RU2210108C2 - Image read-out scanning device - Google Patents

Abstract

FIELD: data readers. SUBSTANCE: scanner that can be used, for instance, to read information residing on relocatable paper or plastic readers has optical guide made in the form of optically transparent planeparallel plate one of whose ends (that facing document being readout) is perpendicular to side edges and opposite end on top of light source is beveled through certain angle to side edges of plate. EFFECT: simplified design, reduced power requirement, enhanced reading speed. 8 cl, 3 dwg

Description

Technical field
The invention relates to scanning devices for reading information, for example, images from paper media, and, in particular, relates to scanning devices for reading information from financial documents, such as banknotes of various types.

State of the art
Various scanning devices for reading information from paper image carriers are known. The most well-known devices are hand-held scanners for reading bar codes, for example, USSR copyright certificate 1837334. This device contains a light source mounted in series along the optical axis, collecting a lens, a slit aperture, and a photodetector combined with a diaphragm. A spherical lens is mounted between the scanned information carrier and the slit diaphragm. Such a device can be made in the form of a rod, such as a fountain pen, and has a fairly simple design. However, the functionality of such a device is limited only by reading bar codes, its accuracy, resolution and speed are insufficient for other purposes. Other handheld devices are also known that are more complex in design and have wider functionality (for example, US patents 5349172 and 5354977, publication of international applications WO 94/19766 and WO 94/19764). In these devices, LED matrices are used to illuminate the information carrier, and CCD matrices are used to perceive the reflected signals. However, such devices do not have sufficient capabilities when it is necessary to read a large amount of information.

 Stationary scanning devices for reading images and printed information are also known. Such devices comprise an optical unit including a light source, a light guide, a system of focusing and / or filtering optical elements, a photodetector and an analog-to-digital signal converter from a photodetector (for example, Russian Federation patent 2032217, US patent 5295196).

 The disadvantages of the known devices are their structural complexity and the excessive energy consumption of the illuminator. The structural complexity is mainly due to the complexity of the optical system, which includes lenses, mirrors, etc. as focusing elements, which are difficult to manufacture and require fine tuning for the correct operation of the device. Moreover, in order to obtain a reflected light flux of sufficient intensity, it is necessary to provide a large amount of light energy for lighting the information carrier.

SUMMARY OF THE INVENTION
An object of the present invention is to provide a scanning device for reading images from moving media, for example, from paper or plastic, and, in particular, from counterfeit documents (such as credit cards, identification cards and banknotes), transported by a transport system, for example, a machine for processing of banknotes, which would have a simple design and provide high efficiency and speed with low power consumption of the illuminator.

 According to the present invention, the scanning device for reading images contains an optical unit including a light source, a light guide and a photodetector, the light guide comprising a transparent element having substantially parallel faces and opposite ends located between parallel faces, one of these ends faces the light source and is located between parallel faces is not perpendicular to them, and the other end faces the scanned image, while the light from the light source enters the fiber through one end is guided on the image after exiting the other end, it is reflected back into the fiber and then directed to the optical waveguide photodetector.

 The use of a fiber of this form concentrates the light incident from the light source on the carrier, and thus makes it possible to use a relatively weak source, while at the same time providing a simple and compact means of separating the transmitted and received light (and, accordingly, the transmitting and receiving devices).

Short list of figures
An example of a scanning device in accordance with the present invention is described below with reference to the accompanying drawings, in which:
FIG. 1 is a diagram of a scanning device for reading images, partly in the form of a block diagram;
FIG. 2 is a ray path diagram illustrating the passage of a light stream through a light guide;
FIG. 3 is a graph of the dependence of the light output of the fiber on the thickness of the fiber (with a lamp radius of 8 mm and a fiber length of 80 mm) without taking into account the influence of the transmittance of the rays at the input and output ends.

Information confirming the possibility of carrying out the invention
The scanning device for reading the image from the medium 1 (Fig. 1) contains a light source, a light guide 2, a swivel mirror 3, a lens 4, and a CCD array or other photosensitive linear array 5, which in this example serves as an image receiver, CCD control unit 6 matrix and analog-to-digital Converter 7 signals of the CCD matrix 5. Typically, the image may be data or drawings, including all open and hidden elements of financial documents, such as banknotes, checks, travelers checks, etc.

The light source in the present example contains six tungsten incandescent lamps 8, preferably filled with halogen, of 15 watts each. Of course, any other type of light source can be used, as well as the number of lamps other than six. Light can be visible or invisible, for example infrared, or combined. The lamps are arranged in a row and placed in front of and almost close to the lower beveled end 10 of the fiber 2. In this example, the fiber 2 is a transparent plane-parallel plate made of glass, one of the end faces of which, in this example, the lower face 10, is beveled at an angle of 45 ^o in relation to parallel faces. The angle of the bevel can vary depending on a number of factors. These factors, described in more detail below, include a detection system, a lighting system, and a glass refractive index. As for the detection system, the prism angle is determined by the location of the mirror 3, the lens 4 and the receiver 5, and the refractive index of the glass, which also has some value. The bevel angle and the refractive index of the glass determine the characteristics of the lighting system (brightness, uniformity, etc.).

 The end face 11, opposite the beveled end face 10 of the fiber, faces the carrier 1 and is perpendicular to the parallel faces, although its angle of inclination may be different from the perpendicular. Figure 1 shows a variant of the device in which to achieve 5 compactness, a rotary mirror 3 is installed. Mirror 3 can be rotated to control the deflection of the beam on the lens 4 and the receiver. Between the lens 4 and the mirror 3 can be installed optical filter. The use of mirror 3 is also optional. The use of a light filter 9 is necessary if only waves of a certain length should be perceived, for example, resulting from fluorescence or infrared radiation.

The bevel angle is selected so as to optimize the light concentration and separation characteristics, but suitable angles are in the range of 27-49 ^° , preferably 41-49 ^° .

 The linear CCD matrix 5, the CCD matrix control unit 6, and the analog-to-digital converter 7 of the CCD matrix signals are electrically coupled and operate mainly to digitalize the outlines of the image obtained on the matrix 5.

In FIG. 2 shows the course of the rays in the fiber 2, where the following notation is given:
SO - the location of the light sources 8;
X is the thickness of the fiber 2;
L is the length of the fiber 2;
z is the angle of capture of the rays by the light guide 2;
g is the angle of capture of rays in the absence of a fiber 2;
and - the angle between the incident beam and the perpendicular to the surface of the beveled end 10;
b is the angle between the refracted beam and the perpendicular to the surface of the beveled end 10;
r; h is the distance of the SO source from the beveled end face 10 in the directions shown;
δ is the scattering angle of the rays incident on the beveled end of the fiber from points located in the upper end of the fiber.

 In this example, r = 4.61 mm and h = 1.525 mm.

 The device operates as follows.

 The light sources 8 emit light that partially enters the light guide 2 through its beveled end face 10 (FIG. 1), passes through it, and then through the end face 11 reaches the surface of the carrier 1, for example, a banknote (shown schematically in FIG. 2), which at this time it moves across the end face 11 with a certain gap from it, which in this example is 0.5-2 mm. In other embodiments, the banknote can be moved along the end face 11 or across it at an angle different from the perpendicular. This gap provides maximum illumination of the information carrier 1, taking into account the scattering of light exiting from the fiber 2, which depends on the actual thickness of the fiber. The presence of a gap guarantees the absence of damage and wear on the surface of the face 11 during operation of the device. The rays reflected from the surface of the carrier 1 again enter the fiber 2 through the end face 11, exit through its beveled face 10 almost perpendicular to the side surface of the plate (as can be seen in FIG. 1) and transmitted to the lens 4. (In FIG. 4, the optical components between the fiber 2 and lens 4 are not shown).

 The following is a mathematical analysis of the system.

In this example, the beveled face 11 of the light guide 2 is located in close proximity to the scanned object, and the beveled face 10 of the light guide 2 has a bevel angle of 45 ^° . In most cases, the bevel angle of the face can vary within
from (90-β ₀ ) / 2 + δ to 90-β ₀ -δ,
where β ₀ is the critical angle of total internal reflection.

For different types of glass, the angle β ₀ ranges from about 40 ^o (light crown) to 34 ^o (dense flint glass). In this example, light crown is used because it has the lowest absorption coefficient. In this case, the bevel angle can vary between (25 ^° + δ) - (50 ^° -δ). When using a lens 4 having a focal length of 8.5 mm and a number f of 1.5, located at a distance of 120 mm, the angle δ is approximately 8.5 / 1.5 / 120 / n radians, which equals 2 ^o , where n is the refractive index of the glass, equal to 1.55.

The optimal value of the bevel angle is in the range between 27 and 49 ^o . In the present example, the angle was chosen equal to 45 ^o , because it is close to optimal and easy to manufacture.

In order for the angle of incidence of the rays emerging from the beveled end 11 to be less than the critical angle of total internal reflection (for the type of light crown used, 40 ^° ), the angle of refraction b must be greater than 5 ^° . According to the law of refraction
sin (a) / sin (b) = n
and then the angle of incidence a on the beveled end 10 must exceed a _min = 8 ^o . Under the conditions described above, all the rays incident on the beveled end face 10 of the fiber will exit the end face 11 after almost 100% reflection from the side faces (walls) of the fiber, that is, there will be no loss of light through the side walls.

The minimum angle of incidence in this design takes place at the beveled end 10 of the fiber 2. With a radius of lamps (sources 8) equal to V = 4 mm, the point SO is at a distance equal to
h = r * tg (45-a _min ) = 2.7 mm.

With a fiber thickness of 3 mm, the capture angle z is
z = (45-a _min ) + arctg ((Xh) / (X + r)) ≈39 ^o .

With a fiber length L equal to 72 mm, the angle g is
g = 2 * arctan (X / 2) / (L + r) = 2.2 ^o .

Fiber efficiency equals
K = K _ri * K _ro * z / g,
where K _ri and K _ro - transmittance on the input and output surfaces of the ends 10, 11, respectively.

 It is noted that the dependence of K on the thickness of the fiber is constant, and the efficiency of the fiber increases with decreasing thickness of the fiber, that is, the thinner the fiber plate, the better. On the other hand, the minimum thickness of the fiber is determined by the angle δ, which is mainly chosen so that the lens aperture is maximum.

When δ = 2 ^o
X _min = δ * π / 180 * 1 = 2.5 mm.
The use of a fiber of such a thickness requires high accuracy in the arrangement of all components of the optical system, therefore, in this example, the fiber thickness was chosen X = 3 mm

In this case
K = K _ri * K _ro * 39 / 2.2 = 17.7 * K _ri * K _ro .

At the entrance to the fiber, the angle of incidence is equal to 45 ^o . At this angle of incidence, approximately 93% of the light energy enters the fiber. The angle of incidence at the exit is 30 ^o . With this angle, approximately 93% of the energy is transmitted. Then
K = 0.93 * 0.93 * 17.7 = 15.

 Figure 3 shows the dependence of the efficiency of the fiber depending on its thickness X.

 Using a flat surface of a fiber allows a relatively cheap way to solve several technological problems: focusing and directing light into the scanning zone and transmitting light to the photodetector. In addition, the use of a flat surface of the fiber allows a simple way to solve the problem of protecting the optical node from dust.

 Theoretical calculations show that fiber 2 increases the illumination of the scanned area by about 15 times compared with direct illumination. Practical measurements showed an approximately 8–9-fold increase in illumination. The difference between theoretical and practical data is explained by the final dimensions of the light sources and the inaccuracy of the adjustment.

 The rays reflected from the carrier 1, after exiting the fiber 2 are reflected from the mirror 3 and then pass through the filter 9 and the lens 4 to the CCD matrix 5. The CCD matrix 5 generates an electric signal proportional to the amount of light incident on it, and therefore proportional to the reflectivity of a fragment of the surface of the carrier 1.

 When using a CCD matrix with line scanning as a photosensitive element 5, a very high speed of reading information can be achieved. The analog signal from the CCD matrix 5 with progressive scanning is then converted by the converter 7 into a digital signal, which for subsequent processing can be input into a computing device, such as a computer, or into a reading display. The device that is the subject of the invention is compact, consumes a small amount of energy and has high speed when reading even grayscale images (drawings). Due to the simplicity of the design, after a slight modification, the device can be easily integrated into any paper processing machine, such as a sorting, issuing or banknote counting machine, in order to determine banknote characteristics (denomination, authenticity, etc.).

Claims (8)

 1. A scanning device for reading images containing an optical unit including a light source, a light guide and a photodetector, the light guide comprising a transparent element having substantially parallel faces and opposite ends located between parallel faces, one of these ends facing the light source and is located between the parallel faces non-perpendicular to them, the other end faces the scanned image, while the light from the light source enters the fiber through one end, sent to the image After exiting from the other end, reflection is reflected back into the optical fiber and then directed by the optical fiber to the photodetector. 2. The device according to p. 1, characterized in that one end is located at an angle, the value of which is in the range of 41-49 ^o , to parallel faces. 3. The device according to claim 1, characterized in that one end is located at an angle whose value is in the range from (90-β ₀ ) / 2 + δ to 90-β ₀ -δ, where β ₀ is the maximum angle of the total internal reflection, and δ is the scattering angle of the rays incident on one end from the other end. 4. The device according to p. 1, characterized in that one end is located at an angle, the value of which is in the range of 27-49 ^o , to parallel faces.  5. The device according to any one of the preceding paragraphs, characterized in that the fiber is made of optical glass.  6. The device according to any one of the preceding paragraphs, characterized in that the photodetector is a linear CCD.  7. The device according to any one of the preceding paragraphs, characterized in that it contains a lens for focusing the light coming from the fiber onto the photodetector.  8. The device according to any one of the preceding paragraphs, characterized in that it contains an analog-to-digital Converter for converting an analog signal from a photodetector to digital signals. Priority on points:
10/07/1997 PP 1.5-8;
10/07/1998 PP 2-4.

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