Detection of whether or not liquid or a liquid droplet (ink) has passed is realized without contacting the passing liquid or liquid droplet, by providing a radiated-wave detector, arranged near the liquid or liquid droplet, for detecting an infrared ray radiated from the passing liquid or liquid droplet,...http://www.google.com.au/patents/US6540318?utm_source=gb-gplus-sharePatent US6540318 - Liquid detection method and apparatus therefor, ink-jet printing apparatus, and ink detection method for the ink-jet printing apparatus
