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An approach was proposed and developed to recover the 3D shape concealed in a shelter with a small hole only using three laser scatterings. This approach extends reconstruction of concealed 3D shape from "around a corner" to "through a small hole". Based on principle of rectilinear propagation of light, a simple geometric mapping tentative theoretical frame independent of scene was proposed to extract 3D information of concealed objects. Experimental setup mainly consists of a nanosecond laser and a single-photon APD, both of which are commercially available. The 3D reconstructions of three hidden objects were acquired with a resolution of centimeters.

A gain-modulated laser range imaging technology is generalized and its range accuracy is deduced. Theoretical results indicate that the range accuracy is proportional to the ratio of gain function to the derivative of the gain function and inverse proportional to output SNR. A gain-modulated laser range imaging system is established in our laboratory. It consists of a pulsed laser which is capable of generating laser pulses with a pulse width of 10ns and a center wavelength of 532 nm, and a receiver which is a digital 256×256 CCD sensor coupled to a GEN II intensifier with a 10nm bandwidth optical filter. Image intensifier is electronically driven and can be set to three modulated gain or constant gain. A range image of the target can be extracted by processing an intensity image with modulated gain and an intensity image with constant gain. Some indoor experiments are performed with sinusoidal, linear and exponential gain functions. The range images of the targets from 52 m to 58 m is taken and analyzed. Experimental results demonstrate the range accuracy with both sinusoidal and linear gain function depends on the relative range but one with exponential gain function independent of relative range. Specially, in the exponential gain function case the relatively small time constant can contribute to relatively high range accuracy.