Final answer:
In epi-fluorescence microscopy, a condenser is not used because it can cause photobleaching, which damages fluorescent dyes due to intense light exposure. Additionally, this type of microscopy utilizes an alternate illumination system that renders a condenser unsuitable.
Step-by-step explanation:
A condenser is not used in epi-fluorescence microscopes because it can damage fluorescent dyes, a process known as photobleaching. This damage occurs due to the intense light typically provided by a condenser, which is necessary for the brightfield microscopy to enhance the illumination of a specimen. However, in fluorescence microscopy, the fluorescence emission from the dyes is more important than the direct illumination on the specimen itself. Moreover, epi-fluorescence microscopy uses an episcopic illumination system where the light source is situated above the specimen and the light shines down onto it, separate from the imaging pathway. Therefore, a condenser, which is designed to focus and increase the intensity of light transmitted through a specimen from below, is not suitable for this type of fluorescence-based imaging setup.