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"The primary method vending machines use to recognize the denomination of paper money is through a magnetic scan; paper currency is printed with magnetic ink, s
by minikites 13y ago
"The primary method vending machines use to recognize the denomination of paper money is through a magnetic scan; paper currency is printed with magnetic ink, similar to the ink on the MICR line of a check, that makes it easily identifiable to machines with magnetic scanners. In addition, each denomination is marked with different fluorescent properties. Many vending machines and other machines that read paper currency use an ultraviolet light to scan the bill, read the fluorescent response and issue the appropriate credit."
- aaronem 13y agoPresumably they were simpler-minded fifteen years ago; either that, or the art major's high-end printer was a lot higher-end than it seemed.
- minikites 13y agoOr a third option: the story is fiction.
- aaronem 13y agoWell, you'll look at it how you please, and I'll do likewise.
- thirdsight 13y agoActually the cheap ones were really naff and relied on correctly sized notes and a mask and backlight with a couple of phototransistors. A pretty picture cut to the right size would work. I think it was enough to prevent casual errors rather than strict validation.
- nappy-doo 13y agoThese are Japanese machines. The US and Swiss makers are optical. There's been consolidation in the industry, so there's really only the US and JAP makers left, and the US maker (MEI) is so far ahead of the rest of the world, there's no point.
- nappy-doo 13y agoIncorrect. The magnetic detectors haven't been manufactured or sold since the early 80s. The simplest machines do use 2 narrow optical detectors, but as their algorithms are considered good enough that you have to basically destroy a bill to use them, what's the point. See my other post about SVM/PCA in this thread.
- minikites 13y agoWhat is SVM and PCA? That sounds interesting.
- nappy-doo 13y agoSupport Vector Machines, and Principle Component Analysis. LMGTFY
- minikites 13y agoWow, that helps. "Principal components are linear combinations of original variables x1, x2, etc. So when you do SVM on PCA decomposition you work with these combinations instead of original variables." "What do you do to the data? My answer: nothing. SVMs are designed to handle high-dimensional data. I'm working on a research problem right now that involves supervised classification using SVMs. Along with finding sources on the Internet, I did my own experiments on the impact of dimensionality reduction prior to classification. Preprocessing the features using PCA/LDA did not significantly increase classification accuracy of the SVM." I can see how that relates to currency detection.
- bunderbunder 13y agoI'm an amateur in machine learning myself so I don't have a lot of knowledge of the details, but allow me to take a guess at what it's about. Support vector machines are a machine learning algorithm that works by taking data points in some (usually) high-dimensional space, and classifies them based on where they lie in relation to a boundary that (mostly) divides the positive examples from the negative ones. So one way a bill detection SVM might work is by using images of the bills are being transformed into points in that high-dimensional space by treating individual pixels as different dimensions, and deciding if they're valid banknotes (and the denomination) based on where in that space a given point falls. Since SVMs are designed to work well in high-dimensional data, you're correct that principal component analysis doesn't normally help them do better. Oftentimes it makes them perform worse. More likely, the reason they're using doing dimensionality reduction is to cut down on the size of the SVM's model. That could help in two ways: If you're using a really massive number of training examples, then dimensionality reduction can help cut down on the time it takes to train the SVM, or the space you need to store your training set. And if you're trying to fit the SVM into an embedded system, then dimensionality reduction would allow you to produce an SVM that runs well on lower-cost hardware.
- rdtsc 13y agoHmm what if the strip was damaged or say brushed against a magnet at some point. Would it reject those bills? Are they still valid bills even with a damaged magnetic signature?
- dsl 13y agoEver had a perfectly crisp dollar bill that a vending machine would just refuse to take? The majority of people will just take another bill out of their wallet and try again, so it is really a non-issue.