Optical microscopes are one of the oldest optical instruments and are used to produce magnified visual or photographic images of small objects. The oldest microscope was the magnifying glass, which was successfully used to observe some large bacteria. In the early 1600s, the first compound microscope with an objective lens and an eyepiece was invented by the Janssen brothers in the Netherlands. All modern microscopes are developed from the compound configuration.
To meet various requirements, many types of microscopes have been developed, for example, polarization, phase contrast, fluorescence, confocal, digital, and nonlinear. Optical microscopes have become an essential tool for biomedical applications, from investigating biological processes and diagnosing diseases to quantitatively measuring biological processes in vitro and in vivo.
Microscope optics is fundamental to many other biomedical imaging systems, such as endoscopy, confocal imaging, fluorescence imaging, and polarization imaging, all of which will be discussed in the following chapters.
Figure 4.1 shows two basic configurations of microscope optics. The exit pupil of the objective lens is set at the rear focal plane; however, this is not an essential requirement for a microscopic objective lens. The advantage of an exit pupil at the rear focal plane is that the objective lens is telecentric in object space, which means that the chief rays are parallel to the optical axis.
Figure 4.1(a) depicts a classic configuration with a finite-corrected objective lens whose image plane is located at a finite distance from the objective lens. The distance between the rear focal plane and the intermediate image plane is defined as the optical tube length. The eyepiece forms a virtual image of the intermediate image at a distance 250 mm from its aperture, where the iris of the observer is located for an optimal viewing condition. Sometimes the eyepiece is designed to image the intermediate image to infinity.