Active Sensor Alignment
It allows vision application designers to reliably identify their optical specifications without having to worry about excessively tweaking and deviating from the original spec when they scale their application. For example, imagine producing multiples of the same vision application that inspects the same product but having to adjust the camera and optical setup to different specifications every time, or worse yet having to return and wait for a replacement camera because the optical specs are outside the tolerance levels. LUCID applies Active Sensor Alignment during the camera’s manufacturing process to minimize unit deviations and ensure quality imaging for each and every Triton and Atlas camera unit.
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In a Perfect World...
In a perfect world, the sensor’s optical center, tilt, rotation, and back focal distance (BFD) would be the same for every camera unit. In this scenario the sensor placement would be specified and verified once and then applied to each future unit during manufacturing. In the real world however, there are slight differences in each of the components that make up a camera, including the angle, center and depth of the camera lens barrel as it relates to the position of the sensor on the PCB imager board.
Challenges of Sensor Placement
Perhaps there is a tiny difference in one of the components, such as varying solder thickness under the image sensor or a tilted image sensor die within its package. While this difference might not be noticeable to the naked eye, their impact on image quality will be. This would lead to a difference in the back focal distance across the sensor area, for example, will produce a blurry corner. The below animations are exaggerated examples of some the issues that vision application designers might encounter with poorly aligned camera sensors.
Passive Sensor Alignment
The traditional method of aligning the sensor with the optical path relies on strict tolerances of the camera components. Camera manufacturers source components from multiple vendors and must ensure they meet their required specifications. The camera is then assembled with the hope that everything lines up. This is called Passive Alignment.
For passive alignment, to catch deficient cameras before being shipped, an image quality test is traditionally applied after the camera is manufactured. While this strategy can be effective, it is very time consuming and costly. Cameras that fail this test are already assembled, and the manufacturer must decide if these cameras are to be sent back and disassembled, inspected, remade, or thrown out. Furthermore, they must also inspect and identify the batches of components that led to these deficiencies. Ultimately, it forces tighter tolerances on the camera components, increasing cost and potentially waste. It is because of this that some camera manufacturers skip the inspection test entirely or instead, widen camera tolerances and ship out the cameras to reduce costs at the expense of quality.
Active Sensor Alignment: Proper Placement at the Source
A much more accurate and efficient way of ensuring proper sensor placement is called Active Sensor Alignment. During the sensor placement process, the placement system measures image center, rotation, tilt and back focal distance, actively adjusting the sensor position based on feedback from visual measurements. The system employs an automated 6 degrees of freedom (6DoF) mechanical unit and a visual inspection unit. The visual inspection unit analyses a visual pattern overlayed across the sensor plane and measures the uniformity of the pattern’s sharpness. For example, if the system measures a corner that is slightly out of focus, it will adjust the sensor’s tilt until it is in focus. Once it calculates the maximum level of sharpness across the sensor plane, it will fix all the components in place.
Using this method allows for accurate sensor placement with a linear alignment and tilt resolution of micrometers. Smaller sensors benefit from precise centering. Having the center shifted even by a little bit can adversely affect the mounting placement of the camera. For larger sensors, minor changes in tilt can cause substantial differences in focal points across the image. For those vision applications that demand a high level of camera accuracy, Active Sensor Alignment is an important manufacturing step that ensures a accurate level of image sharpness than traditional passive alignement.