Neutral Density Filter | Imaging Glossary

View

Definition

A filter that absorbs light approximately equally across the visible spectrum, so it reduces brightness without changing the color of the illumination. Strength is usually given as optical densityLoading..., where transmission T = 10^−OD: ND 0.3 passes about 50%, ND 0.6 about 25%, ND 0.9 about 12.5%. Some manufacturers label by transmission or attenuation factor instead (ND2, ND4, ND8 for the same three).

On a brightfield microscope with a halogen lamp, ND filters are the right way to dim the image, because lowering the lamp voltage changes its color temperatureLoading.... Record which filter was in the light path: two images taken with different filters are not comparable unless exposure was adjusted to match.

Learn by playing

Larry's White Balance Lab

Game · White balance · Color temperature · Neutral density filters

Optical density, in powers of ten

A neutral density filter is specified by its optical densityLoading..., and transmission T = 10^−OD. ND 0.3 passes 50%, ND 0.6 passes 25%, ND 0.9 passes 12.6% and ND 1.2 passes 6.3%. Because OD is logarithmic, stacked filters add: ND 0.3 plus ND 0.6 behaves as ND 0.9.

Some manufacturers label filters by attenuation factor instead: ND2, ND4 and ND8 for the same 50%, 25% and 12.5%. The two systems collide at small numbers, so check the transmission, not just the label, and write the value you used into the acquisition record.

Simplified

A gray filter is rated by how much light it blocks. A 0.3 filter lets through half, 0.6 lets through a quarter, 0.9 about an eighth. Stack two and their numbers add: 0.3 plus 0.6 works like 0.9.

Some brands label them differently, ND2 for half, ND4 for a quarter, ND8 for an eighth. The labels can be confusing, so check how much light each one actually passes and write it down.

Neutral is an approximation

No filter is perfectly flat across the spectrum. Absorptive filters are tinted glass and transmit a few percent more or less at the ends of the visible range; reflective filters use a thin metal coating that is flatter but sends the blocked light back toward the source, where it can bounce and create reflections unless the filter is slightly tilted.

For quantitative color work, set the white balanceLoading... with the filters you will image with already in the light path. Then a small tint in the filter is corrected along with the lamp, instead of becoming a color castLoading... when the filter is added later.

Simplified

Gray filters are never perfectly gray. Most have a faint tint, a few percent toward one color. Some are mirror-like instead, which keeps the color more even but can cause reflections if they sit perfectly straight.

The simple fix: put the filter in before you set the white balance. Then the camera corrects for the filter's tint along with the lamp's.

Where it goes, and why that matters

On a brightfield microscope the filter belongs in the illumination path, usually a slot or turret near the lamp or below the condenser, never in the imaging path. There it only reduces the light reaching the specimen; it adds no optical surfaces between the specimen and the camera, so sharpness and contrast are untouched.

It is the right way to dim a halogen lamp because brightness changes and color temperatureLoading... does not. The exposure timeLoading... is the other clean control. Record which filter was in place: two images taken with different filters are only comparable if the exposure was adjusted to give the same blank-glass brightness.

Simplified

The filter goes in the lamp's light path, before the light reaches the slide, not between the slide and the camera. That way it only makes the light dimmer and does not blur or degrade the picture.

It is the right way to dim a halogen lamp, because the brightness changes and the color does not. Note which filter you used, so images taken on different days can be compared fairly.

Explore TissueFAXS

Imaging Spectroscopy Platform

Precision whole-slide imaging with TissueFAXS, optimized optics, and multi-channel acquisition.

Optical density, in powers of ten

Neutral is an approximation

Where it goes, and why that matters

Related Terms