The light-sensitive surface inside a camera (whether digital or analogue) is illuminated, gradually, when the shutter release is activated. The incoming light may thus impinge on some regions of the surface for much longer than others, depending on the type of shutter involved.
In an SLR, with a shutter which pivots upwards and then down again, those pixels at the bottom of the image are illuminated for much longer than those at the top. A rotary shutter largely fixes this potential problem (which might otherwise result in some of the sensors in the bottom of the image becoming ‘burnt out’.
A light, angled mirror (blue) is used to allow through the lens composition as usual, but in today’s invention, this mirror rotates about the axis as shown, driven by a high-speed motor.
This provides uniform illumination of the sensing surface whilst still allowing a direct view of the scene via the eyepiece. The mirror shaft would obviously need to be balanced to minimise vibration (perhaps by using several, petal-like mirrors).