MicrobeHunter.com Microscopy Forum

The field lens on my Olympus BH2 has a bright perfectly circular hot-spot visible at the centre of the field lens. When the field diaphragm is closed to a minimum the hot-spot is only a pinpoint, when fully open it is about 3mm. I have no trouble achieving Kohler illumination over 10/0.30 - 40/0.95. What does it indicate, what would be causing this and how could it be eliminated?
Thanks
Michael

Would not have expected to see a hot-spot on the Field Diaphragm dust glass. They are more likely to appear on the camera sensor due to the added camera lens in afocal setups.
It might help to identify the source if you post images of your illumination system

Have you placed a business card directly under the condenser to see if it disappears? It may well be a reflection.

I had tried the business card idea, it isn't a reflection from the condenser.
Michael

75RR, As far a I know the illumination is standard BH2 (image attached), I've never tried to open it to inspect it because the hotspot doesn't appear to be interfering with the visual observing or DSLR imaging. I have occasionally seen a faint blue area close to the centre of an image when the afocal set up is misaligned but I have fixed that!

I've attached the image from the BH2 manual but I can't understand what it shows about the light path. Other Olympus documents show item A as a simple stop.

Michael

The fact that the spot diminishes and increases as you close and open the field diaphragm, tells you where the problem lies. You are seeing a reflection of the field window on the field window.
The light is likely reflecting back down the line and then re-reflecting back to the window. If there is any surface film or dust film( a good possibility in an old scope) on any surfaces in the illumination path, that will reduce transmission and increase the possibility of that film/dust acting as a kind of screen, showing a ghost image of certain components or physical structures, depending on the design of the system.

Thanks Apochronaut, I'll start by opening the field lens unit.
Michael

I agree with Apochronaut. If you have removed the possibility of reflection from above, then it may well be a hazed screen on the under side of the field lens presenting an image of the field diaphragm. This optic does get dirty on the underside, though this optic assembly can also eventually show lens separation when the optic bonding glue ages, which may also create a similar effect.
The orientation of field optic in the BHS is easily seen as the two optics bonded together are of a different thickness, thinner optic on top from memory. BHT is a much more difficult story if you were to accidental forget orientation.
Rubber suction cup will be your best friend here!!

Thanks for the suggestions everyone. I'm off for a months vacation now so I'll start dismantling and cleaning when I return!
Michael