Chromatic confocal technology is one of the most commonly used optical methods for three-dimensional surface morphology detection. It is a non-contact, non-destructive measurement, which is very precise, fast, and insensitive to the surrounding environment. Presently, the point and line chromatic confocal technologies require one- or two-dimensional space scanning during detection, resulting in low detection efficiency. Multi-point array or snapshot chromatic confocal measurement technologies have been investigated for improving detection speeds. The axial dispersion lens is one of the important components of the measurement system. The dispersion range of the lens and the image space numerical aperture determines the axial resolution and the maximum measurement angle of the measurement system, and the field-of-view and magnification determines the lateral detection area. A survey showed that the majority of contemporary wide-field chromatic confocal dispersion lenses adopt expensive diffractive elements and aspheric surfaces and produce a narrow field-of-view and dispersion ranges.The objective of this study is to design a wide-field long-axis dispersion lens to enlarge the measurement area and improve the accuracy of a snapshot chromatic confocal measurement system.

The surface white light passes through the dispersive lens and generates the surface axial dispersion. Light, with different wavelengths, is focused at different axial positions, and the same wavelength and different fields-of-view are focused on the same vertical plane. The dispersion lens is required to meet the telecentric conditions in the image and object space to receive the maximum amount of light, reflected from the measured object. The double telecentric dispersion lens with -1 magnification is divided into two parts, front and back group, which are symmetrical at the middle point. The back group is designed first, and the front is its mirror image. The axial dispersion distance is twice that of the back or front group. As suggested by the structural characteristics of the dispersion lens, the Cook three-piece is selected as the initial structure. The axial chromatic and residual aberration evaluation functions are established to obtain the initial parameters of this structure and design the symmetrical half-group lens. After flipping and combining the two assemblies, the whole dispersion lens is obtained. Then, the Cook structure is adjusted by trying back group with different numerical apertures and combining them with their matching front group to obtain a wide field-of-view axial dispersion lens with adjustable performance parameters.

In this paper, a wide field-of-view axial dispersion lens (Fig. 14) was designed. Its performance parameters can be adjusted by replacing the back group, and the effect of parameter adjustment is elucidated. The dispersion lens designed in this paper is characterized by two sets of performance parameters. The first set comprises a 9-mm image height, 4.06-mm axial color difference, 4 F number, and -1 magnification. The second set includes a 2.7-mm image height, 1.2-mm axial color difference, 1.2 F number, and -0.3 magnification (Table 2), and the image quality reaches the diffraction limit. Reasonable manufacturing and installation tolerances were considered, and the mechanical and optical parts were produced. Finally, the lens was adjusted using a center deviation eccentricity measurement instrument for auxiliary group. A ZYGO interferometer was used to test the image quality of the axial dispersion lens. The results show that the wavefront RMS value of the axial dispersion lens with the first set of parameters is 0.053λ-0.075λ, while the wavefront RMS value for the second set of parameters is 0.061λ-0.078λ (Table 3). The first set of performance parameters is deemed suitable for measuring objects with large detection areas, smooth surface structures, and large height differences, whereas the second set is for objects with small detection areas, complex surface structures, and small height differences.

In this paper, a design method is proposed for a wide field-of-view dispersion lens, which not only reduces the design difficulty but also doubles the axial dispersion of the back group of the lens. By replacing the back group, the lens performance parameters can be adjusted, so that the measurement system can be adopted for more applications. The objective of a chromatic confocal lens is to expand the dispersion range, increase the image space numerical aperture, and maintain the near-linear dispersion performance. However, the three parameters are related to the energy utilization of the measurement system and the volume and complexity of the lens. Increasing the image space numerical aperture will enhance the signal-to-noise ratio measurement and will also increase the aberration of the lens while affecting the linearity and dispersion range. A wide field-of-view dispersive lens was developed using the proposed design method, which can utilize two sets of performance parameters. To verify the performance of the designed dispersion lens, an experimental measurement device was constructed, which comprised of a light source, a pinhole array plate, and an imaging spectrometer beam splitting module. The experimental tests demonstrate the feasibility of the proposed design method.