Line-scan cameras adjust to low and variable speeds
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 12/1/2008 2:00:00 AM
 HENNING TIARKS Product manager Basler Vision Technologies |
When building an inspection system for low-end PCBs (printed-circuit-boards), you can obtain high-resolution images for less cost by using line-scan cameras instead of area cameras. But it has been difficult for line-scan cameras to achieve high resolution and good image quality while the object being imaged is moving at very slow or variable speeds.
Basler Vision Technologies’ product manager Henning Tiarks described the challenges of slower and variable speeds and how these can be overcome by designing sensors and camera electronics together.
Q: Why do line-scan cameras have difficulty adapting to slower and variable speeds?
A: In inspection systems that use line-scan cameras, these cameras are positioned above PCBs that move continuously underneath, while they scan boards line by line to build an image. The cameras must be synched with the boards, which in the past demanded constant board speeds. Slower speeds are used mostly for low-cost systems that inspect PCBs targeted at low-cost markets.
High performance at low speeds is challenging for line-scan cameras because low speeds can mean long exposure times. Although longer exposure times can provide more detail, they also mean a greater chance of artifacts—created by light in combination with noise from the camera’s electronics—that degrade image quality. One way to solve this is to make the sensor quickly dump the electrical charge inside pixels in between each line.
Q: How does jointly designing sensor and camera electronics help this situation?
A: To make the sensor perform a fast dump requires a low electrical charge so it can be dumped in time for the next image. This requires designing the sensor and the camera electronics together, as Basler has done in its runner camera series.
We cover the sensor and measure the noise in the image, so there is no influence of light noise from the sensor, only the influence of electronics noise from the camera. The sensor’s frequency varies with variations in heat, as do the frequencies of the camera electronics. If the frequencies inside the camera are dominant, this can create artifacts that can be detected when doing a fast Fourier transformation.
A good design that works in variable heat conditions is therefore essential. This includes taking care of heat dissipation and monitoring temperature inside the camera, as well as choosing electrical components that work stably under different heat conditions. The result is a slowest speed in the runner series of 1 Hz, or 1 line/s, and a fastest speed of 56 kHz, or 56,000 lines/s. Previously, the minimum speed in Basler’s line-scan cameras was 1 kHz, or 1000 lines/s.
Q: What else can be done to improve image quality at slow speeds?
A: In low-noise situations, although a higher bit depth digitizes artifacts as well as image data, proportionately fewer bits are affected, so there is more usable information with fewer artifacts. Most line-scan cameras are 10-bit, but we put two more bits in the 12-bit runner series. This increases gray values from 1024 to 4096, giving greater accuracy and a more finely tuned image.
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