Microscope illumination
Light microscopy

Objects examined in microscopy are rarely luminous on their own, and it is necessary to illuminate them. Given, on the one hand, the small ratio of the diameter Φ′ of the instrument's pupil to that, Φ _o , which the pupil of the eye would have if it were observing the object directly, and, on the other hand, the instrument's transmission factor T reduced by the large number of lenses and surfaces, the clarity of the microscope: C = (T / n ² ) (Φ′ / Φ _o ) ²

The ratio of retinal illuminance when observing through the instrument and with the naked eye is very low...