The H7421-40 has high sensitivity on wavelength from 300 nm to 720 nm. The H7421-50 is sensitive over a wide spectral range from 380 nm to 890 nm. The photomultiplier tube is maintained at a constant temperature by monitoring the output from a thermistor installed near the photomultiplier tube and regulating the current to the thermoelectric cooler. Heatsink with fan (A7423) sold separately