An ultrasonic level or sensing system requires no contact with the target. For many processes in the medical, pharmaceutical, military and general industries this is an advantage over inline sensors that may contaminate the liquids inside a vessel or tube or that may be clogged by the product. Both continuous wave and pulsed systems are used. The principle behind a pulsed-ultrasonic technology is that the transmit signal consists of short bursts of ultrasonic energy. After each burst, the electronics looks for a return signal within a small window of time corresponding to the time it takes for the energy to pass through the vessel. Only a signal received during this window will qualify for additional signal processing. A popular consumer application of ultrasonic ranging was the Polaroid SX-70 camera which included a light-weight transducer system to automatically focus the camera. Polaroid later licenced this ultrasound technology and it became the basis of a variety of ultrasonic products.The SX-70 is a folding single lens reflex Land camera which was produced by the Polaroid Corporation from 1972-1981.In 1947, Polaroid introduced its first consumer camera. The Land Camera Model 95 was the first camera to use instant film to quickly produce photographs without developing them in a laboratory. The popular Model 95 and subsequent Land Cameras required complex procedures to take and produce good photographs. Photographic paper had to be manually removed from cameras, peeled open after 60 seconds, needed several mi utes to dry, and often left developing chemicals on hands. The instructions for the Model 20 Swinger, introduced in 1965, warned that, if not followed, "you’re headed for plenty of picture taking trouble". Pictures from the SX-70, by contrast, ejected automatically and developed quickly without chemical residue. Polaroid founder Edwin Land announced the SX-70 at a company annual meeting in April 1972. On stage, he took out a folded SX-70 from his suit coat pocket and in ten seconds took five pictures, both actions impossible with previous Land Cameras. The company first sold the SX-70 in Miami, Florida in late 1972, and began selling it nationally in fall 1973. Although the high cost of $180 for the camera and $6.90 for each film pack of ten pictures limited demand, Polaroid sold 700,000 by mid-1974. In 1973–4, the Skylab 3 and 4 astronauts used an SX-70 to photograph a video display screen to be able to compare the Sun's features from one orbit to the next. There were a variety of models beginning in 1972 with the original SX-70, though all shared the same basic design. The first model had a plain focusing screen (the user was expected to be able to see the difference between in- and out-of focus) because Dr. Land wanted to encourage photographers to think they were looking at the subject, rather than through a viewfinder. When many users complained that focusing was difficult, especially in dim light, a split-image rangefinder prism was added. This feature is standard on all later manual focus models.