Beam with extended confinement for scanning purposes

Scanner optics, usable for example in a bar code reader, generate a multitude of Gaussian beams derived from a common light source to produce a beam with extended working range. In the preferred embodiment, a 360 degree fan of Gaussian beams is generated by passing a laser beam through at least one diffractive element, e.g., computer generated holographic (CGH) plate, computed to perform the transformation of the illuminating beam into the desired output beam. The beam thus produced, having a small diameter over a large longitudinal distance, is useful for long range scanning.

TECHNICAL FIELD 
The invention relates to optical scanning devices such as barcode scanners, 
image scanners, plotting devices, laser machining devices and more 
particularly to an optical system generating a light beam with an extended 
depth of focus or working range. The principle and implementation 
described for light beams and optical systems are readily converted to 
other electromagnetic or acoustic beams and devices for radar and sonar 
implementation. 
BACKGROUND ART 
Optically encoded information, such as barcodes, have become common. A 
barcode symbol consists of a series of light and dark regions, typically 
in the form of rectangles. The widths of the dark regions, the bars, 
and/or the widths of the light spaces between the bars indicate the 
encoded information. A specified number, widths, and arrangement of these 
elements represent a character. Standardized encoding schemes specify the 
arrangements for each character, the acceptable widths and spacings of the 
elements, the number of characters a symbol may contain or whether symbol 
length is variable, etc. The known symbologies include, for example, 
UPC/EAN, Code 128, Codabar, and Interleaved 2 of 5. 
Readers and scanning systems use electro-optical means to decode each 
symbol, thus providing multiple alphanumerical characters that typically 
are descriptive of the article to which the symbol is attached or some 
characteristic thereof. Such characters are typically represented in 
digital form as an input to a data processing system for applications in 
point-of-sale processing, inventory control, and the like. Scanning 
systems of this general type have been disclosed, for example, in U.S. 
Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,409,470, and 
4,460,120, all of which have been assigned to Symbol Technologies, Inc. 
To decode a barcode symbol and extract a legitimate message using such 
optical scanners, a barcode reader scans the symbol to produce an analog 
electrical signal representative of the scanned symbol. A variety of 
scanning devices are known. The scanner could be a wand type reader 
including an emitter and a detector fixedly mounted in the wand, in which 
case the user manually moves the wand across the symbol. Alternatively, an 
optical scanner scans a light beam such as a laser beam across the symbol, 
and a detector senses the light reflected from the symbol. 
In either case, the detector senses reflected light from a spot scanned 
across the symbol and provides the analog scan signal representing the 
encoded information. 
A digitizer processes the analog signal to produce a square wave where the 
widths and spacing between the square boxes correspond to the width of the 
bars and the spacing between bars. The digitizer serves as an edge 
detector or wave shaper circuit, and the threshold value set by the 
digitizer determines what points of the analog signal represent bar edges. 
The threshold level effectively defines what portions of a signal the 
reader will recognize as a bar or a space. 
The signal from the digitizer is applied to a decoder. The decoder first 
determines the square shape widths and spacings of the signal from the 
digitizer. The decoder then analyzes the widths and spacing to find and 
decode a legitimate barcode message. This includes analysis to recognize 
legitimate characters and sequences, as defined by the appropriate code 
standard. This may also include an initial recognition of the particular 
standard the scanned symbol conforms to. This recognition of the standard 
is typically referred to as autodiscrimination. 
Different barcodes have different information densities and contain a 
different number of elements in a given area representing different 
amounts of encoded data. The denser the code, the smaller the elements and 
spacings. Printing of the denser symbols on an appropriate medium is 
exacting and thus is more expensive than printing low resolution symbols. 
The density of a barcode symbol can be expressed in terms of the minimum 
bar/space width called also "module size" or as a "spatial frequency 
bandwidth" of the code, which is the inverse of twice the minimum bar or 
space width. 
A barcode reader typically will have a specified resolution often expressed 
by the module size that is detectable by its effective scanning spot. For 
optical scanners, for example, the beam spot size is larger than 
approximately the minimum width between regions of different light 
reflectivities, i.e., the bars and the spaces of the symbol. The 
resolution of the reader is established by parameters of the emitter or 
the detector, by lenses or apertures associated with either the emitter or 
the detector by angle of beam inclination, by the threshold level of the 
digitizer, by programming in the decoder, or by a combination of two or 
more of these elements. In a laser beam scanner the effective sensing spot 
typically corresponds closely to the size of the beam at the point it 
impinges on the barcode. The photodetector will effectively average the 
light detected over the area of the sensing spot. 
The distance within which the barcode scanner is able to decode a barcode 
is called the effective working range of the scanner. Within this range, 
the spot size is such as to produce accurate readings of barcodes for a 
given barcode line density. The working range relates directly to the 
characteristics of the scanner components and to the module size of the 
barcode. 
Typically, an optical barcode scanner includes a light source, such as a 
gas laser or semi-conductor laser, that generates the light beam. The 
light beam is optically modified, typically by a lens, to form a beam spot 
of a certain size at a prescribed distance. The optical scanner further 
includes a scanning component and a photodetector. The scanning component 
sweeps the beam spot across the symbol. The photodetector has a field of 
view which extends across and slightly past the symbol and functions to 
detect light reflected from the symbol. The electrical signal from the 
photodetector is converted into a pulse width modulated digital signal, 
then into a binary representation of the data encoded in the symbol, and 
then to the alphanumeric characters so represented, as discussed above. 
Many known barcode scanning systems collimate or focus the laser beam using 
a lens system to create a beam spot of a given diameter at a prescribed 
distance. The intensity of the laser beam at this point, in a plane normal 
to the beam (i.e., parallel to the symbol), is ordinarily characterized by 
a "Gaussian" distribution with a high central peak. The working range is 
defined as the region within which the intensely bright beam spot can 
decode the information after being scanned across the barcode symbol. One 
desires a large longitudinal distance within which range the designed beam 
allows barcode patterns to be accurately scanned. But as the distance 
between the scanner and the symbol moves out of the working range of the 
scanner, which is typically only a few inches in length for most common 
barcode densities, the Gaussian distribution of the beam spot greatly 
widens as a result of beam diffraction, preventing accurate reading of a 
barcode. This widening effect is more pronounced for narrow "pencil" 
beams, necessary for scanning fine barcode patterns. The laws of physical 
optics predict that a uniform beam with a circular aperture of radius "a" 
will spread in free space within a cone with a half angle of 
0.61.lambda./a, where is wavelength of the beam. 
The far field region, where the beam spreads at this rate, starts at the 
distance 
EQU z=.pi.a.sup.2 /.lambda. 
For a beam of Gaussian profile, the field amplitude distribution in the 
plane of its waist (narrowest region) is exp(.sup.-r .sup.2 
/.omega..sup.2.sub.o), where .omega..sub.o is the waist radius. Such a 
beam spreads with a half angle of .lambda./.pi..omega..sub.o before and 
after a corresponding Rayleigh distance (or confocal beam parameter) of 
H=.pi..omega..sup.2.sub.o /.lambda.. 
Because the requirements of the scan beam to be of narrow diameter and to 
be maintained at uniform diameter for a long distance are contradictory, 
present scanning systems must be positioned within a relatively narrow 
range of distances from a symbol in order to properly read it. For 
example, a scanning beam with a wavelength .lambda. of 0.67 .mu.m, and an 
aperture of 15 mils or 0.38 mm (=2.omega.o) will provide a working range 
(=2H) of 340 mm. Finer beams of smaller diameter will have much shorter 
working ranges. 
It has been recently shown (J. Durnin, Exact Solutions for Nondiffracting 
Beams, JOSA A, 4, 651 (1987), also U.S. Pat. No. 4,852,973), that a beam 
with an amplitude profile given by the Bessel function of zero order 
J.sub.o (.alpha.r), r denoting the transverse distance propagates without 
expansion. It is obvious that such a beam is unrealizable in a practical 
optical system, due to its infinite lateral extent. The energy is spread 
out, so that same energy is contained in rings of equal width. Such a beam 
cannot be used for barcode scanning due to the infinitely wide spread of 
energy in its cross-section, as a result of which the detected signal has 
very low contrast. 
Nevertheless, it has been shown that a J.sub.o (.alpha.r) distribution can 
be generated by a circular fan of a multitude of plane waves propagating 
at an angle .theta. with respect to the z-axis, i.e. by integrating all 
plane waves propagating at an angle .theta. with respect to the z-axis. 
One device that comes "close" to that distribution is the "axicon," which 
indeed generates a circular fan of semi-plane waves, but those are not of 
infinite extent. 
It has been shown that axicons provide a limited region where the "quasi 
J.sub.o " distribution is obtained (see J.H. McLeod, "Axicon: A new type 
of optical element", JOSA A, 44, 592 (1954)), G. Indebetouw, 
"Nondiffracting optical fields . . . ", JOSA A, 6, 150 (1989)), A. Vasara 
et al., "Realization of general nondiffracting beams . . . ", JOSA A, 6, 
1748 (1989)). Scanning optics implementing the axicon are described in 
Katz et al., U.S. Pat. No. 5,080,456 and Marom et al. copending 
application Ser. No. 07/936,472, filed Aug. 28, 1992, both assigned to 
Symbol Technologies, Inc. 
An axicon has a region given by a/.theta.=(n-1).alpha. whereby a 
"quasi-Bessel distribution" is generated. In this relation, a=aperture 
radius, n=index of refraction of the axicon material, .alpha.=angle 
between surfaces of the axicon and .theta.=resulting phase front tilt. 
Although the extent of the beam is limited in its cross-section, the 
quasi-Bessel distribution achieved is similar to the ideal "non 
diffractive" beam, thus also suffering from same poor signal contrast, as 
mentioned above. 
DISCLOSURE OF THE INVENTION 
One object of the invention is to produce optic elements that generate a 
wave front energy distribution which provides slim and elongated beams, 
superior to those obtainable by other beam forming mechanisms such as 
lenses and axicons. 
Another object of the invention is to increase the working range of an 
optical scanner. 
A further object is to implement novel optics to extend the range of an 
optical beam for long range scanning. 
A still further object is to produce scanning optics to generate a narrow 
scanning beam having minimum spreading of the beam diameter in the 
longitudinal direction, and improved modulation contrast. 
The above and other objects and advantages of the invention are carried out 
in the invention, by a barcode scanner comprising means for generating the 
effect of a plurality of Gaussian light beams circularly distributed 
around a common axis at a given tilt angle and intersecting each other in 
a common spatial region to form a composite beam of light, means for 
directing the composite beam of light toward information to be scanned and 
causing the composite beam to move along a scan line, and a light detector 
positioned to receive light reflected from the scanned information. In the 
preferred embodiment, the light source is a laser, and at least one 
optical element is positioned in the path of the beam generated by the 
laser to produce a composite beam having predefined amplitude and phase 
distribution. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the invention. The object and 
advantages of the invention may be realized and attained by means of the 
instrumentalities and combinations particularly pointed out in the 
appended claims.

BEST MODE FOR CARRYING OUT THE INVENTION 
As used in this specification and in the appended claims, the term 
"indicia" broadly encompasses not only symbol patterns composed of 
alternating bars and spaces of various widths as commonly referred to as 
barcode symbols, but also any other one or two dimensional graphic 
patterns. In general, the term "indicia" may apply to any type of pattern 
or optically encoded information which may be read or identified by 
scanning a light beam and detecting reflected or scattered light as a 
representation of variations in light reflectivity at various points of 
the pattern or indicia. FIG. 4 shows a bar code 15 as one example of a 
"indicia" which the present invention can scan. 
The present invention is based on our discovery that a beam of narrow 
elongated structure, suitable for scanning purposes, can be synthesized by 
intersecting a multitude of Gaussian beams travelling at a common angle 
.theta. with respect to z, the propagation axis, and distributed uniformly 
on a continuous 360 degree "fan" shown symbolically in FIG. 1. We have 
determined that a quasi zero-order Bessel function J.sub.o distribution 
truncated by a Gaussian beam appears at the waist of the intersection of 
the beams, as will be demonstrated below. Such a synthesized beam profile, 
characterized by minimum spreading of diameter as the beam propagates, 
satisfies long range scanning requirements. 
This analysis will be based on parameters related to scanning a pattern of 
single-dimensional structure, most commonly a barcode, and where a single 
beam is scanned across the pattern and a detector picks up the reflected 
light. The analysis will assume that the reflected light is averaged 
(integrated) in one direction. 
A signal obtained from beam scanned along the x direction over barcode 
strips oriented along the y direction, exhibits a certain resolution 
contrast known as the Modulation Transfer Function or MTF. Evaluation of 
the MTF is readily obtained by integrating the intensity distribution of 
the beam in y direction and calculating the Normalized Fourier Transform 
in x direction: 
##EQU1## 
FTx standing for Fourier Transform in x direction. 
The following analysis will compare the scanning capability of simple 
Gaussian beams propagating on-axis to that of synthesized beams obtained 
by taking a Gaussian beam propagating at an angle .theta. with respect to 
that axis, replicating it around that axis, and summing up the effect of 
such beams as shown in FIG. 1. 
The transverse amplitude field distribution of a simple Gaussian beam 
having a waist .omega..sub.o has the form: 
##EQU2## 
where: r--the radial distance from the propagation axis z, 
.lambda.--the wavelength of the light beam, 
H--the Rayleigh distance 
##EQU3## 
.omega..sub.o --the radius of the waist, A.sub.z --a complex 
multiplication factor depending on the position of the examination plane, 
along the z-axis 
i--.sqroot.-1 
The intensity distribution "I" has the form 
##EQU4## 
The resulting MTF of a scanned pattern (e.g. bars of the barcode symbol) as 
defined in (1) is obtained by substituting r.sup.2 by x.sup.2 +y.sup.2 in 
expression (3) and substituting in (1) 
##EQU5## 
f--denotes the spatial frequency bandwidth of the pattern. 
The effective working range (or depth of focus) is known to equal twice the 
Rayleigh distance 
##EQU6## 
At the extremities of this range (z=.+-.H), the radius of the beam 
.omega.(z) becomes 
EQU .omega..sup.2 (H)=2.omega..sup.2.sub.o (7) 
Thus, the working range exhibiting a minimum contrast (M=MTF.sub.min) for 
the entire spatial frequency bandwidth 0 to f, is derived by substituting 
(7) and (6) into (5) for z=H, 
##EQU7## 
L denoting 1n 
##EQU8## 
The extent of the working range of the synthesized composite beam, which 
represents the subject of this invention is given in the following: 
The amplitude field distribution of the synthesized beam consisting of many 
Gaussian beams covering a continuous fan can be expressed in integral 
##EQU9## 
where .zeta.=resin .theta.+zcos.theta. 
.sigma..sup.2 =r+z.sup.2 sin.sup.2 .theta.-r.sup.2 sin.sup.2 
.theta.cos.sup.2 .phi.-zr sin2.theta.cos.phi. 
A.sub.z,r,z and H are as defined in Eq.(2), and 
.theta.--tilt angle in radians between the axis of any single Gaussian beam 
forming the fan and the common axis of propagation, z. 
The intensity distribution of the beam will now be 
##EQU10## 
For most scanning system implementations .theta.&lt;&lt;1 and r&lt;&lt;H are reasonable 
assumptions leading to the following approximate expressions: 
##EQU11## 
where J.sub.o is the Bessel function of zero order and first kind. 
The achievable working range depends on the minimum acceptable MTF or 
contrast (M), the spatial frequency bandwidth (F) and a proper selection 
of the parameters H and .theta.. 
It is useful to express: 
EQU .theta.=f.lambda.t (13) 
##EQU12## 
where the normalized parameters t and h were found empirically to conform 
to the 
EQU t=1.058-0.532L+0.171L.sup.2 -0.0204L.sup.3 +0.0005L.sup.4 (15) 
EQU h=1.25-5.05L+7.65L.sup.2 -3.17L.sup.3 +0.518L.sup.4 (16) 
with 
##EQU13## 
as mentioned earlier. 
The expected working range is: 
##EQU14## 
where d is a normalized distance found empirically to be: 
EQU d=12.4-28.8L+234.2L.sup.2 -7.81L.sup.3 +1.08L.sup.4 ;0.05&lt;M&lt;0.35(18) 
Thus the increase of the working range corresponding to the synthesized 
beam, compared to that of a simple Gaussian beam is: 
##EQU15## 
FIG. 2 depicts this ratio. 
For M&gt;0.35 (=35%) the synthesized beam becomes very similar to the simple 
Gaussian beam and the working range is thus no more enhanced. On the other 
hand, for M=0.05 the enhancement factor is above 6.5 (650%). 
The first step in the construction of a beam generator according to this 
invention is to define the desired focusing distance F, namely the 
separation between the output plane of the beam generator to the center of 
the working range. In order to take advantage of the full working range 
WR.sub.c,F must be greater than 
##EQU16## 
The amplitude field distribution at the output plane of the generator is 
defined by substituting in the set of equations (9) or the approximate 
equation 
(11) z by -F and the appropriate values for .theta. and H. 
Realization of this amplitude and phase field distribution is achieved, in 
accordance with the invention, by passing conventional light beam of known 
amplitude and phase distribution, e.g. a Gaussian beam, through a 
semi-transparent plate 23 bearing a fringe pattern, such as shown in FIG. 
3. This plate 23 is essentially a diffraction element known as computer 
generated hologram (or CGH) and is based on well-known technology such as 
described in Vasara et al., "Realization of General Non-Diffracting Beams 
with Computer Generated Holograms", J. Opt. Soc. Am. A, 6, No. 11, 1748 
(1989). The CGH is characterized by a pattern of generally parallel lines 
of varying thickness and spacing to control, respectively, the amplitude 
and phase distribution of the diffracted beam passing through it. The CGH 
23 is preferably disk-shaped, and with its concentric series of "bands" 
shown in FIG. 3, converts and incoming conventional beam into the desired 
beam with long working range. 
The fringe pattern shown in FIG. 3 consists of black lines on transparent 
background. It is possible to transform the pattern of black lines to a 
pattern of transparent lines which affect the phase, rather than the 
amplitude, of the illuminating light beam. 
It is advantageous to reduce the variations of the fringe density necessary 
on the CGH by optimally using it in conjunction with a collimated or 
converging beam illumination, readily obtainable with a lens. Thus, 
referring to FIG. 5, a laser source 20 generates a beam 14 that is passed 
through a lens 22 and CGH 23, reflected from scan mirror 17 and impinged 
on a pattern. The arrangement and operation of these elements will be 
described in more detail below with reference to FIG. 4. The synthesized 
beam, at the output of CGH 23, is characterized by a long working range of 
operation, particularly at medium MTF. FIG. 5 is a close view of the beam 
generator in FIG. 4. 
The diffracted beam by this conventional CGH implementation contains only a 
small fraction of the light energy of the original beam (before 
diffraction) and the direction of propagation is not collinear. Those 
deficiencies are readily overcome by cascading more lenses and diffracting 
elements in the path of the beam. For example, shown in FIG. 6, there are 
two diffracting elements in the path of the beam. The first one (23b) 
projects on the second (23a) the light intensity distribution as evaluated 
using the form (10) or the approximate expression (12). In this 
configuration both diffracting elements 23a, 23b may be of the 
"phase-only" kind, whereby the spatial frequency of the fringe pattern 
varies, while locally maintaining equal width for bars and spaces. 
Moreover special phase-only CGH embodiments known as blased gratings, 
binary optics, kinoforms or diamond turned aspheric lenses, all of them 
working at nearly 100% efficiency and on zero order of diffraction 
(collinear output beam), are readily implementable. 
The additional CGH (23b) generates a predetermined intensity pattern on a 
given plane. This element can be manufactured by any one of the 
embodiments mentioned above. FIG. 7 shows one possible way of mounting 
both diffracting surfaces on the same optically transparent substrate, rod 
or prism. 
One can design CGH elements that can be used in a reflection mode as shown 
in FIG. 8. (All CGH elements described earlier can be readily conceived to 
work in a reflection mode.) The CGH's in the shown configuration (FIG. 8) 
operate in the first order of diffraction. Therefore the angle of 
reflection differs from the angle of incidence. 
FIG. 9 shows still another example of a beam generator. The last element 
(29a) is a zero order reflection CGH. It is preceded by another CGH of the 
same kind (29b) that projects the wanted intensity distribution on the 
last element (29a) which modulates only the phase. Between the collimating 
lens and the CGH generating the desired intensity distribution, there are 
two additional reflecting blazed gratings (29c, 29d) working in their 
first order of diffraction mode. The task of these gratings is to correct 
the astigmatism and/or anamorphism of a laser diode beam collimated by the 
lens (22). For a circular laser beam, these elements are omitted. 
All configurations of the beam generator for extended working range beams 
can be implemented for any other electromagnetic wave, belonging to 
visible, ultra-violet or infrared wavelengths as well as in radio waves 
and acoustic waves. Moreover, for radio wave beam forming, use of phased 
array elements could generate directly the required amplitude and phase 
distribution, as needed at the beam generator output plane. 
FIG. 4 depicts a hand-held laser scanner device 10 for reading symbols, 
implementing the principles of the invention. The laser scanner device 10 
includes a housing that is generally of the type shown in the 
above-mentioned patents having a barrel portion 11 and a handle 12. 
Although the drawing depicts a hand-held pistol-shaped housing, the 
invention may also be implemented in other types of scanner housing, such 
as a desk-top workstation or stationary scanner. In the illustrated 
embodiment, the barrel portion 11 of the housing includes an exit port or 
window 13 through which the outgoing laser light beam 14 passes to impinge 
on and scan across the barcode symbol 15 located at some distance from the 
housing. 
The laser beam 14 moves across the symbol 15 to create a scan pattern. 
Typically, the scanning pattern is one dimensional or linear, as shown by 
line 16. This linear scanning movement of the laser beam 14 is generated 
by an oscillating mirror 17 driven by a stepping motor 18. If desired, 
means may be provided to scan the beam 14 through a two dimensional 
scanning pattern, to permit reading of two dimensional information 
patterns. Also, instead of the oscillating mirror, means may be provided 
to move the source of beam 14 and/or target to produce the desired beam 
scanning effect. The mirror 17 may be planar, as shown, or of another 
suitable configuration. 
A manually actuated trigger 19 or similar means permit the operator to 
initiate the scanning operation when the operator aims the device 10 at 
the symbol 15. Use of the trigger switch reduces the power drain since the 
components, such as the laser light source, the scan motor 18, and the 
photodetector and the decoders, can be activated during limited periods of 
actual scanning of a symbol rather than at all times. 
The scanner device 10 includes a laser source 20, e.g., a gas laser tube or 
a semiconductor laser diode, mounted within the housing. The laser source 
20 directs the laser beam 14 through the optical means 9 comprising the 
diffracting elements and optional lenses 22, to modify and direct the 
laser beam onto the oscillating mirror 17. The mirror 17, mounted on a 
vertical shaft and rotated by the motor drive 18 about a vertical axis, 
reflects the beam and directs it through the exit port 13 to the symbol 
15. A photodetector 21 is positioned within the housing to receive at 
least a portion of the light reflected from the barcode symbol 15. The 
photodetector 21 may face toward the window 13. Alternatively, a convex 
portion of the scan mirror 17 may focus reflected light on the 
photodetector 21, in which case the photodetector faces toward the scan 
mirror. As the beam 14 sweeps the symbol 15, the photodetector 21 detects 
the light reflected from the symbol 15 and creates an analog electrical 
signal proportional to the reflected light. A digitizer (not shown) 
typically converts the analog signal into a pulse width modulated digital 
signal, with the pulse widths and/or spacings corresponding to the 
physical widths of the bars and spaces of the scanned symbol 15. A decoder 
(not shown), typically comprising a programmed microprocessor with 
associated RAM and ROM, decodes the pulse width modulated digital signal 
according to the specific symbology to derive a binary representation of 
the data encoded in the symbol, and the alphanumeric characters 
represented by the symbol. 
To operate the scanner device 10, the operator depresses trigger 19 which 
activates the laser source 20 and stepper motor 18, etc. The laser source 
20 generates laser beam 14 which passes through the hologram plate and 
lens combination. The hologram plate modulates the beam to create a 
multiplicity of beams that together represent a synthesized beam which 
does not vary substantially over the working range 24. The hologram plate 
23 directs the synthesized beam onto the rotary mirror 17, which reflects 
the beam outwardly from the scanner housing 11 and toward the barcode 
symbol 15 in a sweeping pattern, i.e., along scan line 16. A barcode 
symbol 15, placed at any point within the distance 24 and approximately 
normal to the laser beam 14, reflects a portion of the laser light. The 
photodetector 21, mounted in the scanner housing 11, detects the reflected 
light and converts the received light into an analog electrical signal. 
The system circuitry then converts the analog signal to a pulse width 
modulated digital signal which the microprocessor based decoder decodes 
according to the characteristics of the barcode symbology rules. 
Still other objects and advantages of the present invention will become 
readily apparent to those skilled in this art from the preceding detailed 
description, wherein only one embodiment of the invention is shown and 
described, simply by way of illustration of one mode contemplated of 
carrying out the invention. As will be realized, the invention is capable 
of other and different embodiments, and its several details are capable of 
modifications in various obvious respects, all without departing from the 
invention. Accordingly, the drawing and description are to be regarded as 
illustrative in nature, and not as restrictive. For example, the concepts 
described in this disclosure are applicable to wave propagation outside 
the optical visible range. Thus, one can apply the principles of this 
invention to the infrared region to generate beams with extended working 
range for two dimensional barcode readers, image scanners, plotters, laser 
machining devices, radar (lightwave or radiowave) and sonar devices, as 
well as other devices and systems capable to operate at minimum MTF values 
lower than 35%.