LED white balance and blue/green artifacts

[sreynolds]

Setup is Wild M7A with Canon M200 mirrorless attached via direct projection using Wild 404891 phototube, light source is AmScope 30W LED dual gooseneck fiberoptic illuminator. I set the white balance to manual on the camera, using a blank white image as the source. The white balance is good, but there is still a strong blue/green aura around objects photographed. Can anybody explain this, and offer a remedy? Example is a small mouse feces, center crop. Sorry....I just used what was handy.

[ldflan]

Axial chromatic aberration, not a white balance issue. If you can stop down the objective, that can help. Reducing the amount of light, and diffusing the light (ping pong balls, say) should also help. Post processing correction for whatever is left.

[MichaelG.]

.

It is, I believe, important to note that the fringe in your image is not consistently coloured.

This leads me to believe that it has more to do with alignment than with the glass.

Edit: __ Here is probably the best page I have seen about ‘purple fringe’ chromatic aberration:
https://learn.captureone.com/blog-posts ... -fringing/
Note specifically that the fringe is the same colour at both edges of the spoke.


As mentioned by Idflan, the white balance looks good … so I would resist the temptation to play with that until the underlying problem has been minimised. You will need careful experimentation, but I suspect the problem might be the illuminator.

Can you try a similar shot with a different light-source ?

MichaelG.

[Scarodactyl]

There can be quite a bit visible in achromatic CMO objectives.

Edit: __ Here is probably the best page I have seen about ‘purple fringe’ chromatic aberration:
https://learn.captureone.com/blog-posts ... -fringing/
Note specifically that the fringe is the same colour at both edges of the spoke.

Axial CA is one color for out of focus areas closer to the lens and one for areas further from the lens which is likely what we're seeing.
Edit: no, looking again this must be lateral in soute of the coloration.

[Alexander]

Considering the different colors on both sides it very much looks like that scope is out of alignment. This is problem not uncommon with older stereo scopes.

[LouiseScot]

Hi

I've had a similar problem using a plan fluor 40x/0.85 objective on my Amscope 720Q, also with a Canon camera, and I initially also thought it could be a camera/white balance issue. viewtopic.php?p=126890#p126890
My images were heavily enlarged (100% crop of original image), though, and the issue was only really visible through the eyepieces if I used a 25x EP. I think the general consensus was that it was most likely linked to the (factory fitted) LED light source since experiments with a halogen bulb were much improved (probably would have been better if I'd thought to use a blue filter under the condenser...). I've since purchased some better LEDs with cri90 but haven't yet got around to trying one. If you can set up halogen illuminator instead of your LED, that might solve your problem.

Louise

[Hobbyst46]

I would suggest looking at the photos provided by the seller of the same Wild model:
https://www.ebay.com/itm/154503118310

They posted photos of a coin at various magnifications. Illuminator was not a LED, it was apparently a fluorescent lamp. No details of the camera , adapters, interface etc. Still, if you copy the photos and enlarge them in any processing software, the amount of fringing is visible. Perhaps it can help.

[jfiresto]

I could not find any coin pictures in that ebay listing. Perhaps ebay's system redirected a link to another, "closest" still active item?

The fringing of the OP's high contrast specimen could very well be lateral chromatic aberration inherent in the CMO design, from looking through the sides of the objective rather than dead center. The posted image appears to be a crop near an outer (vignetted) corner where you would expect the LCA to be greater.

If we can find a common specimen to image, I can compare the LCA of another M7A that was cleaned and checked a few years ago.

[Hobbyst46]

I could not find any coin pictures in that ebay listing. Perhaps ebay's system redirected a link to another, "closest" still active item?

https://www.ebay.com/itm/154503118310
Scroll down, on and on, the photos are visible along the right side of the screen.

[LouiseScot]

I could not find any coin pictures in that ebay listing. Perhaps ebay's system redirected a link to another, "closest" still active item?

The fringing of OP's high contrast specimen could very well be lateral chromatic aberration inherent in the CMO design, from looking through the sides of the objective rather than dead center. The posted image appears to be a crop near an outer (vignetted) corner where you would expect the LCA to be greater.

If we can find a common specimen to image, I can compare the LCA of another M7A that was cleaned and checked a few years ago.

You have to scroll right down!

[jfiresto]

Thanks guys. I found them after disabling an overeager ad blocker.

The coin images may show less LCA if they were shot through a 20X/13(?) eyepiece.

[sreynolds]

Here https://www.stevereynoldsinstruments.com/2022_10_29/ are a couple shots of a US quarter, still with the LED light. I will try to find a halogen source to compare. Quarter at minimum and max magnification, 16x eyepiece, phototube iris at halfway. If you can post comparison shots that would be helpful. Thanks.

[jfiresto]

I could not find a U.S. quarter, so I used a just slightly larger Swedish krona. First the head of Carl Gustaf at 6X

m7a_6x_left_6000K_ring.jpg (163.53 KiB) Viewed 3210 times

Then some detail on the back at 31X

m7a_31x_left_6000K_ring.jpg (205.61 KiB) Viewed 3210 times

EDIT: Here is the back, again, but this time illuminated by a less oblique Schott halogen ring light [that is more like it]:

m7a_31x_left_halogen_ring.jpg (204.58 KiB) Viewed 3210 times

All were taken with an E-PL8 MFT camera, focus peaking and half closed apertures, then decimated to 640 pixels.

[MichaelG.]

Example is a small mouse feces, center crop. Sorry....I just used what was handy.

.
Just for the avoidance of doubt ^^^

MichaelG.

[BramHuntingNematodes]

I have not seen any 30 watt LEDs with particularly even spectra.

[sreynolds]

I did a comparison of the LED with a Xenon-bulb mini-mag flashlight - I'm willing to believe that I need a better light source than the AmScope LED. The LED has distinctly more purple fringing than the Mini Mag, and that flashlight is very marginally qualified for what I am doing with it. I set the camera on auto while balance for the Xenon bulb.

full LED.jpg (272.34 KiB) Viewed 3254 times

full Xenon.jpg (246.21 KiB) Viewed 3254 times

crop LED.jpg (170.52 KiB) Viewed 3254 times

crop Xenon.jpg (187.16 KiB) Viewed 3254 times

[LouiseScot]

That's as I found, albeit with a halogen. I might look into xenon g4 bulbs also.

Louise

[MichaelG.]

I did a comparison of the LED with a Xenon-bulb mini-mag flashlight - I'm willing to believe that I need a better light source than the AmScope LED. The LED has distinctly more purple fringing than the Mini Mag […]

An excellent demonstration !!

<< Long-live the fibre-optic gooseneck, with a filament lamp >>

MichaelG.

[jfiresto]

That is quite an improvement. I repeated my earlier 31X image with a Schott halogen ring light and it is noticeably better,
especially when you flip between the images.

I picked up a Schott KL1500 for cheap that I will try to convert to LED. I was planning to use a high CRI neutral white LED

nichia_5000K_CRI98_led.gif (31.48 KiB) Viewed 3209 times

but I wonder if I might do better with a warm white LED with a much smaller blue peak.

Thank you for the pictures and comparisons – they were very helpful!

[MichaelG.]

but I wonder if I might do better with a warm white LED with a much smaller blue peak.

That would certainly be my choice ^^^

It is tempting to rely on the single CRI figure; but ultimately it is too simplistic
… i.e. different spectra can have the same CRI

The Devil is in the detail

MichaelG.

[Alexander]

I picked up a Schott KL1500 for cheap that I will try to convert to LED. I was planning to use a high CRI neutral white LED

All LEDs on the market today feature a significant hole in the green range. This is the reason why color critical applications like pol-microscopes still have halogen illumination, not LED.

[LouiseScot]

I bought some Osram CRI90 LEDs recently (though not actually tried them out yet!). My intended use is mainly for ordinary transmitted light, brightfield. You could maybe construct a kind of ring light using a number of them plus a diffuser. They are only rated at 2W but 230-260lm depending on colour temperature. This is the emission spectrum they publish for the range:

Osconiq_c2424_cri90.JPG (64.28 KiB) Viewed 3175 times

It's actually biased/shifted towards the red end of the spectrum with no apparent actual hole in the main green part - but clearly a dip/trough around 475nm (cyan). I hope I can get the time to try them out in the next few weeks! There must be other brands that can also do broad spectra without huge blue peaks and gaps? Of course, these Osram LEDs I have cost a fair bit more than the really cheap ones but needs must! One of these days I'll dig out my diy spectrometer and see if I can acquire some actual spectra!

Louise

[BramHuntingNematodes]

I picked up a Schott KL1500 for cheap that I will try to convert to LED. I was planning to use a high CRI neutral white LED

All LEDs on the market today feature a significant hole in the green range. This is the reason why color critical applications like pol-microscopes still have halogen illumination, not LED.

Hmm don't know about that sounds a bit like old fashioned thinking Zeiss currently has several led options for polarizing research scopes

[imkap]

At first I had an obviously bluish light led and blue CA was very obvious, later I obtained a 4000K CREE led and blue CA was obvious again on camera but a bit less, so with white balance setting and condenser moving up and down I was able to eliminate it to some extent or completely sometimes. But with that light some halo appeared, not sure why. The hole through which the light passes on my GFL is small, I felt that the LED might have been too powerful for the small hole. Just a feeling I don't know. BHS does have a much wider diameter glass...

After that I was wondering the same thing and wanted to order a 2700K with a smaller peak in blue. But meanwhile I got a BHS microscope with original halogen lighting so never tried the 2700.

CA is visible with halogen too, but not always. Sometimes it bothers me, sometimes I don't notice it and sometimes I remove it with software.
When I turn up the light near max, there seems to be more CA, not sure why. So I turn it down and set higher ISO. Noticeable mostly in DF which is not original but improvised... Maybe with original Olympus parts (DF condenser) and APO objectives it would go away (almost) completely...

[ldflan]

I think the CA problem is just inherently complex. Like a lot of others, I've tackled these problems in the context of microscope LED conversions a few times and come away with these thoughts as my (definitely non-physicist) take -

1. You cannot really correct axial chromatic aberration only by selecting a different color balance for your light source, or by filtering your light source. Axial CA is the lens system's inability to focus green and violet light in the correct plane, not an imbalance of color. To some degree you can limit the apparent effect of CA by color filtration, but potentially at the cost of a correct white balance. And for the same reason it is true that an excess of blue or violet in the LED color balance seems to accentuate the problem at that part of the spectrum.

2. Some CA is going to be present in almost any lens system, at least under challenging imaging conditions such as high contrast and highly specular lighting. So attention to the amount of light, the diffusion, angle of entry of the light into the lighting system, and (if possible to adjust) especially the objective aperture can each be simple and easy ways to address the problem.

3. Chromatic aberration can also be a the result of internal lens misalignment, so that possibility should be kept in mind.

4. I would defer to an optics expert on this, but it seems to me that if you introduce an LED that has its own collimating lens or reflector (over or behind the LED) into a system that was designed for a halogen or other bulb, you have not reproduced a simple bulb as you intended. Instead you have created an entirely new multi-element lens system that includes the LED's collimating system, and this can and does introduce aberrations not corrected by the original optical design of the lighting system and imaging optics.

5. Many retrofit LED systems also produce something much closer to a point-source of specular light when compared to a filament. This light quality (not color) will only aggravate any chromatic aberrations. The better ones (in my limited experience) seem to use LEDs that do not have their own collimating lens.

6. The camera makes a difference that is hard to predict. When you introduce a digital camera sensor you have added another lens element that can introduce chromatic aberrations. Specifically, digital camera sensors are overlain by microlenses of their own...

This is to say nothing of the relay lens (if any) being used...

[imkap]

Yes, I agree with all stated... It is complicated and probably caused by multiple things in the light path.