Abstract

There are certain applications where it is desirable to image an object embedded in, or hidden behind, a scattering medium. An example in the medical field would be transillumination of human tissue¹,². Among other potential applications one could consider imaging through any highly scattering medium like clouds or plastics. The obstacle to overcome is the multiply scattered light obscuring the image of the object. Several techniques have been employed to solve this problem. All of them are relying on the fact that the highly scattered light is delayed in time with respect to the less scattered light. The forward scattered component of the light transmits more spatial information about the hidden object. Using very short laser pulses(typically less than a few ps) and time gating one can filter out the scattered light which arrives at the detector later in time. Various time-gating schemes have been used including fast gated electronic amplifier², fast streak camera³, optical shutters using Kerr effect⁴, holography with either short⁵ or long⁶ broadband laser pulses, and a number of nonlinear-optical techniques⁷,⁸,⁹. Often submillimeter spatial resolution is desired which requires the use of subpicosecond lasers with the gating techniques. It is desirable to extend the hidden object imaging to longer laser pulses that are readily available with commercial lasers today. For example, with longer pulses, higher energies per pulse can be used within the ANSI exposure limits for human tissue. This can significantly reduce the requirements on image collection. Recently a technique called Chrono-Coherent Imaging⁶ (CCI) was developed, where the short coherence length of a long pulse duration broadband light source is used for gating. The light is split in two beams, where one of the beams illuminates the object. The interference between the two beams is recorded on the film as a holographic image. One of the disadvantages of this method is the degradation of the holographic image due to the scattered light which is also incident on the film. In addition, it is not a real-time imaging technique.

© 1992 Optical Society of America