GB2473883A

GB2473883A - Read head illumination system, for scanning device

Info

Publication number: GB2473883A
Application number: GB0917075A
Authority: GB
Inventors: Gerald Paul Cook; David William Quinton
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2009-09-29
Filing date: 2009-09-29
Publication date: 2011-03-30
Also published as: WO2011041228A1; GB0917075D0

Abstract

An illumination system adapted for use with a read head for a scanning device, such as that used for machine readable documents including passports has an input channel, adapted to receive light from a light source, such as LED lights 41-44, and an output channel 40. Diffusion means 45 are positioned adjacent the output channel to direct light from the input channel toward the output channel and they have a diffusion surface facing outwards toward the output channel. The diffusion surface is shaped (i.e. having a curved convex roughened reflective surface) such that the intensity of the illumination profile of the region of illumination has a minima at the centre of the region of illumination. This minima in intensity compensates for angular losses created by the imaging system used to capture an image of the object within the scanning device.

Description

READ HEAD ILLUMINATION SYSTEM

The present invention relates to an illumination system adapted for use with a read head for a scanning device, in particular, a scanning device for scanning machine readable travel and security documents.

Machine readable travel documents were first introduced in the 1980's, with Australia, Canada and the USA all providing machine readable passports to their citizens. Each passport was printed in accordance with the standards laid out in ICAO document 9303, first published in 1980. The documents contained a two-or three-line strip of code, known as a machine readable zone, or MRZ, which contained detailed information about the holder and the document itself, such as name, date of birth, and passport expiry date. The code also included separator characters and check digits, to minimise the risk of forgery. The MRZ was configured to be readable using either a swipe reader (where the zone is drawn though a slot and consequently across an imaging device with access to optical character recognition (OCR) capacity) or a full page reader (where the full page of the document is scanned and the MRZ read using a scanning device read head with OCR capability).

More recently, so-called "e-passports" (machine readable documents in accordance with ISO 14443) have been issued on a worldwide basis. In addition to a MRZ, an e-passport contains a radio frequency identification device (RFID) chip that holds biometric data in relation to the passport holder. Typically, biometric data can include measurements of facial characteristics of the holder, as shown in the accompanying passport photograph. Both the MRZ and the chip are read to extract information about the holder, which is displayed on a terminal screen to a viewer, for example, a customs officer. Some of the data may be encrypted, thus providing greater security for the holder, and reducing the risk of fraud or forgery.

Again, e-passports may be read using either a swipe reader or a full page reader by the simple inclusion of an RFID reader into a portion of the device. In addition to the MRZ, a full page reader also captures an image of the entire passport page, including the photograph of the holder, by scanning the entire page in a single operation. At the same time as this scanning takes place, the RFID chip is interrogated to disclose the biometric information stored therein.

An example of a swipe e-passport reader currently available on the market is the RTE67O1 series, available from 3M Rochford Thompson (a trading name of 3M United Kingdom plc), The Votec Centre, Hambridge Lane, Newbury, Berkshire, RG14 5TN, United Kingdom. The reader is essentially e'ongate, with an opening for receiving the document to be read running a'ong its top surface. Within the opening is a scanning device for reading data within the MRZ, and any other document provided with machine readable text in accordance with ICAO Document 9303.

In use, the passport is swiped through the opening and illuminated using an infrared light source.

Once scanning is complete OCR software used to process the scanned image before displaying the gathered and processed information to the user on a terminal screen.

In order to scan the data, the scanning device is provided with a read head. Typically the read head is used to scan infrared responsive data. The read head is positioned within the opening and captures data by means of a line scan CCD (charge-coupled device) chip housed within the read head. A lens is used to focus the light reflected back from the surface of the document being read onto the CCD chip. The illumination system comprises an array of LEDs positioned above an input channel, which is reflected out of an output channel by a minor onto the surface of the document being imaged. This creates a substantially strip-shaped region of illumination, aligned with the viewing area of the line scan CCD chip.

For the imaged data to be processed accurately and quickly, the data collected must be of a high quality, as it undergoes conversion from an analogue signal to a digital signal before being analysed. Although small errors in the image data, due, for example, to variations in the thickness of the lens can be treated by calibrating the data using a smoothing function before analogue to digital conversion, some errors need to by minimised by physical calibration of the components of the read head, for example, polishing the lens, but in particular, calibration of individual components is carried out in the illumination system. However, both the lens and the CCD chip have losses that increase with increasing angle of view that may require correction of data and calibration of components in order to be dealt with.

One option is to ensure that the light incident on the document being scanned has an illumination profile across the entire illuminated area of the document that results in uniform exposure to all areas of the CCD chip. The illumination profile is a profile of the intensity of illumination as measured across the width of the illuminated region, and so may have points of maximum, minimum and substantially constant intensity. As discussed above, the document is illuminated by an array of LEDs. Each LED has an illumination profile, approximately Gaussian in shape, which is characteristic and inherent to that individual LED. When all of the LEDs of the array emit light simultaneously, each individual illumination profile is superposed to produce an overall illumination profile for the anay. This tends to be predominantly square in shape, but with an appreciable drop off in illumination towards the outer edges of the profile. Losses within the lens and CCD chip can compound this effect. This non-uniformity can lead to errors in processing the imaged data, which cannot be easily removed or minimised by data smoothing or other programming techniques. To avoid such problems, each LED in the array is carefully aligned and selectively polished by hand, to create flat surface regions that cause the shape and position of the illumination profile of the LED to change. The resulting superposed illumination profile for the array is substantially flat (having a substantially constant intensity), and preferably shows a small increase in intensity in the edge regions to take into account losses from the lens and CCD chip.

However, although this technique gives an improvement in the image, the manufacture of the read head is time consuming and expensive, and requires great technical skill in the person canying out the calibration process. The image data may still require additional processing when converted from an analogue to a digital signal. In addition, the read head is formed as a metal box, making it robust but heavy, thus increasing the overall weight of the scanning device.

This is a concern when using the scanning device as a mobile device, rather than a static desktop device, as weight is a key factor in the ergonomic design of the scanning device.

It would be desirable therefore to design an illumination system and read head that are capable of capturing accurate imaged data, without the need for polishing and aligning the LEDs in the illumination system by hand, and which is lightweight enough to be used in mobile as well as static desktop devices.

The present invention aims to address these problems by providing an illumination system adapted for use with a read head for a scanning device, the system comprising: an input channel, adapted to receive light from a light source; an output channel, adapted to provide a region of illumination to an object, an image of which is subsequently captured by an image capture device, the output channel being angled to the input channel to prevent direct light transmission from the light source; and diffusion means positioned adjacent the output channel to direct light from the input channe' toward the output channe'; wherein the diffusion means comprises a diffusion surface facing outwards toward the output channel, the diffusion surface being shaped such that the intensity of the illumination profile of the region of illumination has a minima at the centre of the region of illumination.

By using a diffusion means to ensure that the region of illumination has a minima at the centre of the region of illumination, any losses within the imaging system used to subsequently image the object are compensated for, and the intensity of the resulting image substantially uniform across the entire illumination profile. The number of errors in both the image capture and any subsequent image processing are therefore reduced, and the image captured is of high quality, with good resolution. Where the image displayed concerns identity information for a document holder, the ability of the illumination system to provide high quality data ensures that the identity can be confirmed and authenticated quickly and accurately.

Preferably, the diffusion surface is curved. The diffusion surface may be mirrored. alternatively and preferably, the illumination system comprises a reflection means, positioned adjacent the output channel, to reflect light emitted by the light source and reflected from the diffusion means out of the output channel. Preferably, the diffusion means produces a diffuse reflection component in the strip-shaped region of illumination.

The illumination system may comprise a three-walled square-channel shaped housing, having a central and two side walls, the input and output channels being at opposite ends of the and the diffusion means being positioned on the central wall. The reflection means is channel adapted to form a fourth wall of the housing, opposite the central wall.

The reflection means is preferably adapted to be inserted between the side walls of the housing.

In this situation, the side walls may be provided with slots adapted to receive corresponding location lugs provided on the reflection means.

Preferably the housing is formed from a plastics material.

Preferably the reflection means is a mirror. The minor may comprise a body of a plastics materia' having a reflective metallised coating.

The diffusion means is preferably formed from a plastics material. Preferably, the curved surface is a portion of the perimeter of a circle, a parabola or an ellipse.

The present invention also provides a read head for a scanning device comprising: a box-shaped plastic housing; and positioned within the housing: an illumination system, a light source to emit light into the input channel of the illumination system; a light capture device adapted to capture light reflected by an object being illuminated, and a lens to focus light reflected by an object onto the light capture device.

Preferably, the light source is an array of light emitting diodes. Preferably, the light captured by the image capture device creates an image of the object having a substantially uniform intensity.

The present invention also provides a scanner comprising the above read head.

The invention will now be described by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic perspective view of a read head including an illumination system in accordance with the present invention; Figure 2 is a diagrammatic top view of the inside of a read head including an illumination system in accordance with the present invention; Figure 3 is diagrammatic perspective view of an illumination system in accordance with the present invention; and Figure 4 is a diagrammatic perspective view of an illumination system in accordance with the present invention with the reflection means removed.

Rather than using individual alterations to the LEDs within the array forming the light source within an illumination system, or relying on smoothing algorithms in the image processing stage, the present invention includes a diffusion means to alter the illumination profile of the light emitted by the LED array before the light exits the read head. This altered profile is corrected to compensate not on'y for variations in the illumination emitted by a light source, but for angie-dependent losses inherent in any imaging system (lens and CCD chip) used to capture an image of the object illuminated by the illumination system. The resulting region of illumination exiting the read head has an illumination profile with a central minima of intensity, but any image captured and processed by an imaging system has a substantially uniform intensity, and can be digitised easily.

Figure 1 is a diagrammatic perspective view of a read head including an illumination system in accordance with the present invention. The read head 1 comprises a rectangular outer housing 2, wherein the two long walls 3, 4 and the top portion 5 of the outer housing are formed from a unitary part. The two short walls of the outer housing 2 are formed by portions of the illumination system 6 and the processor unit 7 respectively. The base portion of the outer housing 2 is removable, and is formed by the circuit board 8 that carries the LED array (not shown) forming the light source of the illumination system 6. The circuit board 8 is held onto the outer housing 2 by means of a series of six retaining lugs 9a -f, which engage with the outer surface of the circuit board 8. Each retaining lug is provided with a sloped upper surface lOa-f, provided so that the circuit board 8 can slide over this surface onto the upper edges of the long walls 3, 4 of the outer housing 2. The retaining lugs 9a -f are flexible, and flex outwards as the circuit board 8 slides over the sloped upper surfaces lOa -f of the retaining lugs 9a -f. When the circuit board 7 contacts the upper edges of the two long walls 3, 4, the retaining lugs 8a -f snap back to their original position, and hold the circuit board 7 in place by means of a retaining surface 1 la -f provided on each retaining lug lOa -f, which contact the upper surface of the circuit board 8, holding the circuit board 8 firmly in place. The circuit board 8 is additionally provided with a connector region 12 to enable an electrical connection to be made to the processor unit 7. The connector has a 12-pin portion for connection to the processor unit, and an 8-pin portion for connection to a CCD (charge coupled device) chip (not shown) that is used to capture images of the object being illuminated. The top portion 5 of the outer housing 2 is provided with a cut out portion (not shown), positioned in the region of the illumination system 6, to enable light to exit the read head 1 incident onto an object and to enter the read head, reflected from the surface of that object, for capture and processing by the processor unit 7. In use, the top portion 5 of the outer housing 2 of the read head 1 is positioned underneath a scanner window (not shown) and the object to be imaged placed on the scanner window, such that light emitted by the illumination system 6 is incident on the object, and reflected back though the scanner window, and imaged in the processor unit 7.

Figure 2 is a diagrammatic top view of the inside of a read head including an illumination system in accordance with the present invention. In this view, the circuit board 8 has been removed to expose the lens assembly 13, the processor unit 7 and CCD chip 14, and the illumination system 6. As discussed above, both the illumination system 6 and the processor unit 7 form the short walls of the rectangular outer housing 2. Located between the illumination system and the processor unit is a lens assembly 13, at a distance approximately two-thirds along the length of the outer housing 2. The exact distance is determined by the focal length of the lens 15, which in this example is 61.5mm, with the overall length and width of the outer housing 2 being 70mm and 32mm respectively, and 18mm in depth. The lens assembly 13 is slotted into the outer housing 2 by means of a pair of opposing grooves 16, 17 located in the long walls 3, 4 of the outer housing 2, which receive projections 18, 19 positioned on the outside of the lens assembly 13. The lens assembly is positioned to focus light reflected from the surface of an object being imaged onto the CCD chip 14. A suitable lens and CCD chip are those available from Schott UK Ltd., Sales Office, Drummond Road, Astonfields Industrial Estate, Stafford ST16 3EL, United Kingdom, sold as N-LAF34, and Texas Optoelectronics Inc. 702 Shepherd Dr. Garland, TX 75042 USA, sold as SL14O1R-LF 128 Pixel Linear sensor alTay, respectively.

The illumination system 7 is shown with the LED anay 20 forming the light source in place.

This will be described in more detail below. The illumination system 7 is formed of two separate parts: a main body 21 and the reflection means 22. The main body 21 forms the inlet and outlet channels (not shown) and the diffusion means (also not shown), and is in the form of a three-walled 23, 24, 25 square channel shaped housing, comprising a central and two side walls.

The central wall 23 forms the short wall of the outer housing 2. The two side walls 24, 25 lie parallel with and inside of the two long walls 3, 4 of the outer housing, and are provided with protrusions 26, 27 that engage with corresponding slots 28. 29 that are provided in the inner surfaces of the long walls 3, 4 of the outer housing 2, enabling the main body 21 of the illumination system 7 to be inserted into the outer housing 2. The side walls 24, 25 of the main body 21 also comprise a pair of grooves 30, 31, into which the reflection means 22 are inserted, forming the fourth and final wall of the main body 21 of the illumination system 7, opposite the centra' wall 23.

The reflection means 22 comprises an open ended rectangular body, having four sides 32, 33, 34, forming a square channel, and a closed base 36. The closed base 36 comprises two sloped walls 37, 38 forming an apex, with the joining point of the apex being located outside of the square channel. The outer surfaces of the sides 32, 33, 34, 35 and sloped walls 37, 38 of the reflection means 22 are provided with a metallic reflective coating, forming mirrored surfaces.

The purpose of the first of the two sloped walls 37 is to reflect light from the diffusion means (not shown) onto the surface of an object being imaged. The purpose of the second of the two sloped walls 38 is to further reflect light that has been reflected from the surface of an object being imaged onto the lens 15 of the lens assembly 13.

Figure 3 is diagrammatic perspective view of an illumination system in accordance with the present invention. This shows the illumination system 6 when removed from the outer housing 2 of the read head 1. The LED array 20 is shown as being in place above the input channel 39 formed by the three walls 23, 24, 25 of the main body 21 of the illumination system 7 to aid in understanding the light paths within the illumination system. An output channel 40 is bounded by the three walls 23, 24, 25 of the main body 21 of the illumination system 27 and the sloped wall 37 of the reflection means 22 that acts to reflect light out of the illumination system and onto an object being imaged. The LED array 20 comprises four LEDs 41, 42, 43, 44 emitting infra red light at a nominal wavelength of 875nm, to image information stored in a document or other object that is security printed in so-called invisible ink. Suitable LEDs are those sold under the name tite-on' HSDL4400 I.R, available from Lite-On Inc., 720 South Hillview Drive, Milpitas, CA 95035, USA. Light emitted by the LEDs enters the illumination system via the input channel 39, and is reflected onto the reflection means 22, where it is in turn reflected out of the output channel 40 and onto the surface of an object being imaged.

Figure 4 is a diagrammatic perspective view of an illumination system in accordance with the present invention with the reflection means removed. Again, the LED anay 20 is shown in position above the output channel 39. This view shows the diffusion means 45 and its position in relation to the output channel 40 clearly. The diffusion means 45 is formed on the central wall 23 of the main body 21 of the illumination means 6. The diffusion means 45 is shaped to compensate for variations in the illumination emitted by the LEDs 41, 42, 43, 44 in the LED array 20. In particular, the diffusion means 45 is curved, with the minimum of the curve being positioned in the centre of the central wall 23 of the main body 21 of the illumination means 6.

The curve extends across substantially the entire width of the central wall 23. In this example, the LEDs were spaced approximately 6.64mm apart and the diffusion means 45 placed approximately 10.5mm away from the LEDs. The resulting radius of curvature was approximately 26mm, and the output channel 40 approximately 24mm in width. The dimensions of the curve cause the reflected light to have an illumination profile matching that of the curve when viewed at a certain distance, which compensates for the drop off in the illumination profile of the LED anay 20, the high-angle losses in the lens assembly 13 and the CCD chip 14. As the curve of the diffusion means 45 has a central minima, so does the resulting illumination profile, such that the minimum intensity is in the central part of the illuminated region and the maximum at the edge regions. Firstly, this minimises the effect of any drop off in intensity at the edge of the illumination profile reaching the diffusion means caused by the superposition of Gaussian light profiles emitted by the LEDs of the LED array. Secondly, by providing an increased intensity at the edge regions of the illumination profile the effect of the high angle losses in the lens assembly 13 and CCD chip 14 are reduced, such that the resulting image intensity across the processed image is substantially uniform. A substantially uniformly illuminated image may be digitised easily, without needing to use complex processing to achieve good quality data.

Although in this example, the curve is an arc, and therefore a portion of the perimeter, of a circle, it may be a portion of the perimeter of an ellipse, a parabola or other curve calculated to produce a substantially constant illumination profile for a particular LED array 20, input channel 29 and output channel 40 arrangement. In addition to modifying the illumination profile of the light exiting the output channel 40, the diffusion means 45 creates a diffuse reflection component into the illuminated region, softening the edges of the illuminated.

Tn use, the light paths within the illumination means 6 can be described as follows. Light is emitted by the LED array 20 and enters the illumination means 6 via the input channel 29. Light travels through the illumination means 6 until it is incident on the diffusion means 45, whereupon it is reflected onto one of the mirrored faces 37 of the reflection means 22 and out of the output channe' 40 onto the surface of the object being imaged. Light reflected by the surface of the object being imaged is incident onto the other of the minored faces 38 of the reflection means 22 and reflected onto the lens 15 within the lens array 13. The lens 15 focuses the light onto the CCD chip 14, and an image is captured. This image is then used for further processing, by conversion from an analogue to a digital format, in the processor unit 7. The digitised image may then be processed using optical character recognition software within the read head, the scanner or elsewhere, as desired.

To make the read head lightweight and easy to manufacture, it is formed from a plastics material.

Suitable plastics materials include polycarbonate and ABS (acrylonitrile butadiene styrene) plastic. By using such materials, the individual parts of the read head 1 can be moulded easily, and assembled simply by hand. To form the diffusion means 45, the plastic is provided with a roughened surface effect to create the diffuse reflection component. The reflection means is coated with an aluminium film to create the mirrored surfaces, and can be spray or dip coated, depending on preference.

The LEDs discussed above are infrared emitting LEDs, as this is the most common prefelTed illuminant for such a read head in passport and other travel and identity document scanners.

Infrared LEDs may preferably have a wavelength in the range of 700 to l000nm. However, the LEDs may be replaced by those capable of emitting visible light (with a wavelength between approximately 500 and 700nm) or ultraviolet light (with a wavelength of approximately less than 480nm).

In the above example, the diffusion means is formed from a plastics material, and light is reflected from the diffusion means onto the reflection means and out of the output channel.

However, as an alternative construction, it may be desirable for the diffusion means to be formed from a curved reflective surface replacing the mirrored surface of the reflection means, and for the diffuse reflective component to be provided by a non-curve, flat and slightly roughened surface provided on the central wall 23 of the main body 21 of the illumination system 6. This would create the same optical effect, and may be desirable with certain LED or other light sources or read head designs or uses.

Both a swipe reader, of the general type described above with reference to the RTE67O1 product, and a read head for such a read may be provided with a read head illumination system in accordance with the present invention. The read head would comprise, in addition to the illumination system described above, a box-shaped plastic housing, a light source to emit light into the input channel of the illumination system, an image capture device adapted to capture light reflected by an object being illuminated, and a lens to focus light reflected by an object onto the image capture device. The light captured by the image capture device creates an image of the object having a substantially uniform intensity. Preferably, the light source is an array of light emitting diodes. Each of these components would be placed within the housing, and positioned within a scanner, as desired.

Claims

CLAIMS1. illumination system adapted for use with a read head for a scanning device, the system comprising: an input channeL adapted to receive light from a light source; an output channel, adapted to provide a region of illumination to an object, an image of which is subsequently captured by an image capture device, the output channel being angled to the input channel to prevent direct light transmission from the light source; and diffusion means positioned adjacent the output channel to direct light from the input channel toward the output channel; wherein the diffusion means comprises a diffusion surface facing outwards toward the output channel, the diffusion surface being shaped such that the intensity of the illumination profile of the region of illumination has a minima at the centre of the region of illumination.
2. illumination system according to claim 1, wherein the diffusion surface is curved.
3. illumination system according to claim 1 or 2, further comprising a reflection means, positioned adjacent the output channel, to reflect light emitted by the light source and reflected from the diffusion means out of the output channel.
4. illumination system according to claim 1, 2 or 3, wherein the diffusion means produces a diffuse reflection component in the strip-shaped region of illumination.
5. illumination system according to any of claims 1 to 4, comprising a three-walled square-channel shaped housing, having a central and two side walls, the input and output channels being at opposite ends of the channel and the diffusion means being positioned on the central wall.
6. illumination system according to claim S wherein the reflection means is adapted to form a fourth wall of the housing, opposite the central wall.
7. Illumination system according to claim 6, wherein the reflection means is adapted to be inserted between the side walls of the housing.
8. illumination system according to claim 7, wherein the side walls are provided with slots adapted ton receive corresponding location lugs provided on the reflection means.
9. Illumination system according to any of c'aims, 5 to 8, wherein the housing is formed from a plastics material.
10. illumination system according to any preceding claim, wherein the reflection means is a mirror.
11. illumination system according to claim 10, wherein the milTor comprises a body of a plastics material having a reflective metallised coating.
12. illumination system according to any preceding claim, wherein the diffusion means is formed from a plastics material.
13. illumination system according to any preceding claim, wherein the curved surface is a portion of the perimeter of a circle, a parabola or an ellipse.
14. Illumination system according to claim 1, or 2, wherein the diffusion surface is mirrored.
15. A read head for a scanning device comprising: a box-shaped plastic housing; and positioned within the housing: an illumination system according to any of claims 1 to 14; a light source to emit light into the input channel of the illumination system; an image capture device adapted to capture light reflected by an object being illuminated, and a lens to focus light reflected by an object onto the image capture device.
16. Read head according to claim 15, wherein the light source is an array of light emitting diodes.
17. A read head according to claim 15 or 16, wherein the light captured by the image capture device creates an image of the object having a substantially uniform intensity.
18. A scanner comprising a read head according to claims 15, 16 or 17.