PURPOSE:To release the working accuracy, to keep a fixed optical axis pitch to the temperature change and to facilitate an easy assembly, by setting the external size of the optical axis direction shorter than double as much as the optical axis pitch for a poloprism having the relation of mirror image for constitution of a series array connected to a poloprism array. CONSTITUTION:Plural poloprisms have outer diameters L of optical axis direction set shorter than double as much as the length alpha of optical axis pitch. A poloprism array 2 containing poloprisms F having the relation of mirror image with said poloprisms unified as elements. These poloprisms and the array 2 are connected each other with a 180 deg. turn so as to obtain the coincidence of direction between optical axes A and A'. Thus a series array is obtained. A gap G is formed between a side faces B12 and D2 of adjacent minor prism parts. This releases the working accuracy, keeps a fixed optical axis pitch regardless of expansion/contraction due to heat and facilitates an easy assembly.
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