Microscope Filters: What Blue, Green and Neutral Density Filters Do

What microscope filters do

Microscope filters sit in the light path, usually between the lamp and the condenser, and change the light before it reaches the specimen. The three most common types do different jobs:

• Blue (daylight) filters make warm, yellowish lamp light look whiter and more like daylight.
• Green filters increase contrast in many stained specimens and can make basic achromat objectives look slightly sharper.
• Neutral density (ND) filters reduce brightness without changing the color of the light.

None of them is essential for casual viewing. Each is a small tool that helps in particular situations, especially with older halogen illumination or when taking photographs.

Blue (daylight) filters

Halogen and other incandescent lamps give a warm, yellow-orange light, and this is more pronounced when the lamp is dimmed. Your eyes adapt to it to some extent, but specimens can look yellowish and cameras record a strong color cast.

A blue filter, often labeled “daylight” or “color balancing”, absorbs some of the red and yellow light so the overall color is closer to daylight. It’s most useful:

• on halogen or tungsten microscopes, when you want natural-looking stain colors;
• for color photography with older illumination, although a camera’s white balance can now do much of the same job.

You may read that blue light improves resolution. Resolution does improve slightly at shorter wavelengths. But a typical light-blue daylight filter still passes a broad range of colors, so any improvement for visual work is small. The main purpose of a blue filter is color correction.

Green filters

Green filters are the most broadly useful of the three, for several reasons:

• Better-looking achromats: standard achromat objectives correct color for two wavelengths, and their best performance is usually in the yellow-green part of the spectrum. Restricting the light to green reduces the colored fringes and softness caused by residual color error. For more on correction classes, see our guide to objective lenses.
• More contrast in red and pink stains: red, pink and magenta structures absorb green light strongly. Under a green filter they appear darker against a bright background. Eosin and other red stains, common on histology slides, can stand out more clearly this way.
• Comfortable viewing: the eye is most sensitive in the green, so a green-filtered image can look clear at moderate brightness.
• Phase contrast: phase rings are generally designed for green light, so a green filter often improves phase contrast images.
• Black-and-white photography: a monochrome image under green light avoids the softness that color error adds.

The trade-off is obvious: the image becomes green, so a green filter isn’t for judging stain colors. Plain colored-glass or gel green filters pass a fairly broad band. Green interference filters pass a narrower band, often centered near 550 nm, and give a stronger effect.

Neutral density filters

A neutral density (ND) filter is a gray filter that cuts brightness evenly across the visible spectrum. It doesn’t tint the light.

Why not just turn down the lamp? With halogen lamps, dimming shifts the color toward yellow-orange. If you want consistent color, for example for photography, you can leave the lamp at a fixed setting and use ND filters to control brightness. With LED illumination, dimming changes color much less, so ND filters are less important, though they’re still handy when even the lowest dimmer setting is too bright at low power.

ND filters are also the right answer when the image is too bright and you’re tempted to close the iris diaphragm to dim it. Closing the iris costs resolution and adds artifacts. An ND filter only reduces brightness.

Reading ND numbers

Two labeling systems are in common use, which can be confusing:

Stacking filters adds their optical densities. ND 0.3 plus ND 0.6 behaves like ND 0.9.

Other filters you may come across

Filter kits and older microscopes may also include:

• Frosted or diffuser filters spread light more evenly, which hides the lamp filament or LED pattern on simple microscopes without Köhler illumination.
• Yellow filters are sometimes supplied to improve contrast in blue-stained specimens, such as those treated with methylene blue, or for black-and-white work.
• Heat-absorbing filters reduce infrared heat from powerful halogen lamps reaching the specimen.
• Polarizing filters are used in pairs (polarizer and analyzer) for polarized light microscopy, to reveal crystals, starch grains, fibers and other birefringent materials.
• Red and other colored filters are mostly for photography or special stains.

Do you need filters with LED illumination?

Most modern amateur microscopes use white LEDs, which are fairly neutral to slightly cool (bluish). With an LED:

• a blue filter is usually unnecessary, and can make the image look too cold;
• a green filter is still useful, for the same contrast and achromat reasons as with halogen;
• an ND filter is mainly needed if the light is too bright even at its lowest setting.

LED color varies by product. If stain colors look off on your microscope, a quick comparison with and without a filter will tell you more than general rules.

Where filters go on a microscope

Filters can sit in several places, depending on the microscope:

• On the light port in the base: many microscopes with built-in illumination have a recessed ring or holder around the light exit where a round filter drops in.
• In a swing-out holder under the condenser: common on microscopes with an Abbe condenser.
• In a filter slider or turret: on better laboratory microscopes, filters are mounted in the base and switched with a lever.

Filter diameters differ between microscopes. Sizes around 32 mm and 45 mm are common, but measure your holder before buying. For how filters fit with the lamp, condenser and mirror, see our guide to which parts of a microscope provide illumination.

Filters vs the dimmer vs the iris diaphragm

Practical tips for choosing and using filters

• Set up your lighting and iris diaphragm first, then add filters. Filters fine-tune the result; they don’t fix poor setup. Our article on microscope illumination covers the basics.
• Try a green filter on unstained or lightly stained specimens before reaching for more elaborate techniques.
• Keep filters clean. Dust and fingerprints on a filter near the field diaphragm can show up as soft blotches in the image.
• Use glass filters made for microscopes rather than thin gels in hot halogen light paths, since gels can fade or warp from heat.
• If you photograph specimens, decide whether to correct color with a filter or with the camera’s white balance, and stick to one approach for consistent results.