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Abstract

A method and apparatus for direct two-dimensional transillumination imaging of a sample immersed in or including a scattering medium at infrared to near-infrared wavelengths. Frequency conversion and time-gating techniques in the infrared and near-infrared range are used to enhance two-dimensional image collection and detection. The apparatus includes a probe radiation source for applying a probe beam in the infrared to near-infrared frequency range to the sample. The probe beam is spread to cover a two-dimensionally extending area of the sample to be imaged. After interacting with the sample, the probe beam is representative of a two-dimensional image of the covered sample area. In one embodiment the apparatus includes a time-gating arrangement for selecting a temporal portion of the probe beam in a narrow time window representative of early arriving photons transmitted through the scattering medium and providing a time-gated beam representative of the two-dimensional image of the sample area. A reference beam interacts with a nonlinear medium to shift the frequency of the probe beam to a different frequency, which may lie outside the infrared to near-infrared range. In many applications of the apparatus the frequency will be shifted upward to a frequency higher than near-infrared. The nonlinear interaction occurs in such a manner so as not to destroy the two-dimensional image carrier by the time-gated beam. A detector operative at the shifted frequency detects the time-gated and frequency-shifted beam to directly record the two-dimensional image.

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