How to Add Darkfield Illumination to an Olympus BH2 Microscope
Click here to learn about darkfield microscopy.
There are multiple ways to add darkfield to your BH2, as described here. The simplest and least expensive method to add darkfield capabilities to a light microscope is to add a simple darkfield patch stop to the condenser, to block the central area of the aperture diaphragm from receiving light, thereby creating the hollow cone of illumination necessary for darkfield.
In many scopes, the design of the condenser is such that a filter slot or carrier placed at the bottom of the condenser is physically close enough to the aperture diaphragm for a patch stop placed within the slot or carrier to provide effective darkfield for low-power objectives (4x – 20x). The design of the BH2 condensers complicate things a little bit, since the internal aperture diaphragm is too far removed from the bottom surface of the condenser mounting dovetail for a slot or carrier to be effective for darkfield.
Fortunately, with the advent of 3D printing, it is a simple matter to fabricate an insert compatible with the BH2 condensers that, when pushed up into the bottom bore of the condenser, positions a patch stop at the appropriate location for effective darkfield.
BH2-DFR Kit
The BH2-DFR Darkfield/Rheinberg Kit
Click here to see a PDF describing the BH2-DFR Darkfield / Rheinberg kit, which was made by the Empire of Dirt Workshop and is freely available for non-commercial purposes. This kit, which is compatible with the BH2-CD Abbe condenser and the BH2-AAC aplanatic / achromatic condenser, will provide darkfield or Rheinberg Illumination (with the proper Rheinberg filters) for 4x through 20x objectives. And maybe even on low NA 40x objectives, with some careful setup.
The various components of this kit can be made on just about any 3D printer. If you don’t have access to a 3D printer, someone you know likely does. If not, please contact the Empire of Dirt Workshop. Oh, and please don’t get overwhelmed when you look at the PDF. There are a lot of parts in this kit, but to do darkfield on your BH2, you only need two of those parts!
The BH2-DFR was designed to place a simple darkfield patch stop up into the bottom bore of the condenser, very close to the aperture diaphragm within the condenser. Close proximity of the darkfield patch stop to the aperture diaphragm is a necessary condition to get acceptable darkfield. Don’t fall for the misinformation out there. Attaching a patch stop to the bottom of your BH2 condenser will not work well for darkfield, since the aperture diaphragm is not physically close enough to the condenser mounting dovetail for this to provide acceptable results.
The BH2-PCD Phase Condenser for Darkfield
Another method to add a simple darkfield patch stop to the BH2 is by using the BH2-PCD Zernike-style phase contrast condenser. This condenser contains an integral patch stop which is suitable for 4x through 20x objectives. If you have the BH2-PC instead of the BH2-PCD (and if you have a choice, you should buy the BH2-PC instead of the BH2-PCD), experiment with using the phase annuli as darkfield stops. Many report success with this method.
Dedicated Darkfield Condensers
If darkfield is required for higher-power objectives, a dedicated darkfield condenser will be needed. Olympus made the BH2-DCD dry darkfield condenser, as well as the BH2-DCW oil-immersion darkfield condenser for the BH2 scopes. Both of these condensers are fairly rare, and each typically will fetch $400 or more when they do show up on eBay.
The BH2-DCD is suitable for use with 4x through 40x objectives. This is a dry condenser and must never be used with oil immersion, or damage to the condenser will result. The BH2-DCW is an oil-immersion condenser suitable for use with 10x through 100x objectives. The BH2-DCW must always be oiled to the bottom surface of the slide, or no light will be visible through the eyepieces. To achieve darkfield using the BH2-DCW and a 100X objective, the objective must have an internal iris diaphragm to allow the NA to be reduced to below the upper NA of the condenser. Refer to the Condenser Page for more information on the BH2-DCD and BH2-DCW condensers.
Third Party Darkfield Condensers
Many people have reported success using Amscope darkfield condensers on their BH2. Refer to the Condenser Page for more information on this option.