How to set up a darkfield microscope

Darkfield microscopy is a useful technique to improve the visibility of objects that are hard to see with regular brightfield microscopy.

It’s best used with samples with very little natural contrast, such as transparent and unstained specimens or crystalline materials.

Here’s how it works

Instead of shining light directly through your sample, darkfield microscopy places a circular blocker known as a ‘stop’ in the way.

The stop blocks any direct light from hitting the sample, allowing only the angled light that can sneak around the edges to get past. This angled light would ordinarily miss the objective lens, leaving the image completely dark (the ‘dark’ in darkfield).

But when that angled light hits a sample, it can be scattered, reflected or refracted far enough that it changes course, entering the objective lens where it can be seen.

The result is a dazzling, brightly lit sample set against a dark background.

Setting up

Almost any brightfield laboratory microscope can be easily converted to perform darkfield illumination.

To get the best results though, you need a darkfield condenser and preferably a centering telescope

CT Centering Telescope


Here’s the general procedure:

  • Lower the condenser.
  • Loosen the locking screw that holds the condenser in place (making sure to support the condenser so it doesn’t fall).
  • Put your darkfield condenser into position and tighten the locking screw.

Once your darkfield condenser is in place, you’ll need to centre it to ensure that the blocker is right in the middle of the light path.

How to centre dry condensers

Centering the condenser is a simple process, although there are some slight differences between dry (low magnification) and oil (high magnification) condensers.

Dry Darkfield Condesner

Here’s the procedure for dry condensers.

  • Rotate the 10x objective into position.
  • Take out one of the eyepieces.
  • Look down the empty eyepiece hole, and you’ll see the darkfield condenser with its round blocker in the middle.
  • Raise the condenser until the blocker almost covers the image, but not entirely. You should see a thin ring of light around it.
  • If the blocker isn’t right in the centre, use the centering screws on the side of the condenser to adjust the position.

How to centre oil condensers

The process is almost the same for oil condensers.

Oil Darkfield Condenser
  • Rotate the 40x objective into position.
  • Remove one eyepiece.
  • Raise the condenser until you clearly see a dark circle through the eyepiece hole.
  • Use the centering screws to move the dark circle to the centre of the image.

Using immersion oil

When you’re using high magnifications, you normally use immersion oil on your slides. But with darkfield microscopy, you also need to put a bit of oil on the condenser. For more information on this read our blog post How to use microscope immersion oil

Here’s how:

  • Take off (or find) the oil darkfield condenser.
  • Put a drop of immersion oil on its lens.
  • Attach your condenser back onto the microscope.
  • Gently raise the condenser until the oil touches the bottom of the slide.

That’s it! You’re all set up for darkfield microscopy.

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