Imagery with constant range lines

Abstract

Method and apparatus for showing the relative range from an active line scan sensor to a target, the changes in range being shown on a recording medium as an alternate series of bright and dark lines. In particular, a modulated laser beam having high frequency components is utilized to illuminate the target, the reflected energy received at the sensor having the same modulation frequency but phase modified by the transit time required to make the roundtrip from the sensor to the target and return. The phase difference between the received energy and the transmitted, or reference, energy is detected by a phase demodulator, the detected phase difference corresponding to range contour lines. A reflectivity signal corresponding to the reflectance of the target is also provided. The range contour and reflectivity signals can be simultaneously recorded on a recording medium. Alternately, the reflectivity signal may be gated by the range contour signal producing a composite image on the medium. Points in the image formed on the recording medium where the phase angles are the same can be made to appear white while the points where the phase angles differ by 180.degree. can be made to appear black. The difference in the range between two black lines is equal to a specific sub-interval of the wavelength of the effective laser modulation frequency, the sub-interval being 1/n where n is the harmonic of the effective laser modulation frequency. The scanning laser may be a mode locked laser, thereby providing a light output having a high requency components and precise frequency and phase stability.