US 6991347 B2
A multi-wavelength light source for use in a banknote validator comprises a housing containing a plurality of LEDs and a window for releasing light emitted by the LEDs. Visible wavelengths have a direct path to a diffuser located in the window. Infra-red wavelengths have to be diffusively reflected by the interior of the housing before they reach the diffuser, and thereby suffer greater attenuation that the visible wavelengths. The diffuser acts as a substantially point-like light source.
1. An apparatus comprising a currency validator that comprises a multi-wavelength light source for illuminating a currency article, the source comprising a housing containing a plurality of LEDs and having a window for releasing light emitted by the LEDs, wherein the light paths from the LEDs to the window differ such that light from at least one LED is attenuated to a greater extent than light from at least one other LED of a different wavelength.
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5. An apparatus comprising a currency validator that comprises a light source for illuminating a currency article, the source comprising a plurality of LEDs and a diffuser receiving light emitted by the LEDs and forming a substantially point-like secondary light source, wherein the light paths from the LEDs to the diffuser differ such that light from at least one LED is attenuated to a greater extent than light from at least one other LED of a different wavelength.
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This invention relates to currency validators generally, and in particular to light sources which illuminate articles of currency, such as banknotes, so that their reflectivity and/or transmissivity in respective wavelengths can be measured at different regions on the articles in order to classify and authenticate the articles.
Such arrangements are well known in the art. It is common to have a light source in the form of a group of light-emitting diodes (LEDs) which emit light of different wavelengths, and to operate the LEDs in succession in order to test a banknote's optical characteristics in respective wavelengths.
One problem is that the intensity of the light output from the LEDs can be very different for different wavelengths. This is particularly a problem if the same sensor is used to detect light emitted by LEDs of different wavelengths, because it is difficult to obtain a sensor with response characteristics of a sufficiently wide dynamic range to cope with all the intensities without sacrificing resolution.
One way of dealing with this problem is to adjust the current supplied to the LEDs in order to make the emitted intensities more similar.
An aspect of the present invention is directed to a different technique which avoids or reduces the need to use current control for rendering the intensities more uniform, and which allows high intensity outputs from the LEDs.
It is known in the prior art to provide a substantially point-like light source to permit accurate focussing of the light on an article. One technique involves using a plurality of LEDs to illuminate an internal diffusing surface of a sphere, the light being emitted by a small window in the sphere. This has the advantage of combining the light produced by more than one emitter, but has the disadvantage that, because of the relatively small window size, the efficiency of the source is very low.
Another aspect of the invention is intended to provide a more efficient point-like light source.
Aspects of the present invention are set in the accompanying claims.
According to a further aspect of the invention, a multi-wavelength light source for use in a banknote acceptor comprises a housing containing a plurality of light emitters, such as LEDs, and having a window for releasing light emitted by the emitters, wherein the light paths from the emitters to the window are such that the light from at least one of the emitters is attenuated to a substantially greater degree than the light from another of the emitters of a different wavelength.
In a preferred embodiment, most or all the light from at least one LED (for example an infra-red LED) can reach the window only after being reflected within the housing, and therefore being attenuated as a result of the reflection. On the other hand, a substantial proportion, or all, the light from other LEDs, for example LEDs of visible wavelengths, can reach the window by a direct path and therefore is not attenuated to the same degree. This means that the current supplied to the infra-red LED does not need to be reduced to a large extent in order to reduce its intensity, which, it has been found, can lead to a reduction in the lifetime of the LED.
Preferably, the housing is, at least on an inner surface, diffusively reflective so that light can reach the window by reflections from this surface.
Preferably, a diffuser is provided to diffuse the light released from the window.
According to a still further aspect of the invention, a diffuser receives light from a plurality of emitters and constitutes a substantially point-like secondary light source. Preferably, the emitters emit light of different wavelengths. Preferably at least one emitter can directly illuminate the diffuser. This aspect provides a multi-emitter light source of good efficiency.
Arrangements embodying the invention will now be described by way of example with reference to the accompanying drawings, in which:
It is known that LEDs of different wavelengths emit light at different intensities. For example, infra-red LEDs emit at a higher intensity than LEDs in the visible range. The arrangement of
The lens 40 may alternatively be arranged to focus an image of the light source 2 on the banknote 44.
The housing contains two packages 6. One of them carries three LEDs, 81, 82, 83, which are located directly beneath the rectangular window 12 and connected to a contact pad 60. The LEDs 81, 82 and 83 emit light of visible wavelengths (for example red, green and blue, respectively). The other package 6 carries three LEDs 84, 85 and 96, of different infra-red wavelengths, connected to a contact pad 62 and positioned away from the region of the window 12.
A diffuser 90 is mounted within the window 12 to act as a secondary light source.
This embodiment operates similarly to the embodiment of
It will be appreciated from the foregoing that the term “light” is used herein to refer not merely to visible wavelengths but also to other wavelengths in the electromagnetic spectrum.