Posted by Micah Simonsen, Last modified by Micah Simonsen
高倍率校准
针对高倍率测量系统的校准可能会带来一些挑战。本文将探讨在小视场条件下获得最佳校准结果的技术方法。
何为“高倍率”？
高倍率测试是指镜头放大倍数大约在1-4倍范围内的测试；例如，对于标准的2/3英寸传感器，对应的视场范围大约在8毫米至30毫米之间。
挑战与技术要点
在小视场条件下，景深会变得非常有限。可能难以甚至无法获得足够的景深来使校准网格充分倾斜。这将导致校准结果中"中心(X)"和"中心(Y)"的参数估计值非常不准确且不切实际。在极端情况下，这些错误的估计值可能会引发"同步错误"警告。
·使用小光圈（高F值）。这将最大化景深；但需注意，在非常高的F值下，镜头的分辨率会受到影响。光圈大于F/8或F/11可能导致图像显得模糊。可能需要在使用大景深的需求与因图像模糊而增加的测量噪声之间取得平衡。
·启用"高倍率"选项。如果无法获得足够的网格倾斜来精确估计中心值，可以在校准对话框中勾选"高倍率"选项。这将强制将中心值设定在传感器的几何中心。这并非最优方案，但可能在原本无法收敛的情况下使校准得以完成，并且比使用非常不准确的估计值要好。
选择合适的校准靶标。此外，为非常小的视场选择合适的校准靶标也可能存在困难。
·对于低至约10-15毫米的视场，使用打印的纸质网格可以取得良好效果，前提是纸张表面光滑覆膜，并且使用高质量的激光打印机。在这些放大倍数下，使用标准复印纸会导致明显的纸张纹理可见；同样，使用低端打印机也会导致靶标圆点形状不佳。这将导致图像无法提取特征点或产生较高的校准分数。
·Correlated Solutions 提供的玻璃校准网格具有非常精确的靶标，适用于小至5毫米的视场。这些网格必须使用漫射、均匀的白光场进行背光照明。确保透明圆点亮度均匀，并且网格的正确定朝向相机（详见网格文档）。如果圆点出现阴影或照明不均，您可能会看到校准分数持续略微偏高，通常在0.2-0.5范围内。
High Magnification Calibration
Calibrating for a high magnification setup can present a few challenges. This article will discuss techniques for getting the best calibration result at small fields of view.
What is 'high magnification'?
A high magnification test is one where the lens magnification is roughly in the range of 1-4x; for example, with a standard 2/3" sensor, this would be fields of view between about 8mm and about 30mm.
Challenges & techniques
At small fields of view, the depth of field can become very limited. It may be difficult or impossible to achieve enough depth to allow for good tilting of the calibration grid. This will result in calibrations which have very poorly estimated, unrealistic values for "Center (X)" and "Center (Y)". In extreme cases you may see a "Sync Error" warning caused by these poor estimates.
·Use small apertures (high F-numbers). This will maximize depth of field; note that, at very high F-numbers, the resolution of a lens will suffer. Apertures above F/8 or F/11 may result in blurry-appearing images. It may be necessary to balance the need for depth of field with the increased measurement noise due to this blur.
·If you cannot get enough tilt to give a good center estimate, you can check the "High mag" option in the calibration dialog. This forces the center values to be at the geometrical center of the sensor. This is not optimal, but may allow the calibration to converge when it otherwise wouldn't, and is better than having a very poor value.
Additionally, it may be difficult to select a calibration target which fits well in a very small field of view.
·For fields down to approximately 10-15mm, a printed paper grid can work well, provided the paper is coated smooth, and a very high quality laser printer is used. Using standard copy paper will result in very visible paper texture at these magnifications; likewise, using a lower end printer will result in poorly formed target circles. This will result in images which fail to extract, or high calibration scores.
·Correlated Solutions provides glass calibration grids which have very accurate targets for fields of view down to 5mm. These grids must be backlit with a diffuse, even white field. Be sure that the transparent dots are evenly shaded, and that the correct side of the grid faces the camera (see grid documentation for details.) If the dots show shadows or uneven lighting, you may see consistently slightly high calibration scores in the 0.2-0.5 range.
