The phrase "focal plane" refers to the specific area where an image is sharply focused when using a camera or microscope. It is the plane in which objects appear clearest and in focus. Full definition
Lightsheet microscopes that project ultra-thin sheets of excitation light through multiple focal planes of a sample may also be useful for providing fast imaging along with optical sectioning (5). (blog.addgene.org)
Analog sensors were replaced with digital focal plane arrays. (space.com)