FAQ: “Do we need to conduct an LED light check? If so, how often and where are the instructions for how to perform that test? (I can’t locate them in the manual) If not, then how will we know if the lights need to be replaced?”
You don't need to conduct a check of the LEDs. Any changes in the overall power or spectral power distribution are negated by the standardization procedure.
This applies to both the ST-HT and D25 NC which are very similar sensors.
FAQ: “How long do the LED lights last?”
The LEDs are warranted for 5 years and based on our in-house testing, they are expected to last much longer than that. LEDs are a very robust light source and as long as they are flashing, they are generally fine.
The performance is being checked. After you standardize (black glass, then white tile), the third step is to read a green diagnostic tile with baseline target values assigned at factory. If your instrument is within tolerance on the green tile, the optical path of the sensor, including LEDs) is considered to be in good working order.
FAQ: “Where are the instructions for replacing the LED lights?”
Replacement of the LEDs is performed at our factory. If the LEDs are not flashing, it is not typically the lamps that is the issue but more often a board (power board, A/D converter) issue. If the instrument comes back to HunterLab for any service, we check the LED source as part of verifying the performance of the unit.