Not sure whether or not this should go here, but this seemed like the most accurate place since my question is about how an instrument works. I have an old unused Waters 440 Absorbance Detector for HPLC that I've scavenged for parts to try to digitize an old polarimeter. What I'm trying to figure out is exactly what the light sensor in the 440 is. Removing it from its housing, it's a small silver tube about the size of a small laser pointer. On its front it originally had what I presume was a filter to cut down what it could see to the UV spectrum, as it seems to be set to detect at 254nm or 280nm normally. Removing the little filter, I see the two little cells that act as the sensors behind another clear lens. The probe has two wires coming from it, ending in BNC connectors that originally plugged into 'Reference' and 'Sample' ports in the back of the 440. What I've done is get a basic post-to-BNC converter and wired the sensor up to a Vernier LabPro that's set to read the differential voltage from the sensor. At a cursory glance, the sensor looks to act like a photoresistor. The brighter light is, the lower the voltage is detected by the Vernier Box. But, well, I'm not sure if it is a photoresistor, as discussing it with my supervisor he noted that the LabPro shouldn't be sending a voltage across the sensor, meaning it couldn't be a photoresistor. So we're back to square one on how this sensor works exactly. The 440 is about 30 years old, so the sensor is also too old to have been a photodiode, I've been told, so that's out too apparently. So, yeah, I'm trying to figure out what kind of light sensor this thing is. I've been able to find painfully little documentation on the instrument (the HPLC has every book for every single piece of equipment it does or has used in the past except the 440) that helps any. Knowing what kind of sensor it is seems like it would be a great help in deciding whether or not it's actually useful. It seems to be sensing light fine at the Sodium-D line that's being used in the polarimeter this sensor has been attached to. Of course, I'm also wondering if there is any fundamental difference in how an automated polarimeter works as compared to how an analogue one such that I'm using does. In what it sees to take a reading, that is. In this polarimeter I'm using it's read by adjusting the moving polarizing filter until a vertical line and the rest of the circular view of the source light are the exact same intensity. Any help is appreciated.