When a collimator is designed, it is needed to choose not only kind and thickness of absorbing material, but also length of collimator for high transmission and small space.

• 准直器的设计不仅需要对吸收材料进行选择,吸收材料的厚度选择也很重要。准直器的透射率和空间的限制对准直器的长度提出要求。