Optical Scanner
This invention relates to optical scanners, particularly scanners for scanning a laser beam across an area.
One way of scanning is to use a raster scan where the light emitted by a light source, or the field of view of a light sensor, is scanned sequentially along a series of adjacent lines in order to scan an area. This requires that the scanner scans along the lines at a high frequency, known as the line scan frequency, anc? scans across the lines at a low frequency, known as the frame scan frequency. The ratio of the line scan frequency to the frame scan frequency is equal to the number of lines per frame and this is often in the order of one or more hundreds.
As a result of this large frequency ratio it is usual to use different mechanisms to scan in the two directions. Commonly a rotating polygonal mirror, that is a rotating polygonal solid with planar mirror faces, is used to produce the line scan while an oscillating mirror is used to produce the frame scan. Such systems are very well known and it is unnecessary to describe them in detail.
Two areas in which it is generally desirable to
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improve optical scanners are scan distortion and scan efficiency. Scan distortion is defined as the deviation of actual scan pattern when projected into the object space from a rectilinear raster and it is desirable to minimise this. Scan efficiency is the percentage of the total operating time which is spent scanning the required pattern and it is desirable to maximise this.
This invention was intended to produce an optical scanning system at least partially fulfilling these desires.
This invention provides an optical scanner comprising a rotating annular mirror having an axis and having a constant surface profile radial to the axis and a varying surface profile circumferential to the axis.
This provides a simple optical scanner which can have a high scan efficiency and low scan distortion. As an example to generate a + 4° scan with 90% efficiency and 10mm beam diameter a rotating polygon scanner would need a diameter of about 2.5m, whereas a scanner according to the invention would need a diameter of about 40mm.
The term optical in this specification covers devices operating in the visible, ultra-violet and infra-red regions of the electromagnetic spectrum.
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Scanning systems embodying the invention will now be described by way of example only with reference to the accompanying diagrammatic figures in which:
Figure l shows a first optical scanning system employing the invention;
Figure 2 shows a rotating mirror used in the system of Figure 1;
Figure 3 is an explanatory diagram explaining the operation of the mirror of Figure 2; and
Figure 4 is a second optical scanning system employing the invention, identical parts having the same reference numerals throughout.
Referring to Figure 1 a laser scanner 1 is shown. This scans a laser beam 2 from a laser source 3 across an area (not shown) .
The scan is a raster scan having 140 lines and a frame scan frequency of 2Hz, so a line scan frequency of 280Hz would be required if the scan efficiency was 100%. In practice the scan efficiency will always be less than 100% so the line scan frequency will have to be increased, for example with a scan efficiency of 80% the required line scan
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frequency rises to 350Hz.
The frame scan is generated by an oscillating planar mirror 4 which oscillates at 2Hz about an offset pivot point 5 and reflects the laser beam 2 to the line scanner 6.
The line scanner 6 comprises an annular mirror 7 rotating about an axis 8 at 16,800 RPM, the mirror 7 is rotated by a motor 9.
The laser beam 2 is reflected from the mirror 7 out of the scanner 1 and into a focussing optic 10 and finally leaves the focussing optic 10 into the area which it is scanned over.
The offset pivot point 5 is located so that as the mirror 4 moves about pivot point 5, for instance to position 4A shown by dashed lines, the laser beam 2 is always reflected to the same point relative to the axis 8, as shown by dashed line 2A. The actual point of incidence of the laser beam 2 on the rotating mirror 4 will of course vary as the mirror 4 rotates about the axis 8.
The rotating mirror 7 is shown in more detail in Figure 2. This is basically a annular cylindrical body 7A having a mirrored annular surface 7B. The annular surface 7B being profiled in the form of a parabola wound around the
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rotation axis 8 which is also the axis of symmetry of the annulus 7A. Although Figure 2 shows the mirror surface 7A formed as a number of flat planes it would in practice be a continuous curved surface. That is, the annular mirrored surface 7B is flat perpendicular to the axis 8 or radially but has a surface profile varying with angle around the axis 8 or circumferentially. If axis 8 is taken to be the Z axis, the surface profile parallel to the Z axis or height of the mirrored annular surface 7B can be expressed as;
Z = K o2
where K is a constant and O is angular displacement about axis Z.
As a result the laser beam 2 is scanned in a plane tangential to the rotation of the mirror 7 at the point where the laser beam impinges on the annular surface 7B.
Looking at Figure 3 a representation of the mirror surface profile against angular position is shown, this is in effect the mirror 7 unwound.
The laser beam 2 whe∑ projected into the plane circumferential the axis 8 strikes the surface 7B parallel to the axis 8. It will generally be at an angle to the axis 8 in the radial plane (perpendicular to the paper in Figure
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3) and this angle is controlled by the movement of oscillating mirror 4. However since the surface 7B of the mirror 7 is flat in this radial direction the framescanning produced by oscillating mirror 4 is entirely independent of the linescan produced by the rotating mirror 7 and vice-versa.
When the laser beam 2B strikes the surface 7B perpendicularly it is reflected straight back. This point can conveniently be used as angle 0 = O or 2 , as the mirror rotates and O increases the angle of incidence of the laser beam 2C, 2D and the surface 7B decreases and so the angle through which the laser beam 2B, 2C, 2D is deflected increases to a maximum at O = + . The reverse process occurs from 0 = to O = 2 as the mirror 7 continues to rotate. The maximum value of deflection angle is set by the value of K in equation 1.
This profile of the mirror surface 7B automatically gives a flyback scan where the beam sweeps along a line at constant velocity and then flips back to its start position instantaneously as the beam passes over the cusp at the 0 = position on the mirror surface 7B. Another scanner configuration is shown in Figure 4, here a laser beam 2 from a laser 3 is scanned across an area by a scanner 11.
The laser beam 2 passes through a central hole in a
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rotating mirror 7 and is incident on an oscillating mirror framescanner oscillating about an offset point 5 as before.
The laser beam 2 is then directed to a point on the rotating mirror 7 rotated by a motor 12 to generate a linescan and passes through a focussing optic 10 to the area to be scanned.
Although this is substantially the same as the scanner of Figure 1 this arrangement of the line and frame scanners 7 and 4 minimises the angles of incidence of the laser beam 2 on the mirrors 7 and 4 and so minimises scan distortion, and also minimises the size of the scanner.
The rotary mirror 7 introduces an aberration in the wavefronts of light reflected from it which is similar to astigmatism. This is corrected by the focussing optic 10 re-imaging the rotary scanning mirror 7 to a real pupil and correcting it at that point for all fields of view.
Alternatively this astigmatism could be corrected precisely on axis and reasonably well elsewhere by providing the frame scanning mirror with a similar but oppositely curved surface.
This source of aberration can be minimised by using a larger radius rotary scanner if this is convenient.
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The laser 2 could of course be replaced by an optical sensor and be used to scan an area for optical sources.
Although the rotating mirror surface profile described is parabolic, a spherical profile could be used instead for small scan angles because a spherical surface equates to a paraboloid for small angles, other profiles could also be used.
The rotating mirror surface described is radial to the axis 8, it could be at any fixed angle to the radius from the axis 8, but radial is simplest.
The rotary mirror is used above as a linescanner for a raster flyback scan, it could be used to produce any scan type by appropriate orientation and surface profiling. The rotary mirror could also be used in conjunction with other scanner types replacing the offset axis oscillating mirror shown.
The use of a beam focussing optic 10 may not be necessary in some circumstances, or it could be replaced with a beam expanding optic.
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