Ok, still waiting for the boss to leave so I'll write a little about this devices.
This type of validators are kinda old (later when I record the video I'll show the newer ones
), they work by comparing known characteristics of bills against the on board "bill table" (thats what's inside the flash prom), I guess that in this old model that's together with the firmware, in modern ones you can choose to update the bill table, the firmware or both.
To build these bill tables manufacturers takes lots of samples of bills and make an average reading of the sensors, thats stored in one slot of the table (let's say the slot for the $5 bill).
When you want to buy something and insert the bill, the validator compares the readings of the sensors and if they are in a certain threshold compared to a value of the table they recognize the value and authenticity of the bill, stack it and send the ACK data or pulses to the machine.
As of why the colors of the LED's, the wavelength, etc, each manufacturer have they own scheme of sensors, light, etc that they trust and advertise as a better way of detecting.
Money controls (nowadays Crane Payment Solutions) makes good validators, I would place it #2 on my top 3 of Bvals for Casinos.
Ah, a little story about the building of bill tables.
Some time ago my country put on the streets a re print of a common bill, but they quietly changed the printing company. I started to get complains from casino operators about the rejection rate of bills increasing, so we discovered the re print, it was quite easy as the bills have a micro logo of the press that makes them and it was a different one
We talked to one of the manufacturers of Bvals and they sent me a software and firmware that would read and dump the sensors readings (sadly it was all encrypted). After sending them this files (one for each of the 4 "sides" of the bill) they told us that we were right and that some characteristics of the paper were different, so they needed to issue a new bill table. The funny part; they asked us if we could send them 200 bills of this type, so we sent a guy to the bank, where he was kept on hold while he was asked questions about why he needed this bills, and they called the central bank of our country, the police and us! It was quite a mess!
Ok that was long! If you made it this far, sorry for my bad english! And please ask away!