Multiple-wavelength range-gated active imaging principle in the accumulation mode for three-dimensional imaging

Multiple-wavelength range-gated active imaging principle integrating spectral information for five-dimensional imaging

The combined multiple-wavelength range-gated active imaging (WRAI) principle is able to determine the position of a moving object in a four-dimensional space and to deduce its trajectory and its speed independently of the video frequency. By combining two wavelength categories, it determines the depth of moving objects in the scene with the warm color category and the precise moment of a moving object's position with the cold color category. Therefore, since each object had the ability to transmit information from different wavelengths, related to the spectral reflectances, it became interesting to identify their spectral signatures from these reflectances. Using a conventional method of spectral classification, it was shown that it is possible to identify objects in a 3D scene from their a priori known spectral signatures and, thanks to this, to reveal especially the fifth dimension in the imaging of the WRAI principle. The experimental tests confirmed that it is possible to record moving objects in a five-dimensional space represented by a single image, thus validating this multi-wavelength imaging method.

Multiple-wavelength range-gated active imaging applied to the evaluation of simultaneous movement of millimeter-size objects moving in a given volume

The combined multiple-wavelength range-gated active imaging (WRAI) principle is able to determine the position of a moving object in a four-dimensional space and to deduce its trajectory and its speed independently of the video frequency. However, when the scene size is reduced and the objects have a millimeter size, the temporal values intervening on the depth of the visualized zone in the scene cannot be reduced further because of technological limitations. To improve the depth resolution, the illumination type of the juxtaposed style of this principle has been modified. Therefore, it was important to evaluate this new context in the case of millimeter-size objects moving simultaneously in a reduced volume. Based on the rainbow volume velocimetry method, the combined WRAI principle was studied in accelerometry and velocimetry with four-dimensional images of millimeter-size objects. This basic principle combining two wavelength categories determines the depth of moving objects in the scene with the warm color category and the precise moment of moving objects position with the cold color category. The difference in this new, to the best of our knowledge, method is at the level of the scene illumination, which is obtained transversally by a pulsed light source having a wide spectral band limited in warm colors in order to get a better depth resolution. For cold colors, the illumination with pulsed beams of distinct wavelengths remains unchanged. Thus, it is possible independently of the video frequency to know from a single recorded image the trajectory, the speed, and the acceleration of millimeter-size objects moving simultaneously in 3D space and also the chronology of their passages. The experimental tests validated this modified multiple-wavelength range-gated active imaging method and confirmed the possibility of avoiding confusion when the object trajectories intersect.

Doppler effect in the multiple-wavelength range-gated active imaging up to relativistic speeds

Having laid down previously the foundations of the combined multiple-wavelength range-gated active imaging (WRAI) principle recording a moving object in a four-dimensional space represented by a single image, it was necessary to know if a Doppler effect could appear in the direction of the radial velocity of the object. This is due to the fact that this imaging principle requires the emission of laser pulses at a certain frequency in relation to this speed. To know the limits, the radial velocity of the object was supposed to go up to relativistic speeds. From a theoretical approach, some temporal behaviors leading to the Doppler effect were shown. To validate this effect experimentally, a setup based on the fictional positions (represented by mirrors) of an object moving at extreme speeds was performed. The results obtained with WRAI confirmed the presence of the Doppler effect, which must be taken into account when the radial velocity of the object is extreme. In this case, some conditions must be met as in the period and the time of the shutter aperture. Singular behaviors have also been shown in the case of relativistic speeds. So despite the Doppler effect, the distance of the fast-moving object and the different moments are well identified according to the different wavelengths in the combined WRAI principle.

Combination of the two styles of the multiple-wavelength range-gated active imaging principle for four-dimensional imaging

Having previously reported the foundations of the multiple-wavelength range-gated active imaging (WRAI) principle in juxtaposed style and in superimposed style, its use in combination of both styles was studied. The juxtaposed style consists of restoring the 3D scene directly. Each emitted light pulse with a different wavelength corresponds to a visualized zone with a different distance in the scene. In superimposed style, each wavelength is superimposed in the scene at a specific moment in time. By combining these two styles and independently of the video frequency, it is possible to deduce the trajectory and direction of the object in 3D space as well as its speed between two consecutive positions from a single recorded image. Furthermore, with the help of range gating, the object visibility through scattering environments is improved. In order to separate both styles in the image, the wavelengths were separated into warm colors for depth and cold colors for time. The experimental tests confirmed the ability to record a moving object in a 4D space represented by a single image, thus validating the combined WRAI principle.

Multiple-wavelength range-gated active imaging in superimposed style for moving object tracking

Having laid down previously the foundations of the multiple-wavelength range-gated active imaging (WRAI) principle in flash mode and accumulation mode, its use in superimposed style for the direct tracking of moving objects was studied. The movement is supposed to be in a transverse plane of the scene. Each emitted laser pulse with a different wavelength visualizes the object at a specific time. Therefore, the wavelength set allows us to freeze the different positions of the object in the recorded image according to the laser emission period. Thus, in each recorded image, the object trajectory can be visualized directly according to these specific moments, allowing us to deduce the speed and the acceleration of the object at the same time. Another advantage, especially when the object trajectory is unpredictable, is that the wavelength progression indicates the trajectory direction. The principle being independent of the video frequency, the object speed can be very high. Furthermore, the range gating allows improvement of the object visibility through scattering environments. Three cases were identified and studied in superimposed style. Although the first case has not been used with range gating, it allowed us to show the possibility of using a traditional color camera. On the other hand, the two cases applying range gating required management of the exposure time. In each case, the temporal behavior was studied, as well as the possible speeds according to the object distance. The experimental test results validated the theoretical part and the possibility to determine the trajectory and the speed of a moving object with the WRAI principle in superimposed style.