberrated due to poor alignment quality, and (2) some reflected light will be fed into the laser cavity, which actually affects the output beam intensity. The reflected beam is definitely something you want to avoid.
If the laser is polarized, it might be an option to filter the returning light in that direction to exclude specular reflection from those shiny surfaces. That reduces the return signal very much, but scans for scattering and the surface geometry matters much less, which may otherwise deflect a focused beam in a different direction. It has the same effect as coating the surface matte.
@Michael Haardt Yes, every diode laser is highly polarized. And, to avoid getting speckles, every diode is narrow band only when it is at low current, around threshold. More current gives more wide bands, thus less speckles. Less speckles may reduce some noise.
This is really well done! I watched an OpenFlexure video and this was suggested. Really shows that a lot of time went into the project and into the presentation. Wishing you the best of success.
Thanks Kent! Good to see you, hope things are well!
Hey, awesome video! I see there already was some comments about reflective surfaces here - and that is indeed the probable cause for the inverted shield structure. When the beam hits the slightly higher point of the shields edge it is scattered into the room and not back into the lense making the signal weaker. This would explain the smaller values in some of the curves peaks too. Alexander Sannikov mentioned in their comment that the metal oxides being diffuseve cause noise in the signal. Indeed if the samples were to be prepared beforehand (like they do for electron microscopes) to be compleately matt with a coating of a white matt substance the laser would behave more uniformly along the entire surface.
Dude your video quality and experimental setups keep on getting better! Awesome stuff, keep it up!
Thanks! Really appreciate it :)
I think the biggest issue is the range of reflectivity. I had this issue with 3d scanning using photogrammetry. Your photos will be greatly affected anywhere there is solder on that PCB, or other curved highly reflective areas. It might be worth trying to air brush a flat color on everything to make the reflective properties uniform, which should normalize your readings and greatly increase the resolution and accuracy of your photos. If air brushing adds too much material to the sample, there might be some gaseous coating options like 2D boron nitride that would only add 1 atom to the height. Good work though. Your channel will do great :)
That project deserves a subscription right out of the box ! Thanks for producing such an interesting content.
Very cool, i worked on a similar project a few years back using DVD optical pickup heads and its internal optical path, which is indeed a confocal arrangement with an electromagnetic vertical adjustment which provides a high degree of precision.. my failure was in the X-Y stage.. might have to revisit with openflexure but it was something that worked as an arduino shield.
I dont know if it was already mentioned in the comments, but. I have some experience in 3d scanning, basically extracting height information from a series of images, and the general recommendation for an object not to bee too white, or too black in albedo and what is sometimes more important not to be too shiny. From your samples you can clearely see that varnished surface of a PCB net you the worst result, and edges of the shiny metallic coin tricked sensor into wrong depth, even though the xy shape was correct. Try taking pictures of embossed areas of paper money and see if it will make a real difference. Amazing video though!
If you're still working on this, consider replacing your photodiode and aperture with a linear CCD array. You can use the reflected beam width as feedback for the next Z position to find the focal height in 2-3 iterations instead of scanning.
Really cool project! Another way to do this is by using a standard cd or dvd laser head. You cannot scan very large areas, but with very high resolution.
Thanks, and cheers for stopping by! Will take a closer look at those DVD units, are there open source designs/plans/software to control them? I'm not sure I have the skills to hack one myself :)
@Breaking Taps I think that styropyro has some tutorials on youtube, and they mostly revolve around an lm317 regulator. Also I had an idea for the optimisation which might be interesting: set a grid over the area you're scanning, and go to each point sequentially and move the z axis to find the height of the point. After the first scan look for areas where there is a large change in height and then go back to get more resolution in those areas. This should result in being able to start with a coarser grid which should make the scan a bit faster, although fine detail may be lost in some areas. This was really cool and I don't need a confocal microscope but now I'd really like to try at some point
@Breaking Taps They are actually not that difficult to control. But, one potential problem is that you need a special 2x2 photodiode to focus properly.
Let me try to explain: imagine a 2x2 grid of photodiodes, with them being oriented at 45 degrees. The optics in there are special so that if you are too far away, the beam becomes elliptical from top to bottom, so the top and bottom photodiodes get more light than the left and right one, but if you're too too far away, the left and right ones get more light than the top and bottom ones. And if you're perfectly in focus, the laser for forms a perfect circle and all photodiodes get the same amount of light. I know Hamamatsu offers photodiodes that can be used for that, but they'd probably cost you a kidney, and the chip that's currently in there doesn't expose the focus data to you. If you want pictures for what I'm talking about, wikipedia has images in the article about the CD player.
But driving the coils themselves is relatively easy, their movement is proportional to the voltage across them.
Actually, a laser unit should be a pretty good confocal stage if I'm not completely mistaken, since you have the exact same thing happening that's happening here, and it's all one unit.
@Breaking Taps dude there are absolutely tons of wonderfully documented open source projects utilizing the high end opto-mechanics in a standard DVD/Blu-ray optical pickup.
a bit of googling, there are a few things on hackaday and elsewhere. Really incredible documented projects out there.
One of my favorites will show up in Google image results… You will see a nice looking finished product with all of the circuitboards in a purple color. They got some really good results with that, and I think A set a fresh eyes like yours could really push the project further.
Billions of dollars of R&D have gone into developing the optical pickups in some of the higher end units. They absolutely engineered those things to within an inch of their life 😂
I’ve got dozens of them, so if you need some parts just let me know and I’ll ship you a box no charge. Also have a bunch of really high-end hamamatsu optical sensors and components from a bunch of equipment I’ve repaired or salvaged. So I may just have something you need. I’ll gladly chip in and help however I can
What a great video! I would be interested to see if you could improve on the amount of time it takes to scan by implementing a gradient ascent technique to try to move the stage closer to the focal point without scanning the entire range. You could also use the surrounding pixels to try to determine a good Z starting point for the gradient ascent at the current pixel. Not sure what kind of effect this would have on processing time though.
After watching this video, I thought this was going to be one of those huge million plus channels that I just hadn't heard of yet. That you only have 25k subs is outrageous to me. Everything here is on par with the best of the best YouTube has to offer.
Congrats on making a convocal! I build microscopes for biological research in my phd for living. Some tips: it seems there are is lot of aberration in the spot at the pinhole, resulting in bad contrast. Use a mirror (in focus) and try tilting and moving the second lens to get a bettet spot. You probably want a kinematic mount for that lens to make the alignment easier.
Love the project! Miss doing this stuff. Man if it takes a week to image a surface you wonder how much temperature change over that time frame would impact the sample. Its a small area you're sampling but the whole object would chmage temp. For efficiency buildings can drop temp overnight. You're measuring such small features I wonder if it might be enough to cause some of those anomalies in the data plots. It's always a challenge coming up with the right algorithms to filter the way you want but not kill good data or pick the wrong point.
Awesome video, I love how you explored everything from start to finish. One method to consider when trying to find the focal point of a given pixel is the acutance. This is essentially a measure of the sharpness of the image using local pixels as a reference point. The likelihood is in regions with a high acutance the image is in focus for that region of pixels.
https://en.wikipedia.org/wiki/Acutance
For small movements like, that, maybe a voice-coil based actuator would probably be better - could you maybe adapt a CD/DVD optical block?
@Nuo X Yes, a piezo stack is used in scanning tunneling and atomic force microscopy for exactly this reason. Unfortunately your scan areas tend to be only a few microns across
@Mr. Brown A small photodiode with a well designed transimpedance amplifier will give higher signal to noise ratio than a photomultiplier for optical power levels on the detector of about 1 nW. With milli-watts of power from the laser diode, there there should be far more power than that through a properly aligned optical setup. Phil C.D. Hobbs book " Building electro-Optical Systems" has a very nice chapter on designing a photo-detector front end.
@excited box that is very clever! A blu ray disc reader is basically a tiny confocal microscope, why build one when someone else has already done all the hard work for you?
@Breaking Taps I'm thinking delta printer mechanism with direct drive hight tpi rods,,, should give a LOT of fine control/height/etc. Also, considered an astronomy cmos cam chip? Pretty high qe. I would think this would increase speed and capabilities. Have step setting for speed (resolution/magnification). Um, maybe not... need to do the math... cool vidy ; )
Taking Excited Box idea a step further, how about having the specimen on a rotating stage and scanning the entire part as a series of concentric circles. Each in focus point would be a vector on that circle on the particular z value of the scan. You could use microsteppers to get good resolution.
Great video, 20 minutes fly by, just the right level of detail. From my experience stacking microscope shots and focus failures, it will be that 'specular highlights' (very shiny surfaces) break your algorithm. You'll be assuming that the thing you are scanning has unchanging topology (which is true), but accidentally assuming that when you move the microscope stage that the pattern of light reflected would not change (untrue), because unless the surface is perfectly matte, the 'brightness' does not 1:1 correlate with 'in focus', it also correlates with 'shiny thing pointing at lucky angle to send light back to sender'. Imagine you were scanning a microscopic disco ball, if you strapped a torch to just above a camera lens and moved it closer and further, you'd see that different panels would suddenly become very bright, and others very dark. It confuses normal cameras with their contrast based focus detection (and even sometimes phase detection). If you want to test what I'm talking about, scan some glitter or a retroreflective tape.
I would spray the item to be scanned with a misting of 'airplane glue' smelling hairspray in the giant cans from 50cm away. It will 'matte' your surfaces sufficiently. If it kills the retroreflection of tape, it will be scannable by your algorithm.
Really interesting! Did you try to process the complete 3D volume in ImageJ (or Fiji) instead of generating the 2D image of Z-profile maxima?
I did! Unfortunately ended up cutting those from the video, due to some technical issues and because the video was running quite long. Here's the "E Pluribus" section of the penny, visualized in VolView: https://imgur.com/a/jwwH17y and here is an alternate version of z-maxima where it takes the top 50 points around each maxima: https://imgur.com/a/YXoShko. And here is the other section of the penny: https://imgur.com/a/MgIjihB
Definitely interesting to look at, and helped debug some issues but made seeing the surface harder.
WOW, so cool man! Got yourself a new sub :) I wonder how much power you got on the objective? You could actually use the exact same set up and turn it into a multi-photon microscope. It would be neat. Just use longer wavelength in the NIR and remove that pinhole. For sample, juste use post-it paper or white paper with a lot of yellow marker. I would really like to see that! Keep the good work :)
Awesome work! Some vibration isolation might help with noise reduction. Air table would be nice but I’ve seen open source AFM and scanning tunneling scopes using a heavy platform sitting on balloons to decouple ambient room vibrations. Good luck! And thanks for sharing this with the world!
This is amazing! It's fantastic that you can achieve this level of detail using 3d printed components. The explanation was very useful and covers a topic which isn't well covered elsewhere. Thank you.
Amazing work. I think there's a lot more you could do on the optimization side - like setting the z scan range based on the height of adjacent points (although if you're post-processessing the data, you'd have switch to doing some of that in real[-ish] time). I think this would particularly help in things like the screw threads (unless you're physically limited in range by something else).
Would it be possible to trade off precision for sample volume? (sounds like it would at least require re-gearing the stage motors)
Wow this is the best channel! BT, great stuff as always! If it doesn't mess with your workflow too much you should consider discussing the upcoming projects at the end of your videos. Also, I'm sure you run into a ton of problems trying to get these types of amazing things working, you should consider asking for help (suggestions and ideas) in the comments. It would help with the YouTube algorithm, it would get people emotionally invested and it would help you make more videos. Apart from the obvious, I think one of the biggest reasons that AVE and Applied Science have been so successful is that they spend a lot of time fostering genuine discord in the comments. Great videos!
That's a good idea, just might start doing that! One thing that makes me hesitant is that I'm never quite sure what projects will end up working out. E.g. I have a few being worked on right now, but it's pretty common for a project to hit a roadblock and pause indefinitely (the confocal was nearly at that point, before I had some ideas to fix the problems). I'd be potentially worried about sharing projects that will never get turned into a video due to circumstances. Think that would be a problem? I don't want to mislead viewers, but maybe I'm just overthinking things here? :)
But that's a good point regarding help - lots of smart people watch these videos and might be able to help me unstick the problem projects, so from that perspective it makes a lot of sense to share a little more proactively. Was just chatting with someone who wanted to see some videos about my failed projects, maybe I can do a somewhat recurring video about work-in-progress and failed projects, see if folks have ideas.
In any case, cheers! Appreciate the feedback! Will think over the best way to do that.
Super cool video! I actually deal with laser scanning confocal micros on a daily basis and its super impressive that you were ale to make one on your own, wow :D probably the next, more advanced step for speeding up the scans would be to use piezoelectric mirrors to deviate the laser beam instead of moving the sample. Also motorized lens in Z axis would be awesome to see :)
Very impressive Mr. Breaking Taps! I hope you continue to refine this. One thought that occurred (and I have no idea if this effect is as I imagine it, nor if it can be utilised) is that the laser diode emits photons in a random direction which you carefully collimate and truncate to a circle through the "hole" before the 2 way half silvered mirror. What if you allowed the random photons to bathe the sample and put that camera back in place of the photodiode with a fine grid of holes rather than a single hole. Would that create a 2D image of the sample at a particular height and then use the z axis on the openflexure platform to scan through the heights creating a series of 2D images that you can then analyse and assemble into a 3D data set?
This is a really good observation! I had a segment in the video about this topic but it got cut due to video length. What you propose is very similar to what real, commercial confocals do. They use a spinning disk filled with pinholes in a spiral pattern, and use a camera sensor instead of a photodiode. This lets an array of pinholes "expose" the sample in parallel at that Z-height, and then the the array of pinhole spots are analyzed on the sensor individually. If you search for "spinning disk confocal" or "Nipkow disk" you'll see how it works. So yeah, you're intuition is spot on :)
I need (want) one!
Awesome work, man!
Great project ! For the data visualisation, you could try to display depth using color and intensity using opacity. This way you visually should attenuate the issue of incorrect local maxima. Use a color scale that has a constant luminosity like one of the cubehelix color scales
This man is absolutely incredible. I wish I was able to retain and use learned information as well as he does. He just is a wealth of knowledge.
You retain information by using it. Memorization is a myth and not real learning.
Fascinating. Great job and good explanation of the optical path. I wonder if it would be faster to run the scan like an atomic force microscope, riding the surface by tracking the amplitude of the photodiode. Move a step in x, then move the z up or down to maximize the amplitude, repeat. Advance y each time you hit the end of x. The assumption is the surface is often flat, or at least has a gentle slope, so instead of scanning the whole z stack for an x,y point, just search around the last height (z) each time you move to a neighboring point.
I would think you could then attempt to recognise the trend from previous points in the direction you're currently moving and predict which way you need to scan in z.
This is really interesting stuff. I just finished my masters on the rotationally coupled imaging of spin coating using a similar optical technology for real-time surface topology observation of the process. If I were to continue I'd definitely integrate some of your mechanics!
Awesome work! I'd suggest you to use "modified functional band depth" instead of linear regression - might improve results. Moreover use Google's Trubo colormap instead of jet/rainbow. It's a lot better for perceptual reasons. Really looking forward to updates :)
This was a great video. I've found having a fiberoptic cable act as the pin hole to be a good solution. Also adding 2 scanning mirror to traverse the XY plane reduces the need to rely on mechanical movement. A photo-multiplier-tube is a great way to increase the electrons per photon ratio. Also using a quarter wave plate and a polarization beam splitter lets you keep more of the precious photons going to the source.
Interesting, I had thought about using a fiber but wasn't sure if that was correct or not. Does it have to be a single mode fiber, or would larger multi-mode fibers work too? I guess larger would be fine, since I'm already using a multi-mode laser and I should care one way or another anyway. Might give that a shot!
Good idea about quarter wave and polarizing beam splitter! Looks like I need to grab a few more optical components to have on hand for the next project :) my box of optics is slowly growing haha
@Breaking Taps - MMF is more forgiving because it's 50µm vs SMF being 10µm. Either is a PITA for alignment / focusing. I think going w/ some wave plates and polarizing beam splitter would be more useful to keep those precious photons going to the meter. Good luck!
The quarter wave plate and polarizing beamsplitter also reduces optical feedback into the laser diode. Optical feedback into a laser diode can cause a lot of noise.
Using single mode fiber gives a smaller effective pinhole and a diverging beam that can then be well collimated with a single achromatic lens, which would remove the need for the input lens in your beam expander. I have just butt coupled a telecom patch cable, with a 9 micron diameter core, to a high power LED and imaged the output onto a cheap security camera and have to turn down the current to the LED to not saturate the pixels on the camera. If you need more power into the fiber, you can buy a "visual fault locator Fiber optic cable tester" which couples mW of light into the core of a single mode optical fiber with no alignment. These are available on Amazon for under $20.
You could also use a fiber-optic circulator or a 3dB fiber optic coupler to replace the beamsplitter, which would remove all need to align pinholes to precise locations.
@Bret Cannon That Y coupler is a brilliant idea. The optics would be one focus lens to bring the fibre end to a parallel beam for the microscope optic. a fibre coupled laser and detector would complete the capture system.
The curved surface at the top of the shield is probably scattering light like a convex mirror. If you had a matte textured object you might get better results.
at college they talked about improving scanning electron microscopes by using a DLP mirror array to be able to scan so much faster. May be worth investigating if you're interested, but I have no idea the technical challenges that this would require to overcome.
This is really impressive! It's great to see people really pushing the limits of DIY science.
Super interesting project! It blends my love of making and metrology. Looking forward to seeing more videos!
Subscribed to Your channel. Very interesting project. Much good info in this comment section as well. The recommended Hobbs book is a really great resource. I am a consulting EE and have built a few similar electro optical systems over the years. Can sometimes be quite a frustrating experience. Some suggestions for possible improvements. I would have put the the photodiode preamp close to the diode. Modulate the laser diode and use a phase locked amplifier. This gives more dynamics and gets rid of various kinds of noise. I think reflectivity messes up Your measurements, perhaps try some easier matte specimens first. Agree that a piezo or voice coil based stage would probably speed up things. I have tried to use low voltage piezo stacks for similar use, but beware of some hysterisis that can be compensated for. Dan Gelbert has a nice video on precision flexures.
this reminds me of how old drum scanners worked, but i cant help but feel there is a better way of measuring depth (and over a much larger area simultaneously) while using a camera sensor and detecting image sharpness.
funny video ! It reminded me one of my internship : I had to build an interferometer to mesure optical elements with 10 nm accuracy for the Z dimension. But instead of laser I used a white light source.
Thanks for the video, it brought back some great memories :)
I love the way you explain! Thank you for shining the light of knowledge all over like that.
Interesting device. I had a similar problem with μm precision movement. a stepper motor or mechanical movement is not the way to go. I opt for precision thermal expansion. Using a known pure metal and the coefficient of linear expansion can be useful for μm precision. Getting to an exact volumetric precision of a metal cube to a μm is relatively easy for a precision engineer. The laser moire effect for surface lapping is a great tool. A cheap tool made from an old DVD disk.
Wow!
Really interesting project. Hope to see more of this soon. Thank you for sharing!
As awesome as this is, is there a lens you would suggest for .01 - .05 mm ? As detailed as your 3d models may come out, I don't need to spend days scanning something. Also from my understanding, many 3d scanners have 2 cameras for depth perception and to measure.
Since the system determines depth based on intensity, maybe build a closed loop controller for the laser diode to ensure stability? Also get a bigger laser!
Marco Reps - 2021-04-21
What's this, I see laser but nothing getting burnt, evaporated or even slightly charred? 🤨
JK that's really cool and I am looking forward to that thing back there in the summer!
Sqwince23 - 2021-04-22
why are you browsing YouTube videos? Don't you have a CNC mill to be recording for us?
dr. Kónya - 2021-04-22
@GermanMythbuster Exactly my thoughts =)
jason hargis - 2021-04-27
styropyro enters the chat
Andrew Hill - 2021-05-03
I read this in your voice ....
Spam Dump - 2021-10-03
In my laser class at UCSD we had a lab where we monkeyed with a CO2 laser. The last section of the lab said to turn it up and burn things. Everyone else in my group was not interested and left to write up the other parts of the lab. I was left alone calling out "Where are you going? They're giving us permission to burn things!" I couldn't understand how they could walk away from carte blanche permission to cook things. (Still don't get that.) And yes, I had great fun scorching a bunch of things.I was particularly fascinated by the fact that I could get burn patterns that showed the transverse modes in the cavity.