Three dimensional or “3D” cameras provide distance measurements to features in a scene being imaged. 3D cameras are used for many different applications in which it is desired to image objects as a function of distance, such as profile inspection of manufactured goods, medical imaging, CAD verification, robot vision, geographic surveying, and film production special effects.
In one method of obtaining a three-dimensional image, known as “time of flight”, one or more light pulses are radiated from a light source towards a scene of interest. The radiated light pulses are generally a non-visible light, such as infrared (IR). The light pulses reflect off the various objects in the scene. A portion of each of the reflected pulses returns to the 3D camera. The time it takes for light to return from an object to a camera is used to determine distance to the object. Light pulses that reflect off objects closer to the camera return to the camera sooner than those that reflect off objects located farther away.
3D cameras used for time of flight measurements generally contain both a light sensitive surface (hereinafter called a “photosurface” or “photosensitive surface”) and a gating means for gating the photosurface on or off. For each radiated light pulse, following an accurately predetermined delay from a time the light pulse is radiated, the camera is gated on for a period of time, hereinafter called a “gate”. Light pulses reflected from objects in the scene leave an image on the photosurface if they are received by the camera during the gate. Since these light pulses have traveled to the object and back in the known delay time between the radiation of the pulse and the gating on of the gate, and since the speed of light is also known, the distance to the object can be calculated. By repeating this process over a range of gate delay times, distances to features in the scene at different distances from the camera can be obtained.
In some 3D cameras, rather than just considering timing between light pulses and gates, amounts of light registered on the photosurface during the time that the camera is gated on is also used to determine distances to objects in a scene. This approach generally requires taking three measurements of light from the scene.
A “timing” measurement is taken by transmitting a light pulse to the scene, and gating on a photosurface on which the scene is imaged with a relatively short gate that is optionally the width of the light pulse. An amount of light registered on a pixel of the photosurface during the gate is a function of a distance from the camera of a region of the scene imaged on the pixel. The timing measurement is also affected by reflectivity of the region being imaged by the pixel. A high reflectivity region reflects more of the transmitted pulse and a low reflectivity region reflects less of the transmitted pulse. Accordingly, features equidistant from the camera that have differences in reflectivity will appear to be located at different distances from the camera.
Timing measurements can be corrected for differences in reflectivity by taking a second, normalization measurement. This measurement involves transmitting a light pulse and gating a photosurface that images the scene with a relatively long gate, optionally three times the width of the light pulse. The long gate enables the photosurface to register substantially all the light reflected from the light pulse by a region in the scene that reaches the camera. The 3D camera obtains normalized distance measurements by dividing the timing measurement by the normalization measurement for each pixel.
A third measurement measures background light. This is ambient light that is present in the scene, and that is present in the timing and normalization measurements. Since background light does not arise from the reflected light pulses, it makes those measurements less accurate. Background light is measured by gating on the photosurface and recording light that registers on the photosurface, when light from a transmitted light pulse is not present. To make the correction, the 3D camera subtracts the background light measurement from the timing and normalization measurements for each pixel.
Generally, a plurality of photosurfaces are used to acquire the various measurements. In some 3D cameras, three photosurfaces are used, a different one for each of the timing, normalization, and background measurements.
3D cameras are often used as components of imaging systems (hereinafter called “3D imaging systems”), which acquire and/or display a picture of a scene being imaged along with distance information to features in the scene. Some 3D imaging systems use a photosurface in addition to those used for providing distance measurements to record the picture of the scene being imaged. Such systems therefore may have four photosurfaces, one for the picture of the scene and one for each of the timing, normalization, and background data needed to calculate distances to features in the scene.
The photosurfaces used in 3D imaging systems use other types of components such as gating means or shutters, filters, and optical devices of varying complexity. Gating means may include, for example, electro-optical shutters or gated image intensifiers. IR filters are commonly used with imaging photosurfaces to prevent distortion of the images they record by infrared light. Other filters may be used, for example, to pass only infrared light to a distance-measuring photosurface. Optical devices include such items as beam splitters, various lenses for refocusing or relaying light, irises, and prisms.
Examples of gated 3D imaging systems that use amounts of light registered on pixels to determine distances to features in a scene are shown in PCT Publication WO 01/18563, the disclosure of which is incorporated herein by reference. Each of the configurations shown include both a video camera or photosurface to provide an image of the scene, and three other photosurfaces, labeled “D”, “B”, and “N”, to provide distance (i.e. timing), background, and normalization information respectively. In FIG. 1 of PCT Publication WO 01/18563 the system includes a beam splitter, two refocusing lens systems, a three-way prism, and a separate shutter for each photosurface. In the same patent, the configuration of FIG. 3 uses a four-way prism to split and direct incoming light.