Frequently, optical systems must perform over a wide temperature range. Due to the thermal expansion and the change of the index of refraction of the lens material with temperature, the performance of the system is affected. This is especially pronounced in the infrared region, where most materials suffer from a high dNâdT, the change of index with temperature. The values for the index of refraction and especially the thermal coefficients varyâin some cases widelyâin the literature and also in the data sheets from suppliers. It is therefore necessary to confirm or recheck these data.
To maintain an acceptable image quality, in many cases refocusing will be necessary. This can be accomplished mechanically or optically. Specifically, the mechanical adjustment can be done manually or by other means, such as feedback servo systems and others. Optically, the compensation can be achieved by selecting suitable optical materials and element powers. This latter method will be discussed with the assumption that the lens system is stabilized at a given temperature; in other words, there are no temperature gradients present.
To close out the chapter, some remarks are made relating to the cold stop and cold shield.
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