The image generated by most projection clocks is large enough that a nearsighted person can see it from a distance without glasses or contact lenses.
Modern high-brightness projection clocks are in most cases analogue and have a halogen bulb backlight.
In most cases, they use a set of rotating and fixed transparent discs with hands and a face.
An LCD is integrated into some clocks to combine analogue and digital information on the projected image.
They usually use lenses although some projectors use the principle of shadow theatre [dubious – discuss], vector or raster scanning.