blue in center of FOV

[92111]

I have a blue in the center of the FOV. I see it bouth in the eyepieces and camera . It is most prominent on using 40x and 100x objects , not prominent on using 10x and 20x objects or dark feild and phase contrast

[92111]

similar problem janvangastel

[MichaelG.]

My guess is that you are seeing the LED emitter, which is a tiny dot at the centre of the lamp.

MichaelG.
.
.
Edit: for clarification
The cluster of emitters is just visible in your image of the LED
... I am thinking that one of these happens to be positioned on, or very near, the opical axis

[92111]

i guess i am seeing the reflection of coating of lens

[MichaelG.]

No one else has responded, so here is a picture of a typical COB LED [larger than yours] showing the individual emitters. These produce blue/violet light, which causes the yellow matrix to fluoresce.

If you have one of these emitters on or near the optical axis, I still think it may be the cause of your blue spot.

COB _LED.jpg (164.76 KiB) Viewed 14619 times

MichaelG.

[mrsonchus]

No one else has responded, so here is a picture of a typical COB LED [larger than yours] showing the individual emitters. These produce blue/violet light, which causes the yellow matrix to fluoresce.

If you have one of these emitters on or near the optical axis, I still think it may be the cause of your blue spot.

COB _LED.jpg

MichaelG.

Now THAT's new information for me, and very interesting...
I had no idea that the COB LEDs worked this way, inducing fluorescence in the yellow matrix. I think your explanation of an emitter near the OA is worth serious consideration, and a strategy to test it. Would simply rotating the LED give any indication of movement of the position of the bright/blue-spot or is it too indistinct perhaps?

John B.

[MichaelG.]

Would simply rotating the LED give any indication of movement of the position of the bright/blue-spot or is it too indistinct perhaps?

Yes, I think it might show some movement, John ... certainly worth a try.

MichaelG.

[MichaelG.]

I had no idea that the COB LEDs worked this way, inducing fluorescence in the yellow matrix.

An extra snippet of information which might prove useful sometime:
If you compare unlit examples of 'White' and 'Warm White' COB LEDs you should notice the colour difference.
The 'White' matrix is the colour of Tarte Citron
The 'Warm White' matrix is the colour of Egg Yolk

... ish

MichaelG.

[mrsonchus]

Very interesting info Michael, thanks.
Is the purpose then of the emitters only to excite the fluorescent matrix or do they also contribute significantly to the 'main' illumination? Do they 'add to the mix' of wavelengths/colours of light emitted by said matrix?


John B.

[MichaelG.]

The blue light produces a strong peak and the fluorescence a much broader one.
... The combined effect is visually white; but it is far peakier than 'black body radiation'

MichaelG.
.
Edit: This previous discussion may be of interest.
viewtopic.php?f=9&t=5686&hilit=black+body

[92111]

So what should I do?
No matter how I change the position of the LED, the position of the blue one does not change.

[Hobbyst46]

So what should I do?
No matter how I change the position of the LED, the position of the blue one does not change.

I would try to diffuse the light. There are several possibilities.
1. A plate of ground glass ("matt" glass or "milky" glass).
2. White tissue paper (did not try it myself but it should work).
3. The best in my opinion is a thin nylon cloth, the type used for filtration, very thin and dense. I bought a 18micron filtration cloth from China, it is very cheap but the minimum amount was 1mx1m.
Of course, an efficient diffuser causes some loss of the light, but these LEDs are so powerful that it can be tolerated.
The diffuser should be placed at a distance of 1-3cm from the LED. Not directly on the LED.

[MichaelG.]

So what should I do?
No matter how I change the position of the LED, the position of the blue one does not change.

I would try to diffuse the light. There are several possibilities. [ ... ]

I agree that this is worth trying first.

More generally though:
You have 'witnessed a crime' and are seeking the culprit ...
Work through the list of suspects; methodically testing each 'to eliminate them from your enquiries'.
The optical configuation is too complex to do this by guesswork.

Rotate and/or shift anything that can be moved, and note which of these movements has no effect.
... eventually you will identify the culprit.

MichaelG.

[ChrisR]

I agree, the problem-finding algorithms have to be adaptable!

When you've asked your mates, tried all the hunches and intuition, used all 5 senses, poked tapped and tried the things which are actually accessible to see if anything happens, then, dammit, you have to be methodical

In the old days people didn't get "stuck", they were just "having a cigarette"...

[MichaelG.]

If you compare unlit examples of 'White' and 'Warm White' COB LEDs you should notice the colour difference.
The 'White' matrix is the colour of Tarte Citron
The 'Warm White' matrix is the colour of Egg Yolk

... ish

This is uncalibrated, but it might loosely quantify that difference:

COB _LED_Whites.jpg (337.39 KiB) Viewed 14534 times

MichaelG.

[92111]

1.after try ground glass and White tissue paper .it does't work,blue guy still there.
2.I changed a new LED.I found new LED make my picture more beautiful,background more white.old led is yellow.
3.1000x without oil.

[92111]

1.try Fluorescent lamp.blue guy still there,but less than LED.
2.if no one know why,don't waste time on this problem
3.1000x without oil.

[92111]

1.when i up the 402a condenser,the blue guy become more and more little and sharp,
2.without 402a condenser,blue guy still there

[92111]

beautiful picture with new LED.400x

[92111]

Finally，I found the blue is LED emitter.

[92111]

but Hobbyst46's method does't work on my diavert,
if there is no other cheap and good method
i want to change to halogen lamp

[MichaelG.]

Finally，I found the blue is LED emitter.

Well done !!

MichaelG.

[MichaelG.]

I have just discovered that there is a special grade of opal acrylic sheet, optimised for diffusing LEDs
https://www.simplyplastics.com/catalog/ ... -110/p-676
Presumably it has more dispersion than the usual opal that is used for fluorescent lamp light-boxes.

This looks very promising ... Has anyone used it ?

MichaelG.

Edit: According to their 'pre-cut discs' page the material is: Perspex Opal 1TL2

[92111]

[quote="MichaelG."][/quote]

HI MichaelG
same LED light source,
blue is prominent on LEITZ DIAVERT,
But i can't see blue on OLYMPUS CH20.
do you know why?

[MichaelG.]

HI MichaelG
same LED light source,
blue is prominent on LEITZ DIAVERT,
But i can't see blue on OLYMPUS CH20.
do you know why?

I think it must be because the illuminator optics differ in detail.
In principle, both should be capable of Koehler, but in reality the settings may be more more sensitive in one than the other.

I remember that your Diavert light source is home made ...
viewtopic.php?t=6472
... [apart from using an LED] is it optically identical to the Leitz original ??

MichaelG.
.
.
http://www.micromagus.net/Microdocs/diavert_manual.pdf

[92111]

is it optically identical to the Leitz original ??

Yes.
hi,
should i close the diaphragm of 402a condenser? i closed it before.
now,when i open the diaphragm of 402a condenser,blue is not prominent .but image become not clear.
found blue phase ring,when condenser at stop 2,1000x

[Hobbyst46]

...

...

Here is a crude demonstration of the diffusion achieved with two layers of thin nylon cloth at about 1cm in front of a LED strip. The reflections in the lower halves of the pics are from a white desk on which the LED strip was placed flat. The LED strip is also shown through a 0.3% reflective-type (mirror type) neutral density filter, to better resolve the individual LEDs.

[MichaelG.]

is it optically identical to the Leitz original ??

Yes.

Thanks for confirming, and for the illustration.
... That should work fine, although the adjustment may be very sensitive with your LED.

I can only suggest that you work carefully through the Köhler/Koehler set-up and 'get a feel' for how the adjustments interact.

It would probably help if you can find a Chinese language tutorial ... explanations are sometimes poorly written, and may not translate well.

If you have an iOS device, this is a nice interactive tutorial : Koehler Tutor by Andrew Barlow
https://itunes.apple.com/gb/app/koehler ... 28178?mt=8

Hopefully, other members can advise further.

MichaelG.

[Hobbyst46]

If I remember correctly, some weeks ago, there was a question from 92111 about which of the two condensers to use, in order to achieve phase contrast on the Diavert. According to the manual it seemed to me that BOTH the 402 and the other condenser were needed, fitted together on the microscope, for proper Kohler illumination and PC.

[92111]

found same problem
http://www.photomacrography.net/forum/v ... highlight=