How to Obtain a Suitable Calibration Plate (如何取得合適的校正板)

目的

一個合適的校正板，對校正流程很重要。對不同的機器視覺裝置特性應該有不同對應的校正板。

我們考慮以下問題，依照 Halcon 文件，提供建議的答案

1. 校正板大小與機器視覺裝置的對應關係
2. 校正板製作精度及材質
3. 我只是想先試試看校正的流程，可以自行製作測試用的校正板嗎？

校正板大小與機器視覺裝置的對應關係

根據 [2] 的描述，校正板至少要是影像可視範圍的 1/4 大小，原文如下

Use a calibration plate that is big enough to fill a large part of the image (at least one fourth of the image's total area).

校正板製作精度及材質

毫無疑問的，製作的校正板具有越高的精度 ± 150 nm (nanometers)，在作校正流程時當然會得到較準確的結果。

精度

Halcon 這樣說

Even more important, these calibration plates are highly accurate, up to ± 150 nm (nanometers), which
is a prerequisite for high accuracy applications. Therefore, we recommend to obtain such a calibration
plate from the local distributor from which you purchased HALCON. [1]

材質

Halcon 這樣說

Detailed information about the available materials, sizes, and the accuracy can be obtained from your distributor. [1]

我只是想先試試看校正的流程，可以自行製作測試用的校正板嗎？

如果你只是想先試試看校正的流程，但是你並沒有購買或製作高精度的校正板，那你該如何？ Halocn 說，你可以用雷射印表先列印出來試試看。

Halcon 這樣說

For test purposes, you can create a calibration plate yourself with the operator gen_caltab. Print the
resulting PostScript file and mount it on a planar and rigid surface, e.g., an aluminum plate or a solid
cardboard.[1]

Such self-made calibration plates should only be used for test purposes as you will not
achieve the high accuracy that can be obtained with an original HALCON calibration plate. Note that
the printing process is typically not accurate enough to create calibration plates smaller than 3 cm.[1]

In case of small distances between object and lens it may be sufficient to print the calibration object 
by a laser printer and to mount it on a cardboard. 
Otherwise -- especially by using a wide-angle lens -- it is possible to print the PostScript file 
on a large ink-jet printer and to mount it on a aluminum plate.[3]

也就是說，你可以印在紙板上或是印出來後把它固定在壓克力版上。那麼列印出來的校正板，它的精確度呢？ Halcon 文件說，若校正板小於 3cm 那麼通常它的精確度都不足。所以建議要 3cm 以上

重點整理

1. 校正板至少要是影像可視範圍的 1/4 大小
2. 校正板應具有高的精度 ± 150 nm (nanometers)
3. 測試用的校正版至少要 3 cm

References

[1] Halcon Document, Solution Guide III 3D Vision Section 3.2.3.1 How to Obtain a Suitable Calibration Plate

[2] Halcon Document, Solution Guide III 3D Vision Section 3.2.4.1 Rules for Acquiring Calibration Images

[3] Halcon Document, HALCON Reference Manual 10.0, camera_calibration (Operator), section How to generate an appropriate calibration plate?

系列文章

[1] Camera Calibration with Halcon (鏡頭校正使用 Halcon)

[2] How to Obtain a Suitable Calibration Plate (如何取得合適的校正板) -- 本文

[3] Rules for Acquiring Calibration Images (Halcon 校正流程中，取得校正板影像的規範)