Some explanation of the way in which optical glass properties change with size will be helpful for finding adequate material specification tolerances of optical elements. Detrimental effects of stress birefringence, bubbles and inclusions, homogeneity, and striae become prominent only for larger glass items. With decreasing item size, especially its thickness, these effects decrease more quickly than linearly. Therefore, for small items some of these effects can simply be ignored (stress birefringence, homogeneity, and striae). This is because the residual wavefront deviations will be, by far, smaller than any requirements, and even smaller than sophisticated measurement methods can detect. Here it is necessary to specify only the use of optical glass in general. The remaining material imperfections, bubbles and inclusions, in small lenses will be treated in a different way than in larger items. They will not be judged by their size and number but just by their presence or absence. In day-to-day business, the value of a small lens will not justify any considerations about allowable bubble content. It will be simply discarded, regardless of the size of the bubble. Thus, this becomes a matter of lens batch yield, not of detailed single lens evaluation. The portion of items to be rejected must remain below a reasonable limit that is agreed upon between the glass supplier and the optical company.
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