Once you have designed your lens, you'll want to know how it will perform once its built. Tolerancing allows you to build in the effects of manufacturing and asssembly imperfections to your lens design, and simulate how it will perform in production.

ZEMAX's tolerancer is one of the most powerful in the world, and supports a flexible tolerancing procedure. First, design your lens, and optimise it. Once you are satisfied with it, it is time to add in the effects of tolerances.

ZEMAX' default tolerance dialog box (above) allows you to set default tolerances for surface and elements. Once the defaults have been set, you can edit them to represent exactly your manufacturing capability. For example, one lens may be a catalogue lens whilst others are made specifically for you. In this case, you need to be able to specify different tolerances for different surfaces.

Many additional toleranced may be defined, including aspheric constants, lens group decentrations and tilts, solve and parameter tolerances, and many more.

Compensators may then be defined. These represent any adjustments that can be made to the design during manufacture or assembly. For example, a mirror may be held in an adjustable mount, and this adjustment may allow you to compensate for manufacturing error of the mirror and other components. Compensators may include distances, tilts and positions of surfaces and groups of elements.

You then need to tell ZEMAX what tolerance criteria to use. ZEMAX supports RMS spot size, wavefront error, MTF and any user-defined merit function.

ZEMAX then conducts a two part analysis. The first part is a sensitivity analysis where each tolerance is considered independently. The optimum value of each compensator is determined along with the change in the merit function. The second part is a Monte Carlo analysis where random systems are generated with user defined statistics and all tolerances are considered at once. ZEMAX then generates a report describing the results. The tolerances can then be modified if required and the process repeated.

The tolerancing report summarises:

	The expected change in performance due to individual tolerances
	The expected variation in performance when a large number of in-tolerance systems are built
	The expected range of compensator adjustment

and gives detailed breakdowns of this data. Tolerance data can be listed field-by-field, and Monte Carlo systems saved to disk for subsequent analysis to help you understand the production behaviour of your systems.