Optical design, then, is not just a matter of sitting down and calculating the layout and profile of light control elements, it includes the assessment of the design to meet the requirements when it is manufactured. This is a significant responsibility for the designer because, if the design is not satisfactory, tools may have to be altered, which can be expensive, and time will be lost, which may be even more expensive, since it may delay the launch of a product. Making mock-ups that will truly represent the performance of the finished product is then an important aspect of the optical design process. Making these has been helped in recent years by the use of three-dimensional CAD drafting. 1 The data file available from this can be used to make a mould from which a reflector can be produced by hydro-forming or vacuum forming a
suitable plastics material, which is subsequently aluminized. Linear prisms can be cut directly by purpose-made cutters, but mock-ups of prismatic refractor bowls with asymmetrical double curvature are difficult to make. If possible, the design should be tested before the moulding tools are hardened.