Varifocal compensated lens for irradiation of flow cells.

The optics of flow cells that are used, for example, in uv absorption detectors for liquid chromatography presents special problems to the designer. The cell must be irradiated in such a way that its volume can be reduced to a small value, typically 10 μliter or less, while maintaining sufficient optical throughput for effective detection of small changes in signal level. Aperture and field stops external to the cell are required to minimize changes in transmission with refractive index of the sample and to prevent rays from being scattered from the walls. The cell and associated optics must be usable over a wide spectral range without degradation of performance by chromatic aberration. Fortunately, observations are usually confined to one narrow spectral region at a time. We have used a varifocal lens to compensate for changes in refractive index of lens elements with wavelengths from 190 nm to 600 nm. The lens images the exit slit of the monochromator (field stop) on the exit window of the cell. This minimizes the volume of the sample required, especially if the cell has a conical shape to conform to the shape of the radiation beam. The paraxial design of the lens is simplified if it is made of two identical plano-convex lenses elements (Fig. 1). The focal length of each element is fo = R/(n — 1). One principal plane is at the vertex of the lens, and the other is separated from it by h0 = t(n — l)/n, where R is the radius of curvature of the convex surface, n is the refractive index which is a function of wavelength, and t is the thickness of the element. If the elements are separated by a distance d, the focal length of the Fig. 1. Nomenclature of a varifocal lens. S is the exit slit of the monochromator. S′ is its image with air and S" with solution in the cell. G′ is the image of the monochromator pupil.