Passing and making counterfeit money is a multimillion-dollar business. With cheap inkjet printers and computers, it's an easy business to enter. Virtually anybody can publish fake money. Retail partners aren't properly equipped to recognize bogus statements and most frequently the invoices aren't recognized as imitation till they get to the bank. You can also purchase counterfeit currency via the internet.
These pens aren't the perfect method to grab counterfeit bills. The pencils utilize iodine to react with the wheat found in newspapers made from wood pulp, but actual cash isn't published on paper produced from wood pulp. You swipe the pencil throughout the invoice and when it reacts with starch the mark will become black and then establish the invoices as an imitation.
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In case the mark stays yellow or clear the invoice is real. This may capture the amateur who prints cash in an inkjet printer using cheap newspaper, but it won't grab the professional counterfeiter who will get a top excellent cotton paper that doesn't utilize starch. A different way to receive an actual paper to publish invoices on would be to bleach ("wash") smaller invoices then use the resulting sterile paper to publish a bigger bill.
There are several better options. The watermark face could be observed by holding up the bill to bright light or sun. The face over the watermark should fit the face on the invoice. Another economical solution is a lava sensor.
Putting the invoice to the lava sensor will immediately identify bogus bills. An additional option is an automated invoice sensor. These operate approximately $300 and you merely set the bill to the machine in the point of purchase and it assesses the invoice and then feeds it back to you.