Free-form measurement method based on differential confocal sensing and CCD image

We propose a free-form measurement method based on differential confocal sensing and charge-coupled device (CCD) image technology for free-form surface measurement with high precision and large range to achieve both sub-millimeter measurement tracking range and sub-nanometer high resolution. The proposed method improves the axial resolution in the micron-meter range by using differential confocal technology and extends the linear measurement range of the sensing feature by using the CCD image method, thereby achieving sub-millimeter linear range and sub-nanometer axial resolution measurements of the free-form surface. Experimental results indicate that when λ = 632.8 nm, objective NA = 0.8 and focusing lens before CCD is f = 50 mm, it has an axial linear tracking range of 310 μm with an axial resolution of 200 nm, and an axial resolution of 0.3 nm within an axial measurement range of 1 μm. Additionally, the laser sensing system based on differential confocal technology and CCD images can detect a tilted surface with an angle of up to 20°, and the root mean square(RMS) measurement repeatability of the free-form surface is better than 9 nm, occupying the large measurement range and ensuring high precision of free-form surfaces with high axial resolution. The method provides a new technical approach for the high-precision measurement of large-range free-form surfaces with widespread use in the manufacturing field.