MicrobeHunter.com Microscopy Forum

I converted my vintage Leitz Dialux from 100-watt halogen to LED. Basically, it worked out well. Good brightness control and plenty of nice 'daylight'. Have set this LED economical to a maximum of 4 watts.
As for the uniformity of the light beam, I am less satisfied. It turns out that the LED is not 1 light source but composed of 12 small LEDs. I have to arrange the condenser completely out of focus to get this pattern invisible.

The LED is mounted in a base. So I can disassemble everything and put in another type of LED. But I don't know which LED to choose. The LED need a fit into a 'base' a 13.35 x 13.35 mm (so same base als the CXA1304)
I can take a wild guess with Cree CMB1304 Pro9. But no idea if that LED is better.

Unfortunately, you have hit-upon an unfortunate truth … Many modern LEDs are of COB [chip on board] construction.
This is very efficient in terms of light output, but not well-suited to Koehler optical arrangements.

I think your best starting-point is to check the dimensions of the filament in the original Tungsten-Halogen lamp, and then search for an LED of the same, or smaller, format.

An alternative might be to use a light-scrambler of some sort [probably an Acrylic cone] to condense the output of the CXA1304 into an evenly- illuminated spot.

MichaelG.

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CXA1304 data-sheet, for convenient reference:
https://downloads.cree-led.com/files/ds ... XA1304.pdf
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P.S. __ if you are under-running the LED, that will degrade the designed evenness of the illumination, because the yellow phosphors will not be fully excited.

You only need 4w? Get a cree xp-l HI emitter. It's a single emitter, not a cob, and it can run up to 10w if you give it 3a. The HI variant does not have a resin dome lens on top which is probably better for replacing a halogen bulb. What base/star it is mounted on if any will vary depending on supplier.

Nikon scopes use what they call 'fly eye' illumination. In addition to a standard diffuser, there is a plastic element in the light path that has a pattern of bumps ('microlens array') that essentially breaks up the image of the filament/LED into hundreds of tiny images. Would something like that be helpful for you?
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Thanks for your responses. The Cree xp-l HI is to be considered indeed (unfortunately not the same 'foot')
Am just cleaning and cleaning the aperture of a Köhler illumination of a Wild microscope (M11).
There they use a condenser lens that is matted. (Just like I saw in a Zeiss microscope)
Leica does not use a frosted lens in the lens set in the light path.
I didn't know the Nikon 'fly eye' approach yet. But interesting to look into that.

The Nikon Labophot 2 filter pack also has a diffuser.
Both are acrylic.
The fly eye gives images a kind of sparkle effect from generating multiple light source points, what is probably described as "pop" in digital terms.

Seen the Nikon Diffuse Lens Filter on the internet but €20 + €40 additional cost me a bit too high to do an experiment with.

A diffuser reduces the intensity of the illumination considerably.
The frosted glass used in the old Zeiss collimator works in addition to Kohler setup and does not replace it.
As Scarodactyl commented above, find a single die LED. Then optimize its position in the optical train to achieve Kohler. Probably the best position would be the same position of the halogen bulb filament.

American Optical used a "Modified Koehler" using a lens that was frosted on one face. I've used a flat plate which I frosted on one side for similar effect with other instruments to provide a more even illumination.

Just fixed the Wild M11 Köhler illumination (grease on the aperture removed). Good time to take a look at this build-up. The condenser consists of 2 lenses as far as I can see. The lens close to the lamp is matted.
OK just leaves the diffusion stuff. Am more interested in a good LED that is not a compound LED.
CREE XP-L HI looks fine.
The puzzle is though. Where to buy ? (in the right colour temperature and high CRI value)
And this LED is an SMD version so you need some kind of intermediate mounting plate.
Am searching . . . Where to purchase this (EU Union) ?

Perhaps something like these? (Found after a quick search with google.de.)

Some more circuit board mounted LEDs. I have bought from them since they are an authorized NIchia distributor.

Thanks 2x. Great websites to puzzle for the right LED on a board.
Found myself this website (+A lot of data and graphs there)
https://www.led-tech.de/de/CREE-XE-G-au ... 000K-CRI90

Placing a diffuser I did not see as the ideal solution. In the end, I decided to dismantle the COB LED and place single LED. Slightly less maximum light output. But now the illumination is even, good.
Extreme light is only needed for special techniques. For normal illumination, the power is set to 10%