By simply taking the fiber-optic light guides and shining the light at an oblique angle onto the specimen on a stereomicroscope or a polarized light microscope will reveal the true color of the material. Some of our scientists at McCrone Associates have customized their microscopes so that the oblique light source is attached to the rear of the microscope, usually underneath the reflected light lamp housing.

Obliquecontrast

SWIR imaging allows the crew of an armoured vehicle to look through haze, fog, sandstorms, snow and glass, providing them with optimal visibility. Typical applications for this SWIR camera technology can also be found in police surveillance, border control and security. In fire departments, for example, it can help crews detect objects or people even in very heavy smoke.

The counterpart to coaxial illumination on the stereomicroscope is reflected light, or episcopic illumination, on the polarized light microscope. Light comes from directly above the sample through the objective and is reflected back up through the eyepieces. Episcopic and coaxial illumination produces similar results.

Oblique lightexample

A few weeks ago, I had the pleasure of moderating one of our webinars presented by Bill Chapin. The webinar was called “Meaningful Particle Analysis Begins with Light Microscopy.” At the beginning of Bill’s presentation he talks about the use of illumination techniques for the stereomicroscope and the polarized light microscope. Bill points out an inherent tendency of microscopists to neglect transferring the use of a good illumination technique from one instrument (the stereomicroscope) to another instrument (the polarized light microscope). In the webinar, Bill bounces between these two instruments making the point that if it’s good for one, why not employ it on the other.

Oblique lightexamination

Using coaxial or episcopic illumination can help to reveal surface textures, such as pitting or striations, that would otherwise go undetected with oblique lighting techniques. Bill tells us that there is virtually no sample where he doesn’t use these two reflected light techniques when he begins an analysis.

Transmittedlight

Try getting creative using different illumination techniques with your stereomicroscope and polarized light microscope—you may find new ways to see your sample more clearly. In some cases, without the proper lighting technique, portions or even entire particle populations of your sample can go undetected, leaving you and your client in the dark. Bill concludes that beyond these vitally important advantages, the right illumination technique also improves sample preparation, thus leading to better and more meaningful data.

Smoke, snow, fog, and certain materials such as glass and silicon are transparent, when imaging in SWIR. That means the atmospheric influence of fog and smoke disappears in the shortwave infrared (SWIR) spectrum. Although capturing images in the SWIR lacks the color content of the visible region, SWIR images -which are formed by light reflected from observed objects- can be interpreted as easily and directly as images in the visible region. A wide range of applications that would be difficult or impossible to perform with visible light are now possible with SWIR.

Taking things one step further, other microscopists attach their fiber-optic illuminator to a momentary contact foot switch. Through this way of controlling the oblique light, you can “hit” the sample with reflected light to gain additional information quickly without removing your eyes from the sample, and still have your hands free for manipulating the sample.

Obliquelighting eye exam

CMOS sensors are excellent devices to meet military needs, but these sensors are usually daylight sensors. A single SWIR camera can be used for both day and night imaging and can function throughout the duration of typical long day and night missions.

Obliquelighting Forensics

24-hour all-weather surveillance is a necessity for many military and emergency services. While thermal imaging can detect temperature differences, a SWIR camera can actually identify the object. Thermal imaging does not offer the resolution and range of imaging that is possible with a SWIR sensor. In a military conflict situation, it is critical to determine whether a threatening vehicle is a civilian truck or a military tank. SWIR can assist in this detection.

A very common illumination technique used with the stereomicroscope is oblique illumination, usually achieved by using two fiber-optic light guides or “goosenecks.” The goosenecks are positioned on either side of the sample at varying angles to produce raking light, or oblique illumination. As Bill points out, this technique is ideal for producing contrast between two colorless materials such as fibers and a filter surface.

Oblique lightphotography

There are two reflected light illumination techniques that Bill says are the most powerful and most underutilized on both the stereomicroscope and the polarized light microscope. In the stereomicroscope, coaxial illumination is a reflection technique in which light comes down from within the body of the stereomicroscope, down through the objective, onto the sample, and then is reflected back up through the same objective, and then to the eyepieces. This type of illumination is essential for revealing extremely thin particles such as glass delamination through an optical phenomenon known as thin film interference. Glass delamination particles are on the order of 50 nanometers thick and would go undetected without using this important technique. As you can see from Bill’s photographs taken from the stereomicroscope, the field of view on the left, using oblique illumination, does not detect these very thin particles. Looking at the same field of view, but with coaxial illumination, particles that were once invisible now stand out dramatically.

If you want to customize things a bit, Bill suggests adding a polarizer under the clear glass stage plate in the transmitted light base of the stereomicroscope. This, coupled with a quarter-wave plate that can be attached to the stereomicroscope objective (required for coaxial illumination and may already be in place), achieves pseudo-transmitted crossed polarized light. This is great for visually separating isotropic from anisotropic particles.

Let’s go through some of these frequently overlooked illumination techniques that Bill describes as invaluable to the microscopist.

Obliqueillumination microscopy

The polarized light microscope commonly uses transmitted light and Bill encourages microscopists to try using it with a stereomicroscope. Using transmitted light with the stereomicroscope, the sample is illuminated from below and light passes through the sample, which is helpful when looking through clear containers like intravenous bags or vials that contain floating particles.

Shortwave infrared (SWIR) light is generally defined as light in the wavelength range of 1.0-2.5 µm. Unlike mid-wave infrared (MWIR) and long-wave infrared (LWIR), which is emitted by the object itself. SWIR resembles visible light in that photons are reflected or absorbed by an object, creating the strong contrast needed for high-resolution imaging. Starlight and background radiation (nightglow) are natural sources of SWIR and provide excellent illumination for outdoor night imaging.

Now go back to your lab and take a look at all of the illumination techniques you are using with your stereomicroscope. Are you using these same techniques with your polarized light microscope? Are there any illumination techniques you use with the polarized light microscope that would be beneficial to you on your stereomicroscope?

Nedinsco offers this new SWIR technology in the OTUS camera products. Depending on your applications, our specialists can advise you on the best optics, coatings, sensors and housing to ensure the best performance possible.