USB microscropsy - Getting the best image quality

[grepper]

USB microscopes are fun to play with, but I have found the images they produce can be so crappy as to barely be useful or informative.  I was curious as to why, especially considering the pretty good image quality produced by the current crop of even inexpensive digicams.

One of the limitations of the scopes is that they are point and shoot, with no manual control of exposure time, f-stop or shutter speed.  That said, the one I have does have circuitry that attempts to maintain reasonable exposure by adjusting for the amount of light entering the thing.  For instance, when a bright light is directed at it the display will white out, and then over a period of several long seconds the sluggish electronics will back off the exposure.  Of course, like any programmed exposure mode in a camera, this can be useful at times and frustrating at others.

Another problem is the very nature of the subject itself, in this case a shiny steel blade.  A steel blade, especially the bevel which is the area of interest, is highly reflective and even with diffused lighting it's easy to have overexposed, blown out spots in the image.

Additionally, apparently steel is not only reflective but also refractive.   Depending on the direction of the lighting, this causes a rainbow effect that produces a very deceptive image.  Tiny scratches are greatly exaggerated, there appear great chasms where none exist, and smooth edges can appear very toothy.  The resulting image can be oddly aesthetically pleasing, but not informative or useful for bevel/edge evaluation.

Anyone with an interest in photography knows the importance of lighting and how it can make or break and image.  Ends up, it is extremely important here.  Due to the point and shoot nature of the scope, the only control the user has toward the resulting image is the manipulation of lighting and the subject background in an attempt to regulate the exposure, shadows and contrast of the image.

I struggled for a long time without success attempting to get acceptable imagery out of the microscope.  The scope has an internal ring of adjustable brightness LED lamps surrounding the lens.  I found them to be a horrible illumination source for close proximity blade photography and it was only after turning them off and using external, directional lighting that I was able to produce useful images.

Anyway, to illustrate the point, I stuck a knife under the scope and took the following images.  The images are crops of about the same area of the blade, without moving the blade under the scope.  The blade is extremely sharp, with a super smooth edge and a highly polished bevel.  It's the same blade that I used in this post and has not been altered since:
http://forum.tormek.com/index.php?topic=3165.msg18296#msg18296

The only difference in the following images is the direction, intensity and amount of diffusion of the light source, and the color and reflectivity of the background.  I have found that with a little bit of playing around it is possible to greatly improve the image quality, and actually produce useful imagery from a cheesy little USB microscope.

[Herman Trivilino]

Additionally, apparently steel is not only reflective but also refractive.   Depending on the direction of the lighting, this causes a rainbow effect that produces a very deceptive image.  Tiny scratches are greatly exaggerated, there appear great chasms where none exist, and smooth edges can appear very toothy.  The resulting image can be oddly aesthetically pleasing, but not informative or useful for bevel/edge evaluation.

I do not think this is refraction. I think it's chromatic aberration.

Chromatic aberration occurs when a lens focuses different colors of light differently. I'm sure you're familiar with the rainbow created when light shines through a prism. This is the same effect.

You can easily observe chromatic aberration with a magnifying glass. Look at something and adjust the position of the lens in an attempt to get the largest possible magnification. You'll notice there's a limit to how far you can go enlarging the image. At some point the image becomes so large it blurs and you see nothing of value. Just as you reach that limit look at the outer edges of the image and you will see colors. That's chromatic aberration.

There are three remedies. One is to use mirrors instead of lenses. This is one advantage of a reflecting telescope, it gives clearer images free of chromatic aberration. The other advantage is the telescope is much shorter, although it's also fatter. This is not a solution for a microscope, though, as far as I know. A second remedy is to use lenses that are not spherical but are instead parabolic. That's not cost effective. The third and most common solution is to design the scope so that lenses have less curvature. This is equivalent to restricting your viewing through that magnifying glass to the center portion of the lens where the least of the aberration occurs. This, however, means less magnification so a combination of lenses (and often also mirrors just so that the geometry of the lens placement works out in a way that allows it all to fit in a smaller container) must be employed. This also adds to the cost.

You mentioned refraction. That occurs only when light passes through something. That's why the two major telescope designs are called refractors (they use lenses) and reflectors (they use a large primary concave mirror). Perhaps the most spectacular and famous reflector is the Hubble Space Telescope. You may have also heard of the Dobsonian reflector. Or Ken's personal favorite, the Astroscan.

[grepper]

True, true.  I had forgotten.  Thanks for the correction Herman.  Appreciated!

Under perfect conditions I get a very small amount of chromatic aberration evident if I zoom in some digicam pics.  It rears its ugly head as purple banding on the edges of dark objects such as tree leaves against a bright, overcast sky.  I see it mostly when the camera is at full zoom and the lens elements are furthest apart.  Not a big fan either in digicam pics or microscope images!

[Jan]

Grepper, thanks for sharing your experience how to get a good quality image of a blade using USB microscope. 

As you have said, suitable illumination is important prerequisite for good quality image. I am wondering about the "whitness" character of white LEDs light. As far as I know the most common white LEDs are de facto blue LEDs coated with yellow fluorescent phosphor which generates the two complementary colours necessary to produce white light. Because the LED light is coherent, it is more prone to interfere with its own reflections which may obscure the observation of scratches, which I consider as quasi periodic grits.

Professional optical microscopes for metallurgy usually use high intensity (50W) halogen lamp.

I am a "scratch hunter". I am interested in the depth of scratches because in my understanding the steel crystalline structure is disrupted to a depth which is several times larger than the depth of the visible scratch itself. The existence of such a deep scratches near the cutting edge is limiting its durability.

The principle is following: when the scratch is parallel with a slip band of a surface grain it can cause a tiny step in the metal surface which may serve as a stress riser where fatigue crack can initiate. 

Jan

[grepper]

That fracture crack is interesting.  For me at least, it is very difficult to product a totally scratch free bevel!  I'm not even sure that at the 5K X - 10K X mag level it's even possible.    But your point is well taken.

You said, "I cannot apply a 500X magnification in reflected light, the objective is very close to the object, circa 2 mm and the object is not well illuminated by the incident light."

Just wondering if you have you ever tried side lighting with a flashlight under the objective?  Even a 2 mm gap leaves a lot of room for light!

[Elden]

   Looking at the picture of Jan' s microscope (http://forum.tormek.com/index.php?topic=3165.msg18417#msg18417, I am not sure that there would be 2mm of space between the high power objective and the work piece. It looks similar to the scopes that we used in college back in 78 - 80 as far as the objectives are concerned. I remember cracking that thin slide cover (it would have been well under 1mm thick) with the high power objective when trying to obtain proper focus a few times. I do not remember if it was 500 or greater power, however.

[Jan]

Elden you are correct, the objective is sometimes almost touching the blade. 

Grepper, I was too optimistic, the gap is much smaller than 2 mm. For the illumination of the blade I use the back scattered transmitted light which source is situated below the microscopic table. The depth of field is small because the available light intensity is insufficient. Not the whole FOV is sharp.

Despite of the limitations I can do some edge observations at the 500X mag level. Attached you can see the edge of Victorinox kitchen knife standardly sharpened 220/1000 and honed on Tormek. After sharpening the knife was used for 1 month.

In the other attachment you can see edge defect of an expensive Santoku knife of VG10 steel. This defect was hardly detectable without a microscope.

Jan

[grepper]

Thanks for checking that out Jan.  I wish my USB scope had more magnification.  I like 500X better than 200X.  I am a big fan of X.  I want lots of X!  The more X the better!

What is the brand of your expensive VG10 knife?  I'm guessing about HRC 60?  The edges of that gnarly, gigantic canyon of an edge defect look pretty smooth and contradict my first thought that it was chipping due to hard, possibly more brittle VG10 steel.

[Jan]

I have got this santoku knife as present. It is Kanetsune Seki 33-Layers Nickel Damascus VG10 Santoku Knife 180mm, HRC 60-61.

https://chefknivesjapan.com/collections/kanetsune-seki/products/kanetsune-seki-33-layers-nickel-damascus-vg10-santoku-knife-180mm-kc-102

Jan

[grepper]

Nice blade you have there Jan!  Being HRC 60-61, how does the edge retention compare with the more run of the mill kitchen knife which generally runs around HRC 58 or so?

Do you find sharpening it more time consuming than the basic kitchen knife?

I notice that site also sells knives with ZDP189 steel.  I've always wanted to have a knife with very hard steel to play with but have always been put off by the price.  Even the Shun knives with VG2 steel which is a little harder, HRC 61-62, get pricey quickly and they are a long way from ZDP189 @ HRC 66 or so.  Sadly I'm sure that I'm never going to shell out $500 for a kitchen knife! 

[Herman Trivilino]

I am interested in the depth of scratches because in my understanding the steel crystalline structure is disrupted to a depth which is several times larger than the depth of the visible scratch itself.

It would seem then, that the practice of polishing out scratches is largely a cosmetic improvement, leaving the damages hidden below the surface!

[Herman Trivilino]

Nice blade you have there Jan!  Being HRC 60-61, how does the edge retention compare with the more run of the mill kitchen knife which generally runs around HRC 58 or so?

Do you find sharpening it more time consuming than the basic kitchen knife?

I believe that chefs who use these knives are in the practice of sharpening them every day.

[Jan]

I am interested in the depth of scratches because in my understanding the steel crystalline structure is disrupted to a depth which is several times larger than the depth of the visible scratch itself.

It would seem then, that the practice of polishing out scratches is largely a cosmetic improvement, leaving the damages hidden below the surface!

Herman, I hope that honing is more than cosmetic operation. Nevertheless in my understanding even below a scratch-free surface they are some groups of atoms in irregular positions as remnants of deep scratches. Those linear defects are called dislocations and are not visible on the surface.

At this point it is important to note that crystalline defects are not always bad. Some of them were introduced intentionally to manipulate the steel mechanical properties. 

In my next life I want to be a metallurgist possessing a small foundry and smithy. 

Jan

[Jan]

Nice blade you have there Jan!  Being HRC 60-61, how does the edge retention compare with the more run of the mill kitchen knife which generally runs around HRC 58 or so?

Do you find sharpening it more time consuming than the basic kitchen knife?

The edge retention of the VG-10 blade is really very good. I am sharpening this knife only twice a year while honing it in between. It takes time to remove some steel from the VG-10 blade, especially when applying very small pressure, and this may be the reason for some microscopic edge defects which I detected only using the microscope. May be not enough steel was removed and some edge defects remained.

What concerns the Victorinox kitchen knife, I received it extremely sharp and was wondering why it cuts so easily. The reason was that the knife came with an edge angle of 20°. The edge retention was limited to one month or so. Then I re-sharpened the blade to an edge angle of 30° and the edge retention increased circa three times.

Jan

[grepper]

Jan-  Concerning the Victorinox blade, even though you specify edge angle, just to be clear, are you talking about the included bevel angle or the half angle?  Either way, it's a little hard to figure.  20° included would be 10° edge angle each side which would be an unusually acute angle for a factory sharpened blade!  Victorinox chef's knives are factory sharpened @ 15° each side.

If you resharpened to 30° included angle, then you brought it back to factory spec.  If you mean 30° each side, that would be pretty blunt, more like a cleaver.  It would however be very durable.

Anyway, it had me scratching my head a little.