Abstract: A turbidimeter for measuring a turbidity of a test liquid including a hollow main body having openings formed at its lower end through which the test liquid is introduced into a measuring optical path within the main body, a semiconductor laser diode arranged in the main body at its upper end and emitting a laser light, a first prism arranged within the main body and guiding the laser light into the measuring optical path, a second prism arranged within the main body and guiding light emanating from the measuring optical path to the upper end of the main body, first and second semiconductor photodiodes arranged within the main body at its upper end such that the light emanating from the second prism is exclusively made incident upon the first semiconductor photodiode, and first and second operational amplifiers arranged within the main body at its upper end and amplifying output signals supplied from the first and second semiconductor photodiodes, respectively.

Abstract: In a turbidimeter for measuring a turbidity of a test solution to be measured i.e. culture solution in a fermentation apparatus, a semiconductor laser diode and a semiconductor photodiode are integrally arranged in a detection portion of the turbidimeter in such a manner that a laser beam emitted from the semiconductor laser diode is made incident upon the semiconductor photodiode through the test solution. Moreover, a protection circuit for the semiconductor laser diode and the semiconductor photodiode is also arranged in the turbidimeter to cut off a current flowed therethrough when an environmental temperature becomes above a predetermined temperature. Therefore, the turbidimeter can be made small in size and light in weight, and the turbidity can be measured accurately over wide range.