Reflected light microscopy is primarily used to examine opaque specimens that are inaccessible to conventional transmitted light techniques. A material is considered opaque if a thin (polished or not) section about 25 micrometers in thickness is non-transparent in the visible light spectrum range between 450 and 650 nanometers. A variety of specimens fall into this category, including metals, coal, wood, slag, rock, plastics, alloys, composites, and bone. Many opaque and transparent specimens have pronounced amplitude or phase characteristics. The individual components of amplitude specimens differ in the amount of light absorption, whereas phase specimens differ only in the refractive indices of the individual features. In practice there is no pure amplitude or phase specimen where either one or the other property is predominant. Opaque specimens are considered phase specimens if the reflection differences between the individual features are below about 10 percent. In this section, we discuss the various mechanisms and optical configurations used to gain contrast in reflected light microscopy.

In its standard configuration, a typical reflected light microscope is readily equipped to examine amplitude (absorption) specimens using brightfield incident light. Natural absorption specimens are distinguished from phase specimens that are converted into absorption specimens by chemical treatment, such as etching and polishing the specimen, or coating the surface with a transparent, thin highly refractive layer. Presented in Figure 1 are reflected light digital images revealing details in a tiny feature intentionally placed on the surface of a computer chip using evaporated metal during the fabrication phase. Figure 1(a) shows the feature in brightfield, while Figures 1(b), 1(c), and 1(d) show the same viewfield in darkfield, polarized light, and differential interference contrast (DIC), respectively. Note the significant differences in contrast that can be obtained using these techniques.
Chemical etching is one of the most popular methods for introducing contrast into metallic specimens. In reflected light microscopy using brightfield illumination, etched structures of the polished specimen become visible due to shadow effects produced by a variety of mechanisms, such as the formation of reliefs, reflections due to cover layers, or reflectance effects arising from etch pits. Careful attention to detail in specimen preparation is important in order to avoid artifacts or misinterpretation of the acquired data. In most cases, a freshly prepared, correctly ground and polished specimen is necessary in order to observe and record important features of interest. Etching can be performed with chemical agents, electrolytic etching (specimen acts as an anode or cathode when bathed in an electrolyte), thermal etching in normal atmosphere or in a vacuum or inert gas. A note of caution should be applied to chemical etching. The process often damages structural elements of the polished specimen where small crystallites are lost. In these cases, the specimen can display only the crystal boundaries, which are not differentiated according to phase changes.
Specimen damage and loss of phase differentiation can be avoided by applying evaporated interference layers to the specimen. This purely physical technique uses a transparent, thin refractive substance (such as titanium dioxide, zinc selenide, or zinc telluride) compatible with the specimen phases, which can be evaporated onto the polished surface. Multiple reflections and interference effects generated by the evaporated film increase contrast so that the specimen can be examined in brightfield reflected light. Among the important parameters to consider when coating specimens is the refractive index of the specimen phase, the refractive index and thickness of the evaporated film, and the wavelength of light used for imaging. In many cases, monochromatic illumination is preferable to broadband white light. An interference filter can be inserted into the vertical illuminator to achieve illumination with a specific color or band of wavelengths.
Contrast can be enhanced in single phases, for example on a polished metal surface, using a gas-ion reaction chamber. The technique employs a residual gas ionized by electron irradiation so that the specimen surface is subjected to the gas in a vacuum chamber. When oxygen is used as the residual gas, the different specimen phases form oxide layers that give rise to colors due to absorption and interference when examined in brightfield reflected light. The degree of contrast enhancement is influenced by the type of residual gas, vacuum pressure, the density of the electron current emitted by the cathode, the discharge voltage, and the temperature of the specimen surface. Contamination can be avoided by heating the specimen, reducing current density, and using oxygen ions in the chamber. This technique can also be applied to non-metallic specimens for general material contrasting. In general, etched and coated specimens can be observed using standard long-working distance reflected light objectives.