What is bright field imaging?

Bright field imaging, or brightfield microscopy, is the most basic and widely used way of lighting a microscope specimen. Light from a lamp passes through the specimen and into the objective. Parts of the specimen that absorb light look darker than the brightly lit background. When people picture a microscope image, such as a stained tissue section, a colored onion cell or a pollen grain, they’re usually picturing brightfield.

How bright field imaging works

On a typical compound microscope:

1. A lamp in the base (usually an LED or halogen bulb) produces light.
2. The condenser under the stage gathers that light and focuses it onto the specimen as a solid cone.
3. The light passes through the specimen. Where the specimen is dense, pigmented or stained, it absorbs some of the light. Where it’s clear, the light passes straight through.
4. The objective collects the light and forms a magnified image, which the eyepiece magnifies further.

The image is built mainly from differences in absorption, with a smaller contribution from light being bent at edges. Absorbing parts look dark or colored, and empty areas look bright. That’s where the name comes from: the field of view is bright.

The same idea also exists in reflected light. On metallurgical microscopes, “reflected brightfield” shows flat polished surfaces as bright and scratches or pits as dark. Our comparison of transmitted vs reflected light microscopes explains the difference.

What a bright field image looks like

• Stained specimens: strong colors and sharp detail. Nuclei, cell walls and tissue layers are easy to tell apart.
• Naturally pigmented specimens: chloroplasts look green, many algae are golden-brown, and insect parts are amber.
• Unstained transparent specimens: faint, low-contrast outlines that are easy to miss. Bacteria and cheek cells in water can be almost invisible.

What bright field is used for

• Stained histology and pathology slides
• Gram-stained and other stained bacterial smears
• Plant sections, pollen, seeds and leaf surfaces
• Pigmented algae, many protozoa and small invertebrates
• Insect parts, hairs and textile fibers
• Rock thin sections in plane-polarized light
• Education. It’s the starting point for almost every microscopy course

Limitations

• Poor contrast on transparent specimens. This is the big one. Most living cells absorb very little light, so they show up weakly or not at all.
• Stains are often needed, which take time, usually kill the specimen and can introduce artifacts.
• The contrast-resolution trade-off. Closing the aperture diaphragm makes transparent objects more visible but lowers resolution. Our guide to microscope contrast explains this trade-off in detail.
• Glare from poor setup. An over-opened field diaphragm, a low condenser or dirty optics all reduce contrast.

Can you view living specimens in bright field?

Yes. You’ll sometimes read that brightfield can only show fixed, dead specimens, but that isn’t true. Pond water organisms, pigmented algae, rotifers and small crustaceans are routinely watched alive in brightfield. The limitation is contrast, not life: very transparent organisms are hard to see. That’s when techniques such as darkfield and phase contrast help.

Getting the best bright field image

A few adjustments make a big difference:

1. Raise the condenser close to the slide. On many microscopes it should sit near the top of its travel.
2. Set up Köhler illumination if your microscope has a field diaphragm. Close the field diaphragm, focus its edge into sharp view with the condenser focus, center it, then open it until it just clears the field of view.
3. Adjust the aperture diaphragm for each objective. A common starting point is to have it fill about two-thirds to three-quarters of the objective’s back aperture (check by removing an eyepiece and looking down the tube). Close it more for transparent specimens, and open it more for stained ones where you want maximum detail.
4. Lower the brightness when viewing faint specimens. Too much light washes them out.
5. Clean the slide, coverslip and lenses. Dust and smudges scatter light and flatten contrast.
6. Try a filter. A blue daylight filter corrects the yellowish color of halogen lamps, and a green filter can increase contrast on some stained specimens.

Bright field compared with other techniques

Brightfield is where nearly everyone starts, and for stained material it remains the best choice. When your specimens are clear, it’s time to look at the other contrast techniques.