13.1 Doublet Tube Lens

A stock 200-mm doublet may serve effectively as a tube lens as shown in Fig. 13.1. The performance on axis is diffraction limited. At a 6.0-mm image field, there are several aberrations. The ray intercepts of the x plane indicate defocus in the form of Petzval curvature. The ray intercepts of the y plane indicate astigmatism and coma. The bulk of the aberration is field curvature. Consequently, the production doublet is effective within a small field with a CCD sensor.

A custom 200-mm doublet may provide a tube lens with flat field (Fig. 13.2). The performance on axis is diffraction limited. At a 6.0-mm image field, there is no field curvature. However, the ray intercepts of the y plane indicate a small coma. The encircled-energy plot indicates nearly diffraction-limited performance. A telecentric lens stop is essential to this design. The custom doublet is an effective design for a CCD sensor with a 12-mm diagonal.

13.2 Doublet-Pair Tube Lens

A typical infinity-corrected microscope employs a doublet pair as shown in Fig. 13.3. The first doublet is plano convex. It provides three features: optical power, correction of spherical aberration, and correction of axial color. The second doublet is a meniscus lens with little optical power. It provides correction of coma and lateral color. The "bending" of the second lens from flat to curved corrects the coma of the first lens.

The doublet pair based on the Abbe number provides excellent correction of geometric aberration (Fig. 13.4). The performance on axis is diffraction limited at the d line. There is only axial color. At a 6.0-mm image field, there is no field curvature. However, the ray intercepts of the y plane indicate a significant lateral color for the g line (426 nm). The encircled-energy plot indicates nearly diffraction-limited performance at the d line at a 6.0-mm field height. A telecentric lens stop is essential to this design. The doublet pair is an effective solution for a CCD sensor with a 12-mm diagonal.