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In order to reorganize the position of objects in a 3D environment, 3D imaging is a physical science that is interested in determining the period of flight of some waves, typically light. Although optical time of flight imagers have been around for a while, the advent of solid state sensors with quick global shutters has made them more portable and amenable to mass manufacture (Song 103). In these solid sensors, light propagation may be stopped in both time and space, and the time of flight can be calculated pixel-by-pixel to precisely reconstruct 3D volumes and scenes. Application involved in the TOF imaging include: virtual keyboards, gaming, and gesture recognition for short range cameras, 3D video monitoring, robotic operation, and security for cameras that are medium range. For the long range cameras, applications include pedestrian and safety avoidance, and for the ultra long range camera’s the applications involved are landscape monitoring and LIDAR telemetry. Also on a similar concept, there are non vision applications like the TOF positron emission tomography; time resolved optical coherent tomography and fluorescence lifetime imaging microscopy among others (Song 103).
For the TOF imaging techniques to determine the position of objects in spaces, so as to build a 3D map of scene, it requires a source of radiation, an imaging device, and a detector. The radiation basically electromagnetic or sonic, therefore they drive the other two mechanisms appropriately. The TOF 3D imaging generally works by illuminating an area with modulated IR light. The distance of the reflected signal can be obtained by measuring the phase changer for every pixel in the sensor that create 3D depth map of the scene (Song 103).
The images below show the principle of the TOF 3D imaging on how it measures the distance between an object and a sensor depending on the time difference between the release of a signal and its return to the sensor after it has been reflected by an object. Various types of the signals can be used with the TOF 3D imaging, the common signals being light and sound. The TaraRanger sensors use light as their signal/ carrier because light can uniquely combine speed, eye- safety, range, and low weight. The use of infrared light reduces signal disturbance, and creates an easier difference from the natural ambient light, leading to the best performing sensors for their particular weight and size.
The principle of the TOF 3D imaging can also be compared to a technology called LIDAR. This is a technology whereby an object is illuminated by scanning laser beams. The light that reflects off the object is than analyzed depending on the wavelength of the laser light that has been used. The LIDAR can gather information about the size of the object and its distance from the laser so as to create an image of the surroundings. By having an collection of tiny LIDARS on the coherent imager, it is easy to simultaneously image varying parts of the scene/object without any mechanical movements within the image (Brooker 154).
The behavior of light is very predictable. If a ray of light could be observed while approaching and reflecting off a flat shiny object like a mirror, the behavior of the light as it reflects from the mirror would follow a predictable law called the law of reflection. The diagram below describes/shows the law of reflection.
In the above diagram, the light that approaches the mirror is referred to as the indirect ray; it is labeled I. the ray leaving the mirror is called the reflected ray marked by the letter R. at the point where the two activities occur: light approaching the mirror and ray leaving the mirror, a perpendicular line can be drawn to the surface of the mirror. This line is referred to as the normal line marked with the letter N. the normal line (N) divides the angle between the approaching ray (I) and the reflected ray into two equal angles (Brooker 134).
The angle between the normal ray and the approaching ray (incident ray) is known as the angle of incidence, while that between the normal ray and the reflected ray is known as the angle of reflection. The angle of incidence and reflected ray are marked with “theta” a Greek letter plus a subscript that reads “theta-i” for the angle of incidence and “theta-r” for angle of reflection. The law of reflection indicates that when light reflects on a surface, the angle of incidence becomes equal to the angle of reflection (Brooker 89).
Song, Zhang. Handbook of 3d Machine Vision: Optical Metrology and Imaging. Boca Raton: CRC Press, 2013. Print.
Brooker, Graham. Introduction to Sensors for Ranging and Imaging. , 2012. Print.
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