Scanning, Optical

a method of investigating rapidly occurring processes, such as the propagation of shock waves, the detonation of explosives, and the development of gas discharges. In contrast to high-speed filming, in which individual frames fix successive phases of a phenomenon discretely, albeit with high frequency, optical scanning provides a continuous photographic record; the optical image of the elements of the front of the process under study moves continuously and at high speed over the surface of a photosensitive layer, such as a photographic film or the screen of an image converter.

In a typical optical scanning design, the intermediate image formed by the first lens is matched with a slit that “cuts” a small section from the image; as the process continues, this image moves along the slit but remains in the slit’s plane. The second lens transfers the image from the slit to a photographic film in the form of a ring outside or inside a rotating drum, whose axis of rotation is parallel to the slit. The time resolution of optical scanning is equal to the time interval in which the image on the film traverses a path equal to its own width. If the film is secured inside the drum, the linear rate of rotation of the film reaches 300–400 m/sec. When the image width is ~0.1 mm, the time resolution may be ~2–3 × 10^-7 sec.

In optical scanning using a mirror, the film is stationary and the moving image is reflected from a rotating plane mirror or reflecting polyhedron. The angular velocity of the mirror may reach 10⁵ rpm; the linear scanning speed with a reflecting dodecahedron may reach 5 × 10³ m/sec, which provides a time resolution up to 2 × 10^-8 sec. This system makes it possible to increase the speed of the relative motion of the film and image.

In optical scanning using image converters, the electron beams carrying the image from a photocathode to a screen are deflected. Electric fields that vary in time according to a linear, circular, or elliptical function are used for this purpose. The afterglow of the screen makes it possible to photograph immediately the entire scanning picture with a conventional camera. The rate of motion of electron beams across the screen (the recording speed) reaches 3 × 10⁵ m/sec, and the time resolution is 10-¹⁰-10-¹² sec.