On Wednesday 26 September 2018 21:45:42 Chris Albertson wrote: > I think what you are seeing is a "speckle pattern" The laser is not > making 40 small beams. it is making one nice uniform beam but > remember the light is all in phase. After a reflection from a > textured surface the light hitting the back your eye adds in phase and > the spots you are are where the light adds in phase and the other > places the wave self-cancels > > see the second paragraph on this page: > https://en.m.wikipedia.org/wiki/Speckle_pattern. > If I am interpreting that correctly, and with the spindle stopped, I should see this "interference" pattern change rapidly as the carriage to target distance changes by a single microstep of the screws driving motor. I don't see that effect, the individual dots remain stationary.
Not discused is the diffraction effect of passing this coherent bean thru a pinhole other than a passing statement that this speckle effect will cause the spots to be larger if viewed thru a pinhole. One naturally assumes the pinhole is in front of the eye, not the lasers exit aperture from a statement like that. Also, all the patterns shown are circular. The patterns thrown on the wall by a laser pointer I've had for years, a laser level, and both of these devices are bars 3 or 4x longer than wide. The dot patterns I am seeing rotate with the device, remaining stationary when the reference is the rotation. IOW go put a magic marker dot on the paper for each dot on a distant target many feet away, then turn the device on its axis by some arbitrary number of degrees. The dots will rotate, and the dots can be re-registered by rotating the target by the same number of degrees. One side of most alu wrapping foil has a frosted finish, so internal reflection of that that light that does not pass thru the pinhole on its first pass but bounces around should result in a larger more diffuse pattern, The other side, being more highly polished should result in a more easily seen circular diffraction, and it will be interesting to see if in the real world, a difference can be seen. And the pinhole material s/b thin. A 1/32" thick piece of brass will have far larger diffraction than a .0005" piece of alu foil for a same sized hole. Interesting series of experiments ahead when the stuff gets here. And someone mentioned using a lappy to view the results, but I'm not sure my old hp with a 1.4ghz turion cpu is enough "iron" to get the job done. I have 640x480 cameras that can focus down to 2 or 3" in order to focus on the target, so it will be tried, and results reported in due time. > If the pattern of the dots stay the same as you move the laser around > then the "speckling" is happening on the lens and some parts of the > beam are flying in a shorter or longer path length. and add the all > add in phase. > > Even if the laser were making 40 spots and the beam as not on-axis, > don't bother to fix it. You don't need to. A 1 depress misprinted > beam will give only tiny error for your purpose. Down in the 2% error > range. and I bet you can aim the beam by eyeball to 1/2 a degree. > That is good enough. > > Even if there are 40 beam hitting your target, so what? Be happy not > you can take 4 measurements at once and drastically reduce the signal > to noise ratio. If you are using software you are like tough an > autocorrelation between images and the is a peak when yu have found a > match. having a complex parter makes the peak of the function more > steam and you have better and more accurate results. 40 points is a > Good Thing, as long as they don't change. > > Again, you beam is fine, it is just that most people forget the laser > is 'coherent" light and all in phase. > > On Wed, Sep 26, 2018 at 4:24 PM Gene Heskett <[email protected]> wrote: > > On Wednesday 26 September 2018 17:22:01 Chris Albertson wrote: > > > ---------- Forwarded message --------- > > > From: Gene Heskett <[email protected]> > > > Date: Wed, Sep 26, 2018 at 12:08 AM > > > Subject: Re: [Emc-users] More on bed wear fix > > > To: <[email protected]> > > > > > > > > > But finding a camera chip big enough to shine these laser's on so > > > that > > > > > > > the laser stays on the imaging chip cannot be found in an > > > > integrated package that will plug into the 15 pin connector on a > > > > pi. The laser beams are 10x the size of the imaging chips even > > > > if they're perfectly aligned. > > > > > > What? If I buy a normal green laser pointer the green spot is > > > about 1mm in dimmer. If I place a 0.01 pinhole in the path of > > > the laser the spot is pretty close to 0.01. As for a camera that > > > plugs into a Raspberry Pi, any USB web cam will do. But I'd use a > > > notebook PC rather then the Pi. What you probably want is not > > > to shine the beam on the sensor but have a glass or plastic target > > > and use a $20 USB microscope to focus on the target. Glass is good > > > because it will reflect 99% of the laser light away and not blind > > > the camera. > > > > > > As you move the carriage the laser spot should move on the target > > > and the USB microscope will image the back side of the target and > > > you can measure the movement by counting pixels, Software can do > > > better by doing some curve fitting. Even if the bed is very bad > > > I doubt the laser spot move even 0.5mm > > > > > > If the laser spot looks huge, perhaps what you have is a small > > > spot that overloads the sensor. > > > > I am looking at the spot with my eyes. I can see the individual > > dots, probably 40 of them. If I could restrict it with a pinhole on > > the laser to block most of the dots, that would be great, but even > > with edm I don't think I could make a pinhole that small. > > > > > The target may need to do some serious > > > amount of attenuation. > > > > I have a polarizing variable filter with a range from ND-2 to ND-400 > > coming. > > > > > And while I said "USB "microscope" I bet a > > > > > > magnifier hot glued to a web cam works. But I did buy a $10 > > > microscope and the image is poor but I can inspect the solder > > > joints on SMT parts, A medium size SMT resister pretty much fills > > > frame, these things are on eBay for $9 to $35 and they all appear > > > to the the same. > > > > Humm, more experimentation seems in order. I have drills down to > > #80, and yards of Reynolds wrap, which might get me down to a usable > > spot size. > > > > > Pretty much everyone who wants to measure displacement from a line > > > uses a laser. It is hard to find a better reference line > > > > > > If your bed where really bad Just put a steel rule in the tool > > > holder and eye-ball the laser spot in the rule, maybe use a loupe > > > to read it. > > > > > > Something is wrong if the laser spot is large. > > > > Its way too big at the source, and 2 of them (different brands) are > > doing it. The pinhole at the src seems like a good starting point. > > > > [...] > > > > -- > > Cheers, Gene Heskett > > -- > > "There are four boxes to be used in defense of liberty: > > soap, ballot, jury, and ammo. Please use in that order." > > -Ed Howdershelt (Author) > > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > > > > _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users -- Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
