Dual-focus optical scanner and such a scanner used as wandtype symbol

Dual-focus optical scanner for scanning an object, provided with optical illuminating means for illumating the object to be scanned with a light point derived from a light source, which illuminating means have their own optics, and with optical imaging means for imaging the object onto a point sensor, which imaging means have their own optics. Said illuminating means and said imaging means are embodied in a coaxial relation with substantially coincident optical axes. The focal plane of the illuminating optics and the focal plane of the imaging optics in the object area are slightly spaced, the separation of said two spaced focal planes being such that the working range of the scanner is the sum of the depths of field of the illuminating and imaging optics, defined according to required spatial resolution. Said optical scanner can be used as a wandtype symbol code reader, such as a bar code reader, for reading the symbol code by relative manual scan movement. The reader includes at least one lens or lens-set (5), and in the optical path of the illuminating means a deflection member (4) is positioned between the one lens or lens-set (5) and the light source (1) for de-flecting the light, scattered from the symbol code, onto the sensor (9).

FIELD OF THE INVENTION 
The invention relates to an optical scanner for scanning an object, 
provided with optical illuminating means for illuminating the object to be 
scanned with a light point derived from a light source, which illuminating 
means have their own optics, and with optical imaging means for imaging 
the object onto a point sensor, which imaging means have their own optics. 
The invention also relates to such an optical scanner, embodied as a 
wandtype symbol code reader. Such scanners are known in practice. 
BACKGROUND OF THE INVENTION 
In such a scanner scanning may be obtained by moving the scanner instead of 
using an automatic scanning geometry. Under application of simple optical 
elements, such as an optical fiber or light collector, the light, in 
general from an incoherent light source illuminates the object and light 
scattering therefrom is collected through, for instance, an optical fiber 
by a sensor. Generally, scanners of this type can operate without imaging 
optics and during the scanning the scanner is contacting the object, e.g. 
a symbol code surface. 
Now in order to obtain non-contact reading some scanners use simple optical 
imaging means. However, due to the relative compact size and its use often 
of a low-cost incoherent light source, the associated depth of field is 
very limited. Moreover, in practical use, the scanner is moved opposite 
the object surface on a distance of several millimeters to several 
centimeters due to the fact for example that the object, e.g. a symbol 
code, is behind a glass or foil cover. The range then in which the object 
has to be investigated or read should be as large as possible. This means 
that the depth of field for the illuminating optics or the imaging optics 
should be as large as possible. Even in the case that a coherent light 
source is used, like a laser for which the depth of field of the optics 
can be less limited, there is always the requirement of having a working 
or reading range as large as possible. 
In general, scanners can be classified in two types of operation. In the 
one type, the so called active mode type, a thin beam pencil is used to 
scan the object. The light scattering therefrom is detected in order to 
derive information therefrom. In order to form this beam pencil a 
focussing optics has to be used. In the other mode type the so called 
passive mode is used. In this type, the object is imaged onto a sensor by 
means of an imaging optics and the information of the object surface is 
detected by this sensor in ambient light with or without additional 
illumination. 
Upon scanning the object like a symbol code with a light beam the size of 
the scanning point on the bar code should be small enough to distinguish 
the narrow symbol or bar. In a passive scanner, the resolution of the 
imaging optics has to be sufficient to distinguish the smallest object 
information or bar width, the size of the spot on the photo sensor should 
be smaller than or equal to the bar width. The higher is the resolution of 
the illuminating optics or imaging optics, the thinner is the bar code 
that can be read. The requirements for high spatial resolution and large 
depth of field cannot, however, be met simultaneously. The high spatial 
resolution of an optical system leads to small depth of field. Said depth 
of field is proportional to the relative optical lens aperture and 
inversely proportional to the size of the image spot. Thereby the depth of 
field is limited by the required spatial resolution. 
SUMMARY OF THE INVENTION 
The invention aims to obviate above problems and to extend the working 
distance or depth of field of an optical scanner in order to improve the 
practical use of such a scanner and to enable the user to handle the 
scanner freely in a non-contacting manner over the object. 
In a scanner as indicated in the introduction this is solved according to 
the invention such that said illuminating means and said imaging means are 
embodied in a coaxial relation with substantially coincident optical axes, 
and that the focal plane of the illuminating optics and the focal plane of 
the imaging optics in the object area are slightly spaced. In such a 
scanner both optics of the scanner are embodied so that the separation of 
the two spaced focal planes is such that the working range of the scanner 
is the sum of the depths of field of the illuminating and imaging optics, 
defined according to required spatial resolution. When this scanner is to 
be used as a wandtype symbol code reader, such as a bar code reader, for 
reading the symbol code by relative manual scan movement, in the optical 
path of the illuminating means a deflection member is positioned between 
the one lens or lens-set and the light source for deflecting the light, 
scattered from the symbol code, onto the sensor. 
In this advantageous embodiment according to the invention the two 
mentioned modes are combined thereby extending the total depth of field of 
the scanner. Within the total depth of field for this new system, if the 
object or symbol code is out of focus of the imaging optics, the 
illumination beam can be used as the scanning beam. In this "active" mode, 
the total scanner or the object, i.e. symbol code surface, can be moved, 
and thereby the illumination beam will scan the symbol code. The light 
scattering from the symbol code will be collected by the imaging optics 
onto the sensor. In the other mode, when the symbol code is out of focus 
of the illumination optics, it can be in focus for the imaging optics. The 
symbol code will still be illuminated by the illumination beam and is 
imaged by the imaging optics. The image focussed by the imaging optics 
will appear on the sensor acting as the image detector. When the sensor is 
small enough, during the movement of the scanner or object, i.e. the 
symbol surface, the "image" of the small sensor will scan the symbol code 
and then detect it. In this case, the scanner seems to work in an image 
scan mode or passive mode. The total depth of field then may be the sum of 
both depth of field of illuminating and of imaging optics because the 
implementation is such that the focal planes of the illuminating optics 
and the imaging optics in the bar code area are slightly spaced apart. In 
such an embodiment, in which the two optics extend substantially coaxial, 
aberration is expected to be small.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Although the explanation in the following is given with emphasis on the 
dual-focus scanner of the wandtype having an incoherent light source, it 
is clear that the invention covers any dual-focus scanner for 
investigating or scanning an object, having either a coherent light source 
or an incoherent light source. 
FIG. 1 shows the diagram of a simple dual-focus wandtype reader according 
to the invention. The light is emitted by a point light source consisting 
of, for instance, a light emitting diode (LED) 1, a lens or lens-set 2 and 
a spacial filter (pinhole) 3. The light from the light emitting diode 
(LED) 1, is focussed by lens 2 onto said pinhole 3. The illuminated 
pinhole can be regarded as a point source. The light from the pinhole 3 is 
directed to a beam splitter 4, e.g. halfmirror 4. The light beam is then 
directed to a lens or lens-set 5 by means of which the beam is focussed 
along its optical axis onto point A in the focal plane in the bar code 
reading space 6. The light scattering from the bar code 7 in this space is 
collected by the lens or lens-set 5 after which it is deflected by the 
beam splitter 4 onto a point-shaped light sensor, which may consist of 
another pinhole 8 and a sensor 9, e.g. a photo diode or photo transistor. 
Through this pinhole 8 the light is collected and detected by the sensor 
9. The position of the corresponding image of the spatial filter 8 is at 
point B in the focal plane in the bar code reading space 6. The distance 
between the points A and B depends on the depth of field for both 
illuminating and imaging optics as follows. 
In the following FIGS. 2a, 2b and 3 are explained. The light from pinhole 3 
is focussed to the point A by the lens or lens-set 5. The plane through 
this point and normal to the optical axis is called the focal plane a. The 
illuminated object at point B, can be sharply imaged by the same lens 5 
through the beam splitter 4 on the pinhole 8. The plane through the point 
B and normal to the optical axis is called the focal plane b. 
FIG. 2(a) shows the case that the bar code, 7, is located in the focal 
plane a. The image, 7', of the bar code will appear on a plane, differing 
from pinhole 8. When the bar code is moving in plane a, the light point 
will scan it, and the light scattering from bar code 7 will be detected 
through pinhole 8 by the detector 9. In this case, optical sensor 9 
detects the light modulated by bar code 7. The scanner in this case works 
in the active mode. 
The spatial resolution of this scanner depends on the size of the scanning 
point. As shown in FIG. 3, when the bar code is shifted from the plane a, 
the size of the scanning point will be increased. As long as the scanning 
point size is restricted within the required limitation (spacial 
resolution in the active mode), .delta..rho..sub.oA, the bar code is still 
detectable. The corresponding shift range, .delta.Z.sub.iA, is the depth 
of the field for this scanner in the active mode. 
FIG. 2(b) shows the case that the bar code, 7, is located in the focal 
plane b. The light from the pinhole 3 is focused to A by the lens or 
lens-set 5 and then, illuminates the bar code 7. The light point on the 
bar code can be large such that by active mode the scanner can not read 
the bar code. However, the sharp image, 7', of the bar code will appear on 
the plane containing pinhole 8. When the bar code is moving in this plane, 
this image 7' will sweep the pinhole 8 and through it, it is detected by 
the detector 9. Here, the detection set (pinhole 8 and optical sensor 9) 
detects the bar code image, 7'. The scanner in this case works in the 
passive mode. 
The spacial resolution of this scanner depends on the size of pinhole 8. 
When the bar code is shifted from the plane b, the image 7' will be 
blurred. As long as the scanning point size on bar code, corresponding to 
the blurred point on plane 8, is restricted within the required limitation 
(spacial resolution in the passive mode), .delta..rho..sub.oB, the bar 
code is still detectable. The corresponding shift range, .delta.Z.sub.iB, 
is the depth of the field in passive mode. 
As shown in FIG. 3, the separation of the two focal planes a and b for this 
scanner is arranged such that the total working range for this scanner 
equals the sum of the depths of active and passive modes. 
In this dual-focus scanning system the illuminating source is assumed to be 
a point source (through the pinhole). As indicated above, the light is 
focussed into a spot at point A, in the space where the bar code is 
situated, by means of lens 5 having an aperture .phi..sub.A. When point A 
is at a distance z.sub.iA from the exit plane or pupil of this lens, 
having a maximum allowable point diameter .delta..rho..sub.oA 
(=.delta..rho..sub.o), the depth of field is estimated by: 
EQU .delta.z.sub.iA =2(z.sub.iA /.phi..sub.A).delta..rho..sub.o. 
Within the range z.sub.iA .+-..delta.z.sub.iA the bar code can be scanned 
by this light source and one can obtain correct information therefrom. 
Outside of this range, as explained above, the scanning point is too large 
for correct reading. The light scattering from the bar code is collected 
by the imaging optics. Through this optics, the bar code is imaged onto 
the sensor which is assumed to be a point sensor through the pinhole 8. As 
indicated above, the corresponding image position of the point-shaped 
sensor is assumed in the bar code reading space at point B at a distance 
z.sub.iB from the exit pupil of the imaging optics. The corresponding 
depth of field in this imaging optics, with maximum allowable image point 
diameter .delta..rho..sub.oB (=.delta..rho..sub.oA), is approximated by 
EQU .delta.z.sub.iB =2(z.sub.iB /.phi..sub.B).delta..rho..sub.o. 
This means that when the bar code is within the range of z.sub.iB 
.+-..delta.z.sub.iB, said bar code can be imaged onto a point shaped 
sensor and can be detected by it correctly. 
By chosing or arranging the distance between points A and B to be 
(.delta.z.sub.iA +.delta.z.sub.iB) the depth of field is then extended to 
be the sum of the depth of fields of both optics. Within this extended 
depth of field, when the bar code is out of focus of the illuminating 
optics, it still can be read by the imaging optics. In this case, the 
illuminating optics provides a beam to illuminate the bar code. In the 
other case, when the bar code is out of focus of the imaging optics, it 
will be scanned by the light from the illuminating optics. The imaging 
optics then just acts as a collector for scattered light. 
With respect to the requirements for reading resolution of the bar code 
reader, the following is observed. The fact that the binary signal of bar 
code of certain spatial frequency can also be contained from the 
sinusoidal signal of the same frequency, leads to the conclusion that the 
spatial resolution of the optical system for bar code reading is required 
to be sufficient to detect only the basic spatial frequency corresponding 
to the narrowest bar code. 
Furthermore it is clear that for obtaining a high spatial resolution in an 
active scanner, the size of the illuminating source or the corresponding 
pinhole, should be as small as possible. Also for a passive scanner, the 
detector or its corresponding pinhole should be as small as possible. A 
further analysis leads to the conclusion that for a circular scanning 
point or spot, the bar code is still readable even in the case that the 
diameter of the scanning part equals the period of the bar code.