Hot Spot when using a Flash with the Labophot Microscope

aidanmoore · Post by **aidanmoore** » Sat Dec 30, 2017 5:32 am

I am wondering if anyone else here has had issues with vignette when trying to couple a flash to a microscope ( Labophot in particular).

I may have done things the hard way to get a vignette-free flash image, so I’m curious if there is a simpler solution.

The Labophot has a very easy access to the transmitted light source at the back of the base, and I simply removed the incandescent lamp and pointed a SB900 Speedlight into the port.

There was quite a bit of vignette on the resulting image:

After looking at the amount of vignette present while moving a fiber light closer to the original position of the incandescent lamp, I was surprised how critical the position of the illumination source is for even light distribution across the resulting image, even if the illumination surface area of the flash is greater than the original incandescent bulb.

Picture here from Quekett for illustration:

The lamp collector in the Labophot can be easily removed and repositioned, so I tried moving the collector assembly towards the flash as in step 2 below.

But there was still vignette.

So I reluctantly cut away the lamp mount to allow the flash unit to fit closer to the original lamp position as in 3 and 4 above. I can always Gorilla glue the severed piece back if I want to restore the microscope.

This time the image was evenly illuminated.

I wanted to be able to quickly model the image before flash, and with the incandescent lamp no longer available I could use a half silvered mirror as described elsewhere here, or just slide the modelling lamp into position.

I chose to use a slider, and built an assembly around a cheap Hiwin linear guideway that moves the flash and modelling lamp into position easily:

The results have been very good, and I’ll post some stacked images using the setup once I’ve found a few good subjects.

Pau · Post by **Pau** » Sat Dec 30, 2017 8:45 am

When you say "This time the image was evenly illuminated" really it isn't, they are just better than your first image likely because the higher magnification at the third image, you have something not right in the illumination train. And it isn't properly centered.
First be sure that the condenser is properly placed and centered. Do you get really even illumination with the pictured LED mounted as shown? And with the original halogen lamp?

I have no experience with your Labophot but I have setting up flash with my Zeiss.
To have good Köhler illumination the best way is to mimic the original microscope illumination. If you have the original lamp house first be sure if there is any other lens between the lamp and the microscope base collector lens, if it is you want it in your modified lamp sources.
You also could measure the distance between the original lens filament and the microscope base: this is the right position for both the LED and the flash tube, not for the flash front surface (although in your case they seem very close)

If you can remove the frosted glass diffuser of the collimator lens (in some scopes it is flat glass while in others it's a the frosted surface of a lens) setting up Kölher is easier: You're able to see the lamp source focused at the same plane of the condenser diaphragm with a phase telescope eyepiece or just removing one eyepiece. Without the diffuser illumination will be worse if the lamp source is improperly placed but equal or better if placed at the right position.

Changing the position of the collimating lens could be useful if you really know what you're doing, in this case it doesn't seem enough

Maybe the easier, although not the best way could be to place a diffuser at the flash surface.

aidanmoore · Post by **aidanmoore** » Sat Dec 30, 2017 9:32 am

When you say "This time the image was evenly illuminated" really it isn't, they are just better than your first image likely because the higher magnification at the third image, you have something not right in the illumination train. And it isn't properly centered.
First be sure that the condenser is properly placed and centered. Do you get really even illumination with the pictured LED mounted as shown? And with the original halogen lamp?

Thanks for catching that.

Below is the 5X image with flash up against the collector lens that I should have placed in the original post.

There is a small amount of vignette that I believe has to do with the coupling of the trinoc port to the camera, so the optical path to the camera may not be perfectly centered as you mention, but I see the same effect through the oculars.

I do get even illumination and Koehler with the original lamp in the original position.

As the distance between the collector lens and the LED source increases there is also increasing vignette, so the issue seems to be with the Labophot light train requiring tight placement of the illumination source against the collector/collimating lens.