As shown in Fig. 10.1, a condenser is a relay lens for the field stop of an illumination system. The illumination field stop is a real object. The condenser creates a virtual image of the illumination field stop at the specimen. The illumination field should be slightly larger than the vision field. Assembly tolerances dictate the amount of additional size of the illumination field beyond the vision field. The condenser also creates a virtual image of the illumination lens stop at the vision lens stop.
10.2 Abbe Illumination
Abbe illumination locates a source at a conjugate of the illumination field with a small NA. Earnst Abbe promoted a small NA of illumination, which facilitates the Abbe sine condition. The small illumination NA of Abbe may be defined as < 0.75 times the vision NA.
Earnst Abbe worked for Carl Zeiss. According to the Zeiss archives, Abbe eventually became the chief executive at Zeiss Optics, through which position he stimulated growth of the company from 25 to 1,400 employees. Abbe promoted numerous social reforms at Zeiss Optics, such as paid vacation, profit sharing, pensions, sick time, and an eight-hour workday.