Light microscopy for materials analysis generally refers to reflected light microscopy. In this method, light is directed vertically through the microscope objective and reflected back through the objective to an eyepiece, view screen or camera. Transmitted light is occasionally used for transparent and translucent materials. For some low-magnification work (stereomicroscopy), external, oblique illumination can be reflected off the sample into the objective. Magnification of the sample image is obtained by light refraction through combinations of lenses, comprising objectives and eyepieces. The minimum feature resolution is about 0.2 µ, but smaller features, as small as about 0.05 µ, can be detected by image contrast enhancement with polarized light, interference contrast, and dark field illumination methods. Images can be recorded on traditional films or as digital files for computer display storage and analysis.


Top

Bright Field Light Microscopy - Produces true color images at magnifications up to about 2000X. The sample surface is uniformly illuminated by indent light rays directed perpendicular to the sample surface. Light reflected back toward the objective lens is collected and focused on the eyepieces to form the observed image. Surfaces that are reflective and perpendicular to the light rays appear bright. Features that are nonreflective or oblique reflect less light back into objective and appear darker.

Polarized Light - Enhanced contrast for features that have anisotropic refractive properties. Two polarizing lenses are inserted into the optical path - one in the incoming illumination and one between the sample and eyepieces. When these lenses are rotated 90 to one another, the "crossed polarizers" result in the subtraction of some portion of the light spectrum by destructive interference. Contrast is obtained between sample features that have different reflective properties. Many metals, including beryllium, zirconium, and titanium, are anisotropic and exhibit grain contrast with polarized light illumination. Thin surface films can also be deposited on samples to obtain polarized light contrast.

Differential Interference Contrast after Nomarski (DIC) - Produces a 3-dimensional image by creating brightness contrast on very minor topographical changes. This technique utilizes crossed polarizers as described above. A double quartz prism is also inserted into the light path to split the incident light into two different paths. This results in two slightly shifted images of the sample on the viewing plane, which produces contrast between features with different heights and topographic orientations. The analyzer can be adjusted to obtain various degrees of interference to enhance selected features or create contrast colors in the image.

Darkfield - Enhanced contrast from subtle topographic features. An occluding disk is placed in the light path, blocking the direct vertical illumination. Peripheral rays in the illumination are reflected such that light reaches the sample at oblique angles. The absence of incident vertical rays results in bright reflectance only from oblique features, such as ridges, pits, scratches, and particles. Reflective features that normally appear bright in bright field illumination are completely black in darkfield illumination. Thus, subtle features that might be completely invisible in bright field are readily observed with darkfield illumination.

Quantification - Microscope image magnification is calibrated against reference standards. Lateral feature dimensions can be measured to an accuracy greater than 0.5 . Computer analysis of digitally-acquired images can measure area/volume fractions, particle sizes, and other features.

Top

• Microstructure evaluation
• Small feature measurements
• Fracture mode preliminary identification
• Corrosion failure inspection
• Surface contamination evaluation

Samples must be opaque or at least translucent for reflected light microscopy. Sample size, shape, and condition depend on the configuration of the microscope. Low-magnification stereo microscopes are small and have long focal length (up to 5 in.), so these can be set up to examine even large samples. Some portable field microscopes can be fixtured directly to large structures. For magnifications of 100X and above, microscopes are not usually amenable to portable use (except field units). Samples for these microscopes are limited to a few pounds in weight, and examination is limited to readily accessible flat surfaces due to the small depth of field at higher magnifications.

Top