Basic principles of microscope objectives.

Microscope objectives are perhaps the most important components of an optical microscope because they are responsible for primary image formation and play a central role in determining the quality of images that the microscope is capable of producing. Objectives are also instrumental in determining the magnification of a particular specimen and the resolution under which fine specimen detail can be observed in the microscope. The objective is the most difficult component of an optical microscope to design and assemble and is the first component that light encounters as it proceeds from the specimen to the image plane. Objectives derive their name from the fact that they are, by proximity, the closest component to the object (specimen) being imaged. Modern objectives, composed of numerous internal glass lens elements, have reached a high state of quality and performance, with the extent of correction for aberrations and flatness of field determining the usefulness and cost of an objective (see Figure 1). Construction techniques and materials used to manufacture objectives have greatly improved over the course of the past 100 years. Today, objectives are designed with the assistance of computer-aided-design systems using advanced rare-element glass formulations of uniform composition and quality having highly specific refractive indices. The enhanced performance that is demonstrated using these advanced techniques has allowed manufacturers to produce objectives that are very low in dispersion and corrected for most of the common optical artifacts such as coma, astigmatism, geometrical distortion, field curvature, and spherical and chromatic aberration (1). Not only are microscope objectives now corrected for more aberrations over wider fields but also image flare has been dramatically reduced with a substantial increase in light transmission, yielding images that are remarkably bright, sharp, and crisp. Three critical design characteristics of the objective set the ultimate resolution limit of the microscope (4). These include the wavelength of light used to illuminate the specimen, the angular aperture of the light cone captured by the objective, and the refractive index in the object space between the objective front lens and the specimen. Resolution for a diffraction-limited optical microscope can be described as the minimum detectable distance between two closely spaced specimen points: