How to avoid extreme highlights from reflecting surfaces

Optimize your illumination system. Extremely applicable in case of image documentation by a digital camera.

Reflected light is polarized light. This physical property is known by everybody who is doing serious photographies with a single lens reflex camera.

Motif: Landscapes, technical buildings, any kind of documentation for newspaper or book.
Solution: A rotatable polarization filter mounted in front of the objective.

Polarization filter of a reflex camera (source)

Rotate the filter until contrast is maximum, reflections are minimum. A new viewing angle requires a new adjustment.

Picture taken with Pol-Filter, interior visible vs Picture taken without Pol-Filter (source)

Polarization filter for upright microscopes

Microscopists may learn from this public use of a polarization filter. For incident light applications dealing with reflecting surfaces, e.g. from metal pieces or crystals, a Pol filter placed between sample and eyepieces/camera is a useful tool. The rotation of this filter again finds the best position for reduced reflections and increased image contrast.

As modern CMOS cameras for light microscopy are sensible to reflected highlights, and in this case tend to overexposure, the correct use of a Pol filter is an essential procedure in digital documentation of reflective samples.

BA310MET with Epi-Illuminator

Note: If the polarizer (P) is removed from the ray path, the rotatable (!) analyzer (A) acts as a single polarization filter. Rotation ensures the best possible extinction of reflections.

Analyzer for BA310MET - slider version

An advice: This procedure also works for “internal” reflections of glass elements mounted within an objective. This is especially valid for low power magnifications (2X, 5X objectives), where the lens surfaces only have a slight curvature, thus “encouraging” reflections of illumination light coming from the light source on its way down through the objective.

Polarization filter for stereo microscopes

Regarding stereo microscopes the procedure is the same. The analyzer is clamped onto the objective of a CMO (Central Main Objective; Galilean type) and is rotated until maximum image quality is achieved.

Analyzer for CMO Stereo microscope, clamp-on version

But here we have a second option. A diffusor system for incident light significantly reduces the highlights reflected by crystals, minerals or metal surfaces.

Diffusor for ring light; clamp-on design (source)

Built your own handmade diffusor for your stereo microscope!

You can build a diffusor by yourself with a stripe of white paper fixed in a roll by a Sellotape. Together with a 1-or 2-arm light guide, combined with a cold light source, this setup gives the chance for a “soft” illumination with less reflections.

Salt crystals, illuminated without diffusor vs illuminated through “hand-made” diffusor

The “handmade” diffusor visualizes a main rule for stereo microscopes:

Illumination is the key issue. We are not talking about Watt-Power, we are talking about illumination angle and the kind of light. Small cause, large effect.