At a cash centre, a machine digests a fresh hopper of notes and flashes a total before the teller has blinked. One note per 0.1 second, counted, checked and sorted - without a single guess. Here is what actually happens inside a banknote counter.
Notes drop from the top hopper onto rubber belts and suction-style separators that peel one sheet at a time. A double-fed pair is the classic failure mode of cheap machines, which is why separation rollers and a thickness check sit at the very start of the path.
Inside the channel, a photocell beam watches each sheet. Overlap and double notes bend the light differently from a single note, so the counter hears the difference before it adds to the total.

Real banknotes carry magnetic ink, IR-absorbing patterns and machine-readable security features. Magnetic heads confirm the ink signature, IR lamps read the pattern, and CIS image sensors profile the note so damaged, taped or printed-out fake sheets are flagged and dropped into the reject bin.
Under a dedicated ultraviolet lamp the security thread and fluorescent fibres light up - the same principle behind handheld and bench UV detectors used on the counter.

All signals fuse into one verdict per note: count, warn or reject. Value counters add denomination recognition so mixed deposits come out as a total, batch mode stops the machine at 100 notes per bundle, and the external indicator reflects exactly what the sensor line saw.
Simple quantity and value counting lives in the front-loading counter range; UV and light-box verification sits with the DC-5000; full series and condition sorting - the imaging step scaled up - is what the BS currency sorters do pocket by pocket.
Why does a counter reject clean-looking notes? Worn magnetic ink or a faded IR pattern reads as a signature mismatch; the note may be genuine but unfit for machines.
Can one machine count mixed currencies? Multi-currency models hold note libraries per currency; the teller picks the set or the machine auto-detects, depending on the series.
How often do sensors need cleaning? In daily cash-room use, dust from old notes coats the optical beam; a wipe-down weekly keeps reject rates low - see the specification table on each product page for duty details.