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

The chromatic confocal measurement method implements a large range of longitudinal absolute measurements without using a mechanical scanning measurement system. The technique is realized by focusing a broad spectrum of white light with varying wavelengths on different planes or focal points. This measurement technique uses the properties of an optical longitudinal aberration rather than eliminating it. Unfortunately, designing confocal sensors to produce a large range of longitudinal chromatic aberration to realize the axial separation of light with different wavelengths is still a challenge. Choosing a reasonable initial structure of longitudinal chromatic aberration is the key to solving this chromatic confocal sensor design problem.

There are two typical methods of constructing the initial structure: the first is by zooming the focal length to generate an initial structure that can generate dispersion. The second is by solving the corresponding dispersion equation. However, at least one set of doublet lens should be adopted despite how the initial structure is determined.

A previous research gave suggestions on the distribution of the optical focus of the doublet lens and the selection of glass materials. However, it doesn’t provide detailed information about the influence of doublet lens on dispersion and the effect of focal distribution and material selection on dispersion. In a separate study, a group of air gap double glued lenses was adopted as an achromatic lens to produce collimated parallel light and spheric lens to collect light of varying wavelengths at different positions on the axis to form longitudinal chromatic aberration. The achromatic lens aims at the extremum of the wavelength range and doesn’t correct the entire wavelength range. Consequently, it makes dispersion linearity control difficult.

In another study, a lens structure with positive and negative lenses was deduced. The negative light near the light source was used to produce image space dispersion and the negative lens near the measured object to magnify the dispersion. Despite the method helping improve the dispersion range of the sensor, the negative lens group is restricted to producing a specific dispersion, and the effect of the dispersion with different characteristics on the sensor performance is often ignored.

In full realization of the possibility of using double lenses to eliminate chromatic aberration, and that double lens can be employed to increase the longitudinal chromatic aberration, researchers Zilong Zhang (PhD candidate) and Professor Rongsheng Lu from the Hefei University of Technology, China, delved into the effect of the double lens with varying optical power distribution on the beam. They proposed two forms of dispersion and compared their influence on the optical properties of the chromatic confocal sensor. Their research work is published in the journal Optics and Lasers in Engineering.

The research team presented two forms of initial structures of dispersion objective for chromatic confocal sensor based on doublet lens. Then, they grouped the doublet lens by the optical power distribution into three forms of surface structures. They also analyzed the dispersion features generated by each structure and proposed L-type, S-type, and two forms of dispersion. They deduced the effect of dispersion type on the dispersion range and numerical aperture of the chromatic confocal sensor. They also constructed two kinds of dispersion objective initial structures on the doublet lenses with varying dispersions.

The authors divided the doublet lenses into 36 categories going by the doublet lenses power distribution and dispersion features of glass materials. 31 structures generated S-type dispersion, and seven structures produced L-type dispersion. Four unique structures were reported, generating two forms of the dispersion according to the designed surface parameters. In addition, the authors recorded two structures with minimal dispersion and, therefore, unsuitable for chromatic confocal sensor design.

Mr. Zhang and Professor Lu constructed the expanded lens of the chromatic confocal sensor using a doublet lens with both the L-type and S-type dispersion, then combined the convergent lens of these parameters to construct the initial structure of two forms of dispersion objectives. The outcomes showed clearly that the initial structure employed the doublet lens that produced L-type dispersion was useful to design sensors with a large dispersion range. However, the initial structure using the doublet lens that produced the S-type dispersion was beneficial for developing sensors with higher numerical aperture.

The findings of this study will go a long way in helping designers choose a reasonable initial structure to circumvent the design difficulty going by the design parameters of the chromatic confocal sensor.