Secondary color correction and tolerance sensitivity: What can you get away with?

As many authors have documented, it is possible to correct secondary color without using special glasses, if there are substantial separations between lenses or groups that are chromatically uncorrected. The trick is to use the separations to “induce” secondary color by allowing the rays of different colors to separate from each other before being refracted by the group that follows. This approach works, but the use of separated and uncorrected groups that correct each other raises the question of tolerance sensitivity, because misalignments between the groups causes imperfect correction of the aberrations. It is generally good practice to correct aberrations within groups, rather than allow the groups to “crosscorrect” each other. On the other hand, the use of special glass types to control secondary color directly is often either discouraged for cost reasons, or simply not allowed because of thermal shock sensitivity. Moreover, some optical systems (particularly projector applications) require extremely good secondary color correction – often to a small fraction of a pixel. The important question is how much secondary color can be induced before the increased tolerance sensitivity negates the advantage of the color correction. In this paper, we examine the as-designed and as-built performance of several sample systems that rely on separated groups for the correction of secondary color, and compare the performance to that of systems designed without regard to secondary color correction.