HP 81520A Silicon Optical head
The HP 81520A is a high sensitivity visible range optical head that connects to the 81533A or 81533B which is a plug-in module that fits the 8153A and 8163 and 8166 Agilent/HP series mainframe. It also connects to the 8152A Optical Average Power meter. The 81520A optical head is made of a 5mm Si (Silcium) sensor element and covers the range of 450-1020 nm wavelengths in a range of +10 to -100dBm. Various connector interfaces are available for that head.
Click the PDF Specification for further information.