What a patch stop does
A patch stop is an opaque disc placed under the microscope’s condenser to turn ordinary brightfield into darkfield. It blocks the center of the light beam, so the specimen is lit only by a hollow cone of steeply angled light. Those rays miss the objective, so the background goes black. Anything in the specimen that scatters or bends light sends some of it into the objective and shows up bright.
To make one, you cut a clear disc that fits your condenser’s filter holder, stick a black disc of the right size exactly in its center, and drop it into the holder with the condenser iris fully open. The trick is getting the black disc the right size for your objective, which is covered in Step 2 below.
A homemade stop costs almost nothing and is one of the easiest ways to try darkfield before deciding whether it’s worth buying more equipment. For the pros and cons of the technique itself, see our article on dark field microscopy and how it compares with phase contrast.
Will it work on your microscope?
A patch stop works best on a compound microscope with:
- A substage condenser, usually an Abbe condenser, that can be raised and lowered.
- A filter holder or swing-out tray below the condenser. This sits close to the condenser’s front focal plane, which is where a stop works best.
- Low and medium power objectives. Darkfield with a stop is usually easy at 4x and 10x. Whether it works at 40x depends on your condenser and objective. It’s sometimes possible, but often the background stays gray.
The rule behind this is that the condenser must deliver light at a wider angle than the objective can accept. In optical terms, the condenser’s numerical aperture (NA) must exceed the objective’s NA. Low-power objectives have low NA, so this is easy. Higher-power dry objectives have higher NA, and a simple dry condenser struggles to get beyond them. 100x oil objectives need a dedicated oil-immersion darkfield condenser and usually an objective with a built-in iris, which a homemade stop can’t replace.
If your microscope has no condenser, only a disc or iris diaphragm under the stage, a stop can still produce a rough darkfield effect at the lowest power, but results are usually weak.
Materials
- Clear base: thin clear acetate, a transparency sheet, clear plastic from packaging or a CD case, or an old clear glass filter that fits your holder
- Opaque material for the stop: matte black card, black electrical tape, black paper or black paint. A coin of the right size also works.
- Cutting tools: scissors or a craft knife, a compass or circle cutter, and hole punches in several sizes
- Ruler or calipers
- Glue stick, double-sided tape or a small amount of clear glue
- A fine marker for labeling
Matte black is better than glossy, which can reflect stray light. Electrical tape is handy because it’s thin, opaque and self-adhesive.
Step 1: Measure the filter holder and cut a clear base
Swing out the filter holder and measure its inside diameter. If a blue or frosted filter came with the microscope, measure that instead. Sizes vary between models, so measure rather than assuming.
Cut a clear disc to that size. Use a compass to draw the circle, and keep the center point marked, because you’ll use it to center the stop. A snug fit stops the disc sliding around and helps keep the patch centered.
Step 2: Work out the stop size for your objective
The stop must be large enough to block all the light that would otherwise go straight into the objective. A larger objective NA needs a larger stop. You can find the size by trial and error, but this method gets you close quickly:
- Put a slide on the stage and focus with the objective you want to use, such as the 10x. Set up Köhler illumination if your microscope allows it.
- Remove an eyepiece and look down the tube from a few inches away. You’ll see a bright disc, the objective’s back aperture.
- Slowly close the condenser’s aperture diaphragm (iris). Watch for its edge to start cutting into the bright disc, then open it again until the edge just disappears.
- Measure the iris opening at that setting from below, using a ruler or calipers.
The stop should be slightly larger than this opening. The measurement is rough, so make several stops in small steps around it, test each one and keep the best. Label each one with the objective it suits.
Expect to need a larger stop for the 10x than for the 4x. If you try this with a 40x objective and the iris opening you measure is close to the iris’s fully open size, a dry condenser probably can’t give a black background at that magnification.
Step 3: Make and center the stop
- Cut or punch a black disc of your chosen size. Hole punches give clean, round edges, and a set of leather punches covers a useful range of sizes.
- Draw a small circle of the same size around the center mark on the clear base to help you position it.
- Stick the black disc exactly in the center. Even a small offset gives uneven lighting, with one side of the field brighter than the other.
- Check that the edges are clean and there’s no glue smeared over the clear area.
If you have several stop sizes, a strip of acetate with stops spaced along it lets you slide between sizes quickly, if your filter holder allows a strip to be pushed through. Otherwise, make a separate disc for each size.
Step 4: Set up the microscope for darkfield
- Open the aperture diaphragm fully. A closed or partly closed iris cuts off the hollow cone of light, and you’ll see little or nothing.
- Insert the stop in the filter holder and swing it into place.
- Raise the condenser close to the top of its travel.
- Turn up the lamp. Most of the light is blocked, so you’ll need much more brightness than for brightfield.
- Focus on the specimen, then adjust the condenser height up and down slightly until the background is darkest and the specimen brightest.
- Check centering. If one side of the field is brighter, or a bright crescent appears, nudge the stop or adjust the condenser centering screws if it has them.
- Close the field diaphragm, if your microscope has one, only as far as the edge of the field of view. This can reduce stray light.
Some people get a darker background at higher powers by putting a drop of immersion oil between the top lens of the condenser and the underside of the slide. Abbe condensers rated at an NA above 1.0 only reach that rating with oil contact, so this can help. Check your condenser’s documentation first, and clean the oil off afterward.
Other ways to mount a stop
- Slider or strip. As described above, a strip of acetate carrying several stop sizes lets you switch quickly.
- On the condenser’s bottom lens. On some condensers you can rest a small stop directly against the lowest lens. Avoid adhesives on the glass.
- On the light port. On microscopes without a filter holder, placing a stop over the light exit under the stage can give a partial darkfield effect at low power. It’s less effective because the stop is not near the condenser’s focal plane.
- 3D-printed holders. Some hobbyists print holders that clip under specific condenser models. If you have a printer, this can make swapping stops easier.
- Commercial stops and sliders. If your microscope maker sells a darkfield stop or slider for your model, it may be inexpensive and save effort.
Troubleshooting
| Problem | Likely cause | What to try |
|---|---|---|
| Gray background instead of black | Stop too small, condenser too low, or objective NA too high for the condenser | Use a larger stop; raise the condenser; drop to a lower-power objective |
| Very dark field with almost no specimen light | Stop too large, iris not fully open, or lamp too dim | Use a smaller stop; open the iris; increase brightness |
| One side brighter, or a bright crescent | Stop or condenser off-center | Re-center the stop or the condenser |
| Bright spot or haze in the middle of the field | Condenser height wrong or light leaking through or around the stop | Adjust condenser height; check the stop is fully opaque |
| The field is full of glowing specks | Dust, scratches, bubbles or fingerprints | Clean slides, coverslips and lenses; avoid bubbles in the mount |
| Glare and no clear detail | Specimen too thick or crowded | Use a thinner, sparser preparation and a thin coverslip |
| Works at 10x but not at 40x | The condenser can’t exceed the 40x objective’s NA | Try a larger stop or oiling the condenser; otherwise use a darkfield condenser |
Darkfield is very sensitive to dirt. Every particle scatters light and glows, so clean glassware makes a bigger difference here than in brightfield.
Good first specimens
- Pond or aquarium water: protozoa, rotifers and algae, which are often easy to find and very striking against black
- Diatoms: fresh or cleaned shells
- Hair, fibers and cotton threads
- Salt or sugar crystals from a drop of solution dried on a slide
- Yeast in a little water, at 10x and, if your setup allows it, 40x
- Cheek cells, which show their outlines clearly
Keep preparations thin. A small drop of water under a coverslip usually works better than a deep, crowded mount; see how to make a wet mount slide without air bubbles.
Going further
Once you have a working stop, the same sizing method lets you make colored Rheinberg illumination filters, which replace the black disc with colored gels. If you find yourself using darkfield often, especially at higher power, a dedicated dry or oil-immersion darkfield condenser is the next step. To see where darkfield fits among other methods, read our overview of contrast in microscopy.