Final answer:
A confocal microscope uses a pinhole to block out-of-focus light, allowing only light from the focal plane to be detected, creating clear 3D images particularly useful for thick specimens.
Step-by-step explanation:
A confocal laser scanning microscope maintains high image quality by using a pinhole to only allow light from a specific focal plane to reach the detector. This method relies on pinhole apertures and wave optics to exclude out-of-focus light. As a laser scans the specimen, only light from the focal plane is collected through the objective lens, then a secondary pinhole blocks any light not coming from the focused point. This process happens successively for multiple z-planes, creating a series of two-dimensional images that a computer constructs into a three-dimensional image. This technique is especially beneficial for thick specimens, allowing for clear, in-depth imaging while minimizing blur from out-of-focus light.