Visible Camera Physics and Background

In this research, two different types of visible cameras were used to collect detailed data on the LPBF process. The first camera is a high-speed camera that collects details of each melt pool, and the second is a low-cost visible camera that is able to collect melt pool data with low exposure time and surface data with high exposure time. The function of the

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visible camera is to collect light intensities over a spatial region. This happens when a photon of light is collected by the visible camera. The camera has a focal plane array that is 1 cm x 1 cm as shown in Figure 2.7. This focal plane array represents the resolution and pixel values. As the photon enters the camera the photon strikes this focal plane, gaining an X and Y location as well as intensity. This happens repeatedly, creating larger intensities and a more densely populated focal plane. The intensities are saved in their X and Y location, which is then turned into the image seen.

Figure 2.7: Focal plane array shown. The area of the pad represents the area of the image [59].

This happens every layer and increasing the exposure time will increase the density of photons on the focal plane, resulting in a bright image. Visible cameras are used for process monitoring because they are easy to understand and use, and they collect useful data regarding the surface and melt pool behaviors. The two different visible cameras will be disused to show their relevance and processes.

2.1.2.1 High-Speed Camera

During the LPBF process, a high-energy laser strikes metal powder causing a transfer of energy, often violently. The majority of the energy is channeled into melting the

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metal powder, however not all of the powder particles conform into the desired part. Few of the particles receive the energy and eject into the environment. This phenomenon is called particle ejection. The particle ejections give information on the process parameters that helps conclude if the parameters are adequate. One way of capturing these ejections is through a visible camera. The first concept of using a visible camera came by high-speed cameras. A Phantom V9.1 high-speed camera is able to capture events at extremely high speed. High-speed cameras are widely used due to high capture rates resulting in as much information and detail as possible. Typical frame rates are in the thousands per second, depending on the camera. In this research, the Phantom camera collects data at 10,000 frames per second. This results in one-two frames in every melt pool.

With high speeds, many different nuances can be detected. For example, as the particle ejections fly into the surrounding atmosphere, the high-speed camera is able to determine their speed This is done by measuring the length the particle streaks over the course of a few frames. The downside to the larger amounts of detail collected through the high-speed camera is the files sizes and saving times. The saving times are on the scale of minutes, and during an LPBF minutes relates to multiple layers. This is not conducive to comprehensive defect detection, nor is it viable for closed loop applications. The files sizes are also cumbersome to deal with, causing many storage devices to become full quickly. One alternative to this is to find the ratio of frames per seconds to saving times that could lead to closed loop applications.

2.1.2.2 Low Cost Visible Camera

Low cost cameras have much slower frame rates resulting in lower cost. It is important to determine the adequate frame rate. If the frame rate is to slow then not enough

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data is collected to have any confidence in the inspection, and if the frame rate is too long it leads to larger files and longer saving times similar to the high-speed camera. The low- cost camera that is used in this research is a Basler acA1920-155 um. The Basler camera has a frame rate of 75 frames per second with a resolution of 1928x1208. The meaningful data collected with the high-speed camera is possible to detect with the low-cost camera. The information gained by using a visible camera relates to the physical set up of the experiment. The visible camera is responsible for detection of small changes that other sensors cannot detect. For example, during an LPBF build occasionally powder will not cover the entire part. This will not be extremely evident in the 3-color spectrometer data but will stick out brightly in the visible camera data. This cross reference of data emphasizes the strength of the multi-sensor approach.

2.1.2.3 Visible Camera Approach

There are two main approaches for using the visible camera. The first is collecting melt pool information with low exposure time. Exposure time is the variable that controls how long the aperture of the camera remains open to collect photon data per frame. Collecting melt pool information by analyzing the melt pool geometry and determining if the melt pool is interacting with or creating a defect is mainly determined visually. The second approach is using high exposure times to evaluate the surface and melt pool behavior. With high exposure time, the camera is collecting data for nearly the entire layer. Figure 2.8 is an example of the data collected using the long exposure technique. The result is a long streak representing the laser. The light collected is much brighter and as a result, the signal saturates. This can be avoided by introducing neutral density filters to block the intensity of the light. The neutral density filters do not block information from reaching the

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sensor but merely dampen the intensity to show more detail. Collecting data in this way is very useful because it allows the capture of the entire layer, to be later stitched together for a comprehensive view of each layer. This data collection approach used in unison with the other sensors generates useful information. A similar approach to this uses layer imaging to map out the entire part and analyses where defects occurred from lack of powder and spreader defects, which enables the tracking and locating of defects in an LPBF part [60].

Figure 2.8: Long exposure technique for low cost visible camera.

The long exposure time is useful in that it helps to recover some of the data that is lost when the frame rate drops. In both cases of high and low exposure time, the data collected is valuable to identify part quality of an LPBF build. The low exposure rate creates individual information on melt pools, while the high exposure rate explores the surface of the part and is able to detect surface changes.