Initial structure of dispersion objective for chromatic confocal sensor based on doublet lens

Dispersion objective is the crucial device of chromatic confocal sensor (CCS). The dispersion range and numerical aperture (NA) determine the measurement range and the maximum measurement slope of CCS, respectively. In this paper, we present two kinds of initial structures of dispersion objective for CCS based on doublet lens, which are suitable for the design of dispersion objectives with large dispersion range and high NA, respectively. We categorized the doublet lens by the distribution of optical power into three kinds of surface structures, divided the distribution area of the glass material into four quadrants according to the refractive index and Abbe number, and then combined to obtain 36 kinds of doublet lens structures. We analyzed the characteristics of the dispersion produced by each structure, and proposed L-type, S-type, and two types of dispersion. The influence of dispersion type on the dispersion range and NA of CCS was deduced, and two kinds of dispersion objective initial structures were constructed based on the doublet lenses with different dispersions. Taking the single wavelength spherical aberration of starting wavelength (400nm) less than 1μm as the optimization goal, the 10 sets of initial structures were optimized and the dispersion range and NA variation direction before and after optimization were compared and evaluated. The design results shown that the initial structure based on the doublet lens that produces L-type dispersion is suitable to design sensors with larger dispersion range, while the initial structure using the doublet lens which produces S-type dispersion is more conducive to designing the sensors with higher NA.