Member postings for Michael Gilligan

Regarding the slits … Could some kind soul please check my homework ?

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MichaelG.

There is a useful and reasonably ‘accessible’ explanation of Single Slit Optical Diffractions by Olympus, here: **LINK**

https://www.olympus-lifescience.com/en/microscope-resource/primer/lightandcolor/diffraction/

The formula given there is, I believe, a practical simplification of some ‘hard sums’ …

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MichaelG.

Edited By Michael Gilligan on 16/08/2023 06:41:15

S K

It is quite possible that I am nearly brain-dead at the moment
[ if so, please excuse me if what follows is drivel ]

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I think both your 50 micron and 160 micron ‘slits’ are probably too wide to function as slits … and they will each therefore be functioning as pairs of single edges, and producing a pair of overlapping patterns.

From memory: To really be a slit, the width needs to be in the same order of magnitude as the wavelength

MichaelG.

John,

Unless you have a data-sheet for the filter you are using, I can only guess … But I think what you have there is, at best, a ‘brick-wall’ low pass filter not a band-pass filter [which would normally need to be dichroic].

I am too tired now, but I will try to find some reference documents tomorrow.

MichaelG.

[ UPDATE ]

I have just located this … which may be important for future tinkering:

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The IR diode of course [by the nature of these things] is essentially a single wavelength emitter [950nm]
BUT

The silicon photodiode is sensitive from 400 to 1200 nm

Therefore … Stray light problems might be considerably reduced by inserting a suitable narrow-band-pass filter in the beam path.

MichaelG.

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Edit: __ the word silicon has been struck through, in deference to S K  

Edit: __ HOWEVER … please see the words in Sharp’s text, which can be dowloaded here:

https://proto-pic.co.uk/content/GP1A57HRJ00F.pdf

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Edit: __ for technical reference
https://opticalfiltershop.com/shop/bandpass-filter/nir-bandpass-700nm-to-1100nm/nir-bandpass-filter-950nm-fwhm-100nm/ 

Edited By Michael Gilligan on 15/08/2023 19:54:49

Edited By Michael Gilligan on 15/08/2023 20:00:18

Edited By Michael Gilligan on 15/08/2023 20:08:18

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Very common practice on Tumble Driers … for obvious Production-Engineering reasons

MichaelG.

Thanks for the endorsement, Dave

I was impressed, but it’s good to know that I am not alone in that.

MichaelG.

Given where you found it … I think it might be worth exploring whether it could be a convenient aid to sketching the layout of clock wheel trains.

MichaelG..

In reverse order, S K

Please don’t worry about my lens … the optical requirements are well known to me and I was only describing adjustments that I have previously made [in the hope of attracting sufficient interest that someone might help me with the programming to make it work as a switch].

I was curious about the laser and the slit because your laser is red

The Sharp opto is an IR device and I think it reasonable to assume that the clever people at Sharp would have designed its optics accordingly.

My question was part of my general quest for understanding. … So thanks for answering.

MichaelG.

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You have my sympathy, but …

[given what you have already demonstrated about diffraction], I have to ask why ?

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MichaelG.

This is unlikely to stir anyone’s blood … but it’s informative and quite well presented:

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Edited By Michael Gilligan on 15/08/2023 07:17:22

Sorry … I’ve been trying to remember a product name, and my Google-Fu is evidently weak

Years [decades] ago we used some product [probably made from horse-hair] which came wet, and rolled-up in a tin.

This was put on the floor, under the machine feet, and after a day or two the liquid was gone and the remaining ‘felt’ made a conforming, and slightly flexible, layer which adhered to both floor and machine.

The machine could be prised off with a crowbar, but for normal use was considered fixed.

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Great stuff, if only I could remember the name !

MichaelG.

Looks like it might be one of those “which side of the pond” issues : **LINK**

https://en.wikipedia.org/wiki/Cotter_(pin)

which [Heaven forfend!] may lead to debate about ‘English as a living language’

< sigh >

MichaelG.

Package drawing:

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I think at least one of these will need careful dissection

… the slit width doesn’t look anything special.

MichaelG.

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Edit: __ just found this, which is possibly a close relative:

https://br.rsdelivers.com/product/sharp/is485e/sharp-is485e-photodetector-amplifier-unit-through/1775969

… Check the data-sheet, downloadable from that page

Edited By Michael Gilligan on 14/08/2023 22:36:12

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I have read it twice this evening, John … and I remain astonished

The biggest ‘take-away’ for me is that the Sharp opto is seriously compromised by ambient light,

[quote] It quickly became apparent though that even ambient daylight has a large effect on precision, so it is best to assume that the sensor should be shaded as well as possible. [/quote]

Left to its own devices [so to speak] the performance is quite amazing; and this, of course, is using a simple IR LED that provides ‘flood-lighting’ !!

All the clever stuff must therefore be credited to the sensor itself … Mmm !!

I have the package drawing, and have purchased a small quantity of these devices to play with study

… in due course [which may be a while yet] I will try to better understand the optics.

MichaelG.

That’s a very impressive performance you recorded for the Sharp Opto, John

… I would be interested to see the the raw data, if you have it to hand.

I am embarrassed to admit that I may not have fully understood your test procedure:

Are you saying that, if we call the true position zero, almost all your individual measurements were within the range -0.0006 to +0.0006 mm ? [i.e. zero +/- three standard deviations] … if nothing else, that’s an amazing performance from your CNC system.

Regarding the camera idea: many alternative lenses and sensors are of course available … my ten micron resolution is just an example, using a very modest webcam to demonstrate the concept.

MichaelG.

Please just simply read the notes that I made for John

Yes the lens would be in place

No there is no likely problem with blur

All it attempts to do is detect the peak travel of the pendulum … at which instant the pendulum would be stationary.

With a notional ten micron resolution, I think it should be useful.

Over and out

MichaelG.

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Sorry … I have only just read this, I was busy writing what I promised for John.

You appear to have completely missed the point of my suggestion.

MichaelG.

As promised, John …

This is very simple stuff, but if I have glossed-over anything, just shout

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Here is the board from a little webcam:

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I removed the lens assembly [ you can see its footprint] to reveal the sensor chip:

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The small brick-red rectangle is the active area.

I have never managed to identify the actual chip, but it appears to be a 1/9” sensor with 640x480 active pixels on a 2.5 micron pitch.

All of which is largely irrelevant, except that it means the focal length of the lens is small, and therefore it can be focussed over a wide range with only short adjustments.

The camera is UVC [Universal Video Class], which means that it will run ‘driverless’ on most any modern computer.

Conveniently, that includes the Raspberry Pi, on which it works nicely with the GUVC application.

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Now … [assuming that we have the complete camera before us] first remove the decorative ‘trim ring’ and confirm that the lens screws freely in and out.

Plug it into your computer of choice, with whatever viewing software you have, and adjust the focus so that a -1/4” wide target just over-fills the frame-width … at which point, each of the 640 pixels is representing 10 microns.

[sorry about the mixed units, it just works out tidily that way].

You now have a microscopic view of an adequately wide field, with approximately 10 micron resolution … this could, of course, be meticulously calibrated, but I see no need in the present application.

… I can leave you to calculate how good that would be in milliradians at your chosen radius.

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Your mission, should you decide to accept it, is to find a way to grab the data, and then apply whatever logic to trigger an impulse at the chosen point whenever the swing has decayed by whatever amount you choose.

Matthias Hipp will be honoured, I will be grateful, and this guy should be impressed:

**LINK** : http://www.rogerj.co.uk/clock.htm

MichaelG.

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If I survive the next several hours in the garden, I will jot down a few thoughts this evening.

Doing it really well might be a little complicated, but as a demonstration of concept the optical side can be done with a cheap USB webcam. … My stumbling-block has been not knowing how to grab the the data.

More anon

MichaelG.