| http://www.w3.org/ns/prov#value | - Preferably, each of said light emitting portion and said light receiving portion is a plate member in an elliptical shape, said first access opening having an elliptical profile with a major axis slightly shorter than a minor axis of said light emitting portion, said second access opening having an elliptical profile with a major axis slightly shorter than a minor axis of said light receiving port
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