Structured light has become a prominent 3D surface measurement technique that is acknowledged for its high speed and accuracy. As the essential component of any structured light scanner, the industrial and scientific light engines are usually either quite expensive or have low brightness. As such, many hobbyists employ commodity projectors (bit.ly/VSD-SL3D). With a large diversity of commodity projectors available on the market, scanners of different scales can be built, ranging from pico projectors to theater projectors with the size of target objects ranging from several millimeters to several meters.

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PolarizingFilterFilm

To see how polarized sunglasses work, it is convenient to think of light as a wave moving along a string. Just like a wave on a string, a light wave wiggles transversally to its direction of motion. And just like a wave on a string, the plane in which the string oscillates can have different orientations. For example, the oscillation can vibrate up and down, side to side or in any combination of the two directions. If the string is made to oscillate at, say, a 45 degree angle, this can be mathematically decomposed into a combination of horizontal and vertical oscillation components in equal amounts. A string oscillating at, say, 75 degrees has more vertical component than horizontal component. The angle of transverse oscillation is called the "polarization angle" (see Figure 1). By the way, when we say that a certain light source is "unpolarized" we mean that it emits waves at random angles so that on average there is no preferred direction of polarization. Direct Sunlight, light bulbs and candles are examples of unpolarized light sources. Almost no natural source of light is polarized at the source; polarization happens after some sort of interaction with matter. Therefore, for almost all sources of light, it is as though we wiggled the string in one transverse direction to generate a wavelet along the string then unpredictably changed to another transverse direction, and so on (see Figure 2). The waves come out swinging in hodge-podge polarization angles, but all travel along the direction of the string.

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To understand how polarized sunglasses block horizontal polarization, it's important to know how the electrons in the molecules of the sunglass filter behave. Electrons are set to oscillation by the incoming light wave and therefore some of the wave energy of the light gets transferred to the electrons to be dissipated or reflected by the electrons. Polarizing filters used in sunglasses contain molecules which make it is easy for the electrons to oscillate in the horizontal direction (long direction) thus dissipating more horizontally polarized light energy. On the other hand the long molecules make it hard for electrons to oscillate in the vertical direction (short direction)—thus diminishing the electrons' dissipation of vertically polarized light waves. (see Figure 5).

For developers or designers interested in assembling their own scanner, sample project files have been made available at bit.ly/VSD-3DSL.

UVfilter

Ying Yu is a PhD student, and Daniel L. Lau is a Kentucky Utilities Professor and Certified Professional Engineer. Both are with the Department of Electrical and Computer Engineering, University of Kentucky (www.uky.edu).

Figure 6: Polarized sunglasses block horizontally polarized light (red) but transmit vertically polarized light (blue). Image source: microscopyu.com

In the 3D scanner, the projectors run at 120 fps using either an 8 or 24 frame structured-light sequence, depending on the required scan accuracy. The system can make 5 to 15 scans/second, and according to our experimental data, the scanner achieves a scanning accuracy with less than 2 mm standard deviation at 1 m range in its 5 scans per second mode.

Figure 2: An unpolarized wave. The direction of polarization changes randomly along the wave. Sunlight and most other light sources are unpolarized but become partially polarized on reflection. Image source: astronomy.nmsu.edu

polarizing filter中文

The Fresnel equations quantitatively show how unpolarized light becomes partially polarized after reflection from a dielectric surface--such as water or glass. The equations are mentioned here just so the reader can see the horizontally polarized component of a wave differs in its reflection coefficient from the vertically polarized component. Unequal reflection coefficients lead to unpolarized light becoming partially polarized. Interestingly, the Fresnel equations predict the existence of an angle for which the glare is completely polarized, not just partially polarized. In other words, glare coming from this angle (known as the Brewster angle) can be completely blocked by an ideal polarizing filter. The effect is dramatic. (see Figure 4).

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Figure 1: "Polarization angle" is the angle of the transverse direction of oscillation of a wave. Here, horizontal (0 degrees) and vertical (90 degrees) are shown. Angle in between can be thought of as a mixture of the two directions with different mixture ratios. The grey rectangle with a vertical slot represents a polarized filter designed to transmit the vertical oscillations of the string and block the horizontal ones. Image source: cnx.org

Circularlypolarized light

P-polarizedlight

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The controller supports two working modes depending on the source of the structured light patterns. One mode internally generates the structured light patterns using the FPGA, while the other passes through the structured light patterns from the graphics interface on a PC, which could range from an Intel (Santa Clara, CA, USA; www.intel.com) Core i5 PC, off-the-shelf laptop, NVIDIA (Santa Clara, CA, USA; www.nvidia.com) Jetson, and even a Raspberry Pi (www.raspberrypi.org). When generated internally, Line 2 of the camera is configured as programmable output and is set high by the operator. The FPGA then sends structured light patterns to the projector and triggers the camera on the corresponding vertical syncs. Upon completing a scan, the operator sets Line 2 of the camera to low, telling the FPGA to stop sending structured light patterns and sending all black patterns instead.

Ari Siletz is president of CCDMETRIX. His company specializes in automated vision system inspection and metrology. With a background in both optical and software engineering, Ari has been developing instruments for the the ophthalmic and optical coating industries since the 1980s. Writing is one of Ari's serious hobbies. He is a published author whose short stories have appeared in numerous literary anthologies. He lives in Sebastopol, California.

Figure 3: Viewing the same scene with and without a polarizing filter. The reflected sunlight in the left image is partially polarized. The right image is taken with a filter that blocks horizontally polarized light. Image source: photography.ca/blog

Figure 5: Molecules in a polarizing filter are long in one direction and short in the perpendicular direction. Electrons can freely oscillate along the length of the molecule, absorbing or reflecting the light energy, while they are unable to oscillate very far along the short direction. The "E-field" arrows in the Figure show the direction of polarization. The small spheres labeled "e-" represent electrons. Note in the Figure how the horizontally polarized wave (top) emerges) from the its electron interaction with its amplitude reduced, while the vertically oscillating wave (bottom) comes through with undiminished amplitude. Image Source: voer.edu.vn

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A team from the Department of Electrical and Computer Engineering, University of Kentucky, has developed FPGA-based projector controllers to build industrial-quality 3D scanners using commodity projectors. As featured below, one such system is a dualprojector structured light scanner composed of two Optoma Corporation (New Taipei City, Taiwan; www.optoma.com) ML750ST projectors, each of which has 500 lumens and a projection area with an approximately 36” diagonal. In addition to the FPGA-based controller, the system also features a Basler (Ahrensburg, Germany; www.baslerweb.com) acA1920-150uc USB 3.0 camera, which features the 2.3 MPixel PYTHON 2000 global shutter CMOS sensor from ON Semiconductor (Phoenix, AZ, USA; www.onsemi.com) and a 150 fps frame rate.

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Sunlight can become partially polarized by the scattering of air molecules or by reflecting off something like a lake. This means that after scattering or reflecting the sun's waves oscillation angles, they are no longer random in all directions but have a preferred direction on average. In the case of a horizontal surface—like a lake or a road—the preferred direction is horizontal. This horizontally vibrating reflected sunlight is the nuisance we see as glare, and this is why polarized lenses are so useful to beach goers and motorists: they block glare. The polarized filters on these lenses preferentially block the horizontal component of light oscillation while transmitting the vertical component. The result is a darker image but with better contrast (see Figure 3).

Polarizing filters Physics

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Figure 4: Glare reflecting at Brewster angle from a window. The light producing the glare is highly polarized allowing the polarizing filter (on the right window) to virtually remove all of it. For fresh water, the Brewster angle is about 53 degrees, so peak performance of polarized sunglasses on a calm lake happen when the sun is at an angle of about 37 degrees from the horizon (90-53= 37). Image source: boundless.com/physics

Figure 6 below illustrates a simplified summary of everything we just discussed about polarization: glare reducing sunglasses!

In practice it's difficult to get the long molecules all lined up in one direction, but somewhat lined up molecules are still effective in making a polarizer. One way to accomplish this is to put long-chain molecules on a piece of transparent stretchable material then heat and pull the stretchable material. The long molecule chains, originally in random orientations, will more or less line up in the direction of the pulling.

Polarizing film Roll

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When passing HDMI video from the host PC to the projector, Line 2 is still set low by the operator while patterns from the PC are passed through the scanner to the projector. The difference here is that the FPGA reads the red channel of the top-left pixel as a one byte of identification data indicating the number of the frame. By reading this top-left pixel as a handshake, the FPGA can decide if and when to trigger the camera with vertical syncs.

PolarizingFiltersheet

In this way, the operator can synchronize their pattern set to the camera without any loss of video and without dealing with the potential replication of frames by the graphics interface. There is no specific requirement for the graphics interface. Most PC graphics interfaces should work as long as they support OpenGL SL 3.3 or higher. Besides conducting the handshake for the system synchronization, the software running on the PC also sets up parameters under which the camera operates, such as exposure time, trigger source, resolution, etc.

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