High dynamic range imaging for laser weld monitoring
Welding-view camera technology
Cornelius Sawatzky
Welding creates bright light that under certain unprotected conditions can potentially be harmful to the human eye. In the case of open arc welding processes such as MIG or TIG, the ultraviolet (UV) rays emitted can quickly burn human skin and eyes. In open environment situations, radiation from specific laser beams can be hazardous to an operator observing a laser weld in progress. These conditions often make it necessary to have cameras installed at the weld head so that a weld in progress can be displayed on a screen remotely in a safe and often remote location.
Less useful camera setups
Using cameras for viewing welding processes improves the operator's working environment and also adds additional benefits. Overall productivity is often improved by reducing set-up time as it allows the operator to adjust or manipulate the weld setup with controls from the same remote location, eliminating trips into and out of the welding-apparatus envelope. Camera systems can also be helpful for troubleshooting because visual results can be compared from one condition to another by using recorded video. Video recording is ideal when trying to compare notes from one configuration to the next or when archiving for future references.
When someone has a need to use a camera, the first instinct may be to pick up a camera from a local shop and then put a dark glass in front of it like the one in a welder's helmet. What is often not realized is just how phenomenal the human eye is at being able to see and differentiate details in a great deal of light range conditions. Most digital cameras have a limited range of light brightness that they can capture into an image. When someone takes a regular camera and applies some dark glass to it, then uses the camera to look at a weld application, one of two things usually happens:
- They get a great view of the weld puddle because they have darkened the image enough, but they cannot see anything else around the weld arc/puddle because it is too dark; or
- They can see some detail immediately around the weld arc such as the seam and material fit-up, but all other details related to the weld puddle are completely saturated white, providing little meaningful information.
A few decades ago, someone applied a localized dark-spot filter into the objective lenses so that only the weld arc would be darkened. This allowed the camera to see a darkened view of the weld arc while seeing the rest of the surroundings with a more normalized view. This technology became known as spot filtering, and results were better than anything else available. Spot filters are still commonly used today and work well for many applications. The primary challenge with a spot filter is that it has to be precisely mechanically aligned with the weld arc. Any misalignment of the camera to the weld arc results in part of the image being saturated or getting washed out by the bright light.
More recently, developments in high dynamic range (HDR) imaging has started to be used in welding-view cameras. The first approach was to use a camera to take multiple images in series, each with different exposure levels, e.g., one image that is very dark to get a good view of the arc and successively brighter images that provide a good view of the workpiece background. Software is then used to combine these images using the appropriate sections from each series of pictures to produce one HDR image. The challenge with this approach is the slow output frame rate, resulting in a choppy and time delayed video output.
Logarithmic HDR
The most recent development in welding-view camera technology is to use a logarithmic sensor to produce an HDR image at the sensor source in hardware. A logarithmic sensor works similarly to that of a solar cell. When a small amount of light hits the cell, it starts to produce a voltage. As the light level increases a little, the voltage increases similar to that of regular cameras. The difference comes in when there is a lot more light such as with the weld arc. With bright light, the rate at which the voltage increases is reduced (FIGURE 1). The benefit in this approach is that it takes much more light before the sensor reaches saturation while still being able to see details in low-light areas. This also happens to be closer to how the human eye perceives light. Because the sensor is naturally producing this logarithmic output at a normal frame rate, the output can immediately be sent to the display, eliminating the choppiness and delay in the video found in competing technologies.
To compare the effects of a standard and a logarithmic output, see FIGURE 2. In FIG. 2a, one can see the flame, some of the objects in the flame, and the background area around the flame. The details in the flame are limited because the brightness has reached the maximum capability of the sensor. In FIG. 2b, where a logarithmic approach was used, the same details as seen in FIG 2a are visible. However, there are additional details in the flame visible, making this a much more usable image.
When applying the logarithmic sensor in welding applications, a good viewing image is produced where the background workpiece and the molten weld puddle are clearly visible (FIGURE 3). This example is a 4-kW fiber laser welding mild steel in a bead-on-plate configuration. The camera is mounted externally from the laser beam delivery, making it slightly off axis. The texture on the workpiece, the molten puddle, and the solidification chevron are clearly seen without the use of any additional external lighting. Sample welding videos can be seen on YouTube: www.youtube.com/XirisAutomationInc.
Welding-view camera technology has steadily progressed in recent years as the market is becoming more aware of camera technology. Various workplace health and safety improvement standards have also increased the need for implementing camera systems. Many remote welding applications such as ID cladding or closed work cells have the need to visually see the live weld in progress and often need the ability to record the visual information for later review. Welding-view cameras will continue to evolve and the market will continue to grow as automation technology grows. ✺
Cornelius Sawatzky ([email protected]) is the sales manager for Xiris Automation Inc.
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