In its most basic form, an episcopic illuminator, also known as a vertical illuminator, is a light source and beamsplitter. They often have provision for aperture and field diaphragms as well as filters. Some are designed for epi-fluorescence, so those have a place for one or more fluorescence cubes.
A well appointed vertical illuminator has two iris diaphrams to control NA (aperture stop) and to limit the field (field stop), as well as a place to insert various filters, such as polarizers, a diffuser, color filters and ND filters. Here we see one with a 50 W halogen lamphouse attached, as well as a rotatable polarizer and fixed analyzer.
Many Nikon epi illuminators have provision to use the brightfield/darkfield (BD or ED) objectives, with the high NA darkfield annulus as a separate light path. Normal brightfield can be selected, blocking the outer annulus with a slider. Here we see inside the unit with the slider partially engaged.
This is what it looks like disassembled, with the BD slider exposed.
And here it is looking through the brightfield side (left) and darkfield (right).
Some units do not have irises, but have slots for 25 mm filters.
And here is one with a field diaphram, but the end is broken off, leaving the aperture stop, but with no way to actuate it.
Some of them are set up to accept a fiber light.
The epi illuminator for the Model S is very small, but has apertures and a slide-in, slightly rotatable, polarizer. The slider on the right in the last two photos is the beamsplitter, so it may be removed for diascopic illumination. I think that this unit has a lens, which makes it appear to be 160 mm tube length, so 160/0 or 160/- would be the common objective type here.
For epi-fluorescence, there has to be a place for a dichroic beamsplitter with excitation and emission filters, as well as a beam block. In the Diaphot, the cube is in the body of the microscope, and the light is injected from the back through the episcopic illuminator.
For epi-fluorescence on Labophot and Optiphot models, a special illuminator is used that has slots for two cubes (or four for the Quadfluor), as well as built in ND filters and an iris, as well as the beam block. Some units have a place for a polarizer slider at the top (in the emission path), and some lack this feature.