Optical focussing sensor

The invention relates to focussing sensors for measuring the deviation between the point of convergence of a beam of radiant energy and the surface of an object receiving this beam. According to the invention, a degree of asymmetry is first introduced into this beam, after which the image of this convergence point if formed on photoelectric sensors enabling the distortions of this image to be measured. These sensors supply signals which are dependent upon said deviation and enable it to be measured. This invention relates to optical focussing sensors by which it is possible to measure the focussing deviation of a beam of radiant energy of which the point of convergence has to be kept in coincidence with the surface of an object which it is desired to illuminate by a spot. An optical sensor of this type may be used in the production of an optical reader intended to read for example the video information recorded in the form of lines of variable length and spacing forming a track on the surface of a support such as a disc. On account of the high density of information thus recorded, the elements characteristic of the information are extremely small and, in addition, it is necessary for the support to be moved past the reading head, which comprises a projection lens, at high speed. The focussing precision of this lens has to be extremely high to obtain a reading spot of sufficiently small dimensions to be able to resolve these characteristic elements. Now, the instability in the movement of the support, in particular along the axis of the convergent reading beam which defines the reading spot, exceeds the limits of this precision to a considerable extent. Accordingly, it is necessary to use elements for controlling the position of the convergence point which enable fine coincidence to be obtained in spite of this instability. In particular, a focussing control loop is used for keeping intact the sharpness of the reading spot. It is known that the error signal required for the operation of the focussing control loop can be obtained by using an auxiliary light beam which, on emerging from the projection lens, forms an inclined parallel beam of which the position, after reflection at the support, followed by another passage through the lens, is detected by photoelectric cells which supply said error signal. It is also known (French Pat. No. 74 01283) to use a stigmatic cylindrical lens which gives a reading beam of which the spot on the support is only substantially circular at the exact focussing point. Beyond this point, the spot has an elliptical shape of which the extension increases with the distance from the focussing spot and of which the orientation is different according to whether the support is situated in front of or behind the circular focussing point. A photoelectric cell with four quadrants enables the change in the shape of the reading spot to be detected and supplies a corresponding electrical signal. Although these solutions function correctly, they require auxiliary means of which the cost is appreciable and their adjustment fairly delicate. In accordance with the present invention, it is provided an optical focussing sensor for measuring the deviations between a reflecting surface and the focussing point of an incident beam formed with coherent light rays, said sensor comprising: PA1 A projection lens for focussing said incident beam at said focussing point nearly said reflecting surface, and focussing in an image point the reflected beam delivered from said incident beam by said reflecting surface; PA1 means for introducing an asymmetry into the spatial distribution of said rays at any point of their path which is not situated at a convergence point; and PA1 photoelectric means for intercepting said reflected beam, measuring said asymmetry, and delivering an error signal; said photoelectric means being located at a point where is located said image point when said focussing point is located onto said reflecting surface, whereby said error signal is null when said deviation is null.

The reading device comprising a sensor according to the invention, which is 
diagrammatically illustrated in FIG. 1, is shown in section along a plane 
defined by the principal optical axis X.sub.1 X.sub.2 of the device and a 
secondary optical axis X.sub.3 X.sub.4 parallel to the direction of the 
velocity vector of the read point P.sub.2 on the support 14, this support 
being perpendicular to the axis X.sub.1 X.sub.2. This device comprises a 
convergence lens 11, a semireflecting mirror 12, a lens 13, a mask 15 and 
two photoelectric cells 16 and 17. 
In the interests of simplicity, the description is confined to the case 
where the device and the phenomena arising out of it are symmetrical in 
relation to the sectional plane X.sub.1 X.sub.2 /X.sub.3 X.sub.4, which 
means that the tracking error is zero. 
The device receives a parallel light beam of circular cross-section, with 
an axis X.sub.3 X.sub.4, defined in the sectional plane by the end rays 
R.sub.1 and R.sub.2. This beam is emitted for example by a laser. It is 
focussed by the convergence lens 11 at a point P.sub.1 which acts as light 
source for the remainder of the device. 
From this point P.sub.1, the beam continues in the form of a divergent beam 
which is reflected by the semitransparent mirror 12 positioned 
perpendicularly of the sectional plane and forming an angle of 45.degree. 
with the axis X.sub.1 X.sub.2 and with the axis X.sub.3 X.sub.4. 
The lens 13 then focusses this incident beam upon the surface of the 
recording support 14 at the point P.sub.2 which is thus the image of the 
point P.sub.1 formed by the optical system consisting of the mirror 12 and 
the lens 13. 
The surface of the support 14 is reflecting and returns the light beam 
towards the lens 13. However, the amount of light returned and its spatial 
distribution are dependent upon the shape of the lines which represent the 
recorded information. 
The reading light beam reflected by the support is thus taken up by the 
lens 13 and then focused through the semitransparent mirror 12 at a point 
P.sub.3 which, like the points P.sub.1 and P.sub.2, is of necessity a spot 
of finite dimensions. As can be seen by applying the classical rules of 
geometric optics, this point P.sub.3 is symmetrical with P.sub.1 relative 
to the plane of the mirror 12. 
Since the point P.sub.3 is the image of the point P.sub.2 formed by the 
lens 13, the distribution of energy in the light spot situated at P.sub.3 
is only dependent upon the distribution of the energy reflected by the 
different parts of the light spot situated at P.sub.2. 
This distribution is symmetrical in relation to the sectional plane because 
the tracking error has been assumed to be zero. It is asymmetrical in the 
sectional plane about the axis X.sub.1 X.sub.2 for the high frequencies of 
the recorded information because, since the points P.sub.1 and P.sub.2 are 
spots of finite dimensions, when the length of the lines carrying the 
information becomes short, correspondings to high frequencies, or when one 
of the ends of a line passes through the light spot situated at P.sub.2, 
thus corresponding to transients giving high frequencies, the quantity of 
light reflected varies along this spot in the direction corresponding to 
the axis X.sub.3 X.sub.4. For the same reasons, but in reverse order, the 
distribution of light is symmetrical in the sectional plane about the axis 
X.sub.1 X.sub.2 for the low frequencies of the recorded information. 
The distribution does not depend upon any degree of asymmetry in the beams 
when the focussing conditions are observed. In particular, the mask 15 
which intersects a portion 18 of the focus reading beam does not therefore 
introduce any asymmetry into the image situated at P.sub.3. 
The photoelectric cells 16 and 17 are situated in the vicinity of the image 
point P.sub.3 formed from the point P.sub.2 by the lens 13. To this end, 
they are positioned in a plane normal to the axis X.sub.1 X.sub.2 and 
passing through P.sub.3 and are situated on either side of a plane 
perpendicular to the sectional plane and passing through the axis X.sub.1 
X.sub.2. They support signals S.sub.1 and S.sub.2 which, when filtered by 
means of a low pass filter, become identical signals because they 
correspond to a distribution of light which, as we have seen, is 
symmetrical at low frequency. 
When, as a result of instability in the movement of the support 14, the 
support moves away from (FIG. 2a) or approaches (FIG. 2b) the lens 13, the 
result obtained above is no longer valid. 
However, irrespective of the position of the support 14, the point P.sub.2 
remains fixed. However, it becomes virtual when the support 14 approaches 
the lens 13, the real focussing point then being the point P.sub.4 which 
is symmetrical with the point P.sub.2 relative to the reflecting surface 
of the support 14. Accordingly, it is justified, for explaining the 
phenomena, to use the simplified FIGS. 2a and 2b from which the lens 11 
and the mirror 12 have been omitted. 
In the case of FIG. 2a, the incident light beam emanating from the point 
P.sub.2 forms on the support 14 a light spot which in the sectional plane 
is defined by the points A.sub.1 and A.sub.2. This light beam is reflected 
at the support 14 and the reading beam thus obtained is focussed by the 
lens 13 at the point P.sub.3. This point P.sub.3 is the image formed by 
the lens 13 of a virtual point P.sub.5 which is symmetrical with the point 
P.sub.2 relative to the reflecting surface of the support 14. Since this 
point P.sub.5 is situated further away from the lens 13 than the point 
P.sub.2, the point P.sub.3 is closer to the lens 13 than the cells 16 and 
17 and the reading light beam, after having converged at P.sub.3, then 
diverges to form on the cells 16 and 17 a light spot defined in the 
sectional plane by the points B.sub.1 and B.sub.2. This spot is not the 
image of the spot A.sub.1 A.sub.2 because, since it is further away from 
the lens 13 than the point P.sub.2 and closer than the point P.sub.5, its 
image has to be formed between the cells 16 and 17 and the point P.sub.3. 
FIG. 2a thus clearly shows that a mask such as 15 so inserted as to break 
the symmetry of the beam relative to the axis X.sub.1 X.sub.2 in the 
sectional plane produces an asymmetry of the spot B.sub.1 B.sub.2, as a 
result of which the cell 16 receives less light than the cell 17. 
Accordingly, the signals S.sub.1 and S.sub.2 obtained under the same 
conditions (low-pass filtration) as before are no longer equal, S.sub.2 
being greater than S.sub.1. 
In the case of FIG. 2b, the incident light beam which converges towards the 
virtual point P.sub.2 forms on the support 14 a light spot which is 
defined in the sectional plane by the points A.sub.1 and A.sub.2. This 
convergent light beam is reflected by the support 14 and the reading beam 
thus obtained converges at the point P.sub.4 from which it continues in 
the form of a divergent beam which is then focussed by the lens 13 at the 
virtual point P.sub.3. This point P.sub.3 is the image formed by the lens 
13 of the point P.sub.4. Since this point P.sub.4 is closer to the lens 13 
than the point P.sub.2, the point P.sub.3 is further away from the lens 13 
than the cells 16 and 17 and the reading light beam is thus intercepted by 
these cells on which it forms a light spot defined in the sectional plane 
by the points B.sub.1 and B.sub.2. This spot is no longer the image of the 
spot A.sub.1 A.sub.2 because this image would have to be formed between 
the cells 16 and 17 and the point P.sub.3. 
FIG. 2b thus clearly shows that, in such a case, the mask 15 results in the 
formation of a signal S.sub.2 weaker than the signal S.sub.1. 
Accordingly, the difference between the signals S.sub.1 and S.sub.2, duly 
filtered, gives a signal which varies in dependence upon the position of 
the support 14 relative to the lens 13 and which disappears by changing 
sign when the support 14 passes through its normal reading position 
corresponding to the exact convergence of the light beam incident upon the 
support 14. A signal such as this may be used as an error signal for 
controlling a focussing servomechanism which, for example, keeps the 
distance between the support 14 and the lens 13 constant. 
The foregoing description has been made with reference to a light beam 
which is symmetrical in relation to a plane perpendicular to the sectional 
plane and passing through the axis X.sub.1 X.sub.2. We shall call this 
symmetry left-to-right symmetry due to the orientation of the FIG. In 
order to obtain an error signal, we introduced the mask 15 producing a 
left-to-right asymmetry of the convergent reading beam situated behind the 
lens 13 in the path of the light rays by blocking out part of that beam. 
It is the left-to-right asymmetry which is required for obtaining this 
error signal, and it may be obtained by any means, in particular by 
inserting a mask at any point of the path of the light rays provided that 
the blocking effect thus obtained it not itself symmetrical. However, this 
mask cannot be positioned at any point of convergence of the beam passing 
through the device because, since the asymmetry of the beam does not 
produce any asymmetry of the images, it follows that any asymmetry in the 
images does not produce any asymmetry in the beam. 
Accordingly, it is paticularly advisable to position this mask in the plane 
of the pupil of the projection lens because, in this case, it is possible 
directly to use this pupil as the mask and, since it is generally 
circular, the required asymmetry may thus be introduced by decentring the 
illuminating beam. In this case, however, the reasoning based on geometric 
optics is no longer sufficient. 
FIG. 3 illustrates a device by which it is possible to focus a light beam 
on the reflecting surface of an object 34 and which comprises a sensor 
according to the invention. This device is very similar to that 
illustrated in FIG. 1, except that the positions of the light source and 
the photocells have been interchanged in the interests of clarity. This 
arrangement comprises a light source 30, a convergence lens 31, a 
semireflecting mirror 32, a projection lens 33 arranged in a mobile 
mounting 35, two photoelectric cells 36 and 37, a subtractor 39 and a 
motor 40. It is shown in section along a plane defined by the optical axis 
X.sub.1 X.sub.2 of the lens 33 and a secondary axis X.sub.3 X.sub.4 
symmetrical with the axis X.sub.1 X.sub.2 relative to the plane of the 
mirror 32. 
The light source 30, for example a laser, emits a parallel light beam 
defined in the sectional plane by the end rays R.sub.1 and R.sub.2. This 
beam is circular and cylindrical about an axis represented by the median 
ray R.sub.3 and the distribution of luminuous energy is symmetrical about 
that ray. The axis of the beam is parallel to the axis X.sub.1 X.sub.2, 
but is offset relative to that axis by a distance .DELTA. so that R.sub.3 
remains in the sectional plane. 
The lens focusses the light beam at a point P.sub.1 situated on the axis 
X.sub.1 X.sub.2. 
From this point P.sub.1, the beam continues in the form of a divergent 
beam, passes through the semitransparent mirror 32 and arrives at the lens 
33. Part of the beam, represented by the vertically hatched zone 41 and 
defined by the rays R.sub.2 and R.sub.4, is intercepted by the mounting 35 
of the lens 33. 
The lens 33 then focusses the beam into the virtual point P.sub.2 which, as 
shown in the FIG., is situated below the reflecting surface of the object 
34, the FIG. showing the device in a state where the motor 40 have not yet 
finished bringing the lens 33 back to a position where the point P.sub.2 
coincides with the surface of the object 34. 
After reflecting at the surfaces of the object 34, the beam effectively 
converges at the real point P.sub.4 from which it continues in the form of 
a divergent beam which is taken up by the lens 33 and emerges from it in 
the form of a convergent beam which, after reflecting at the mirror 32, 
coverges towards a point situated on the axis X.sub.3 X.sub.4 beyond the 
cells 36 and 37. 
Accordingly, the cells 36 and 37 finally intercept the beam before it is 
focussed again, with the result that a light spot defined in the sectional 
plane by the points B.sub.1 and B.sub.2 is formed on the cells. 
In order to determine which of the two cells 36 and 37 is the more 
illuminated, it is not advisable to a assimilate the zone 41 intercepted 
by the fitting 39 with the zone 18 blocked out by the mask 15 in FIGS. 1 
and 2. This can be seen by referring to FIG. 4 which is a view in section 
along a plane perpendicular to the axis X.sub.1 X.sub.2 and situated at 
the level of the lens 33. The axis X.sub.3 X.sub.4 is the same as that in 
FIG. 3, but is projected onto this plane. The circle 43 defines the 
external contour of the lens 33 and the internal contour of the mounting 
35 of which the external contour is defined by the circle 45. The circle 
42 defines the cross-section of the light beam incident through the plane 
of the FIG. whilst the rays R.sub.1 to R.sub.4 are represented by their 
trace in that plane. 
It can thus be seen that these three circles defines three zones: 
a vertically hatched zone 41 corresponding to that part of the beam which 
is intercepted by the mounting 45; 
a central zone 44 which corresponds to that part of the beam which passes 
through the lens 33; 
a horizontally hatched zone 46 which corresponds to that part of the lens 
which is not illuminated by the beam and which defines a virtual blocked 
beam 46 in FIG. 3. 
The light beam emerging from the lens 33 may thus be presented as emanating 
from the choice of one of the following three incident light beams: 
the real beam illuminating the zones 41 and 44 blocked out by the real mask 
41; 
a virtual beam illuminating the zones 44 and 46 blocked out by the virtual 
mask 46; 
a virtual beam illuminating the zones 41, 44 and 46 blocked out by the real 
mask 41 and the virtual mask 46. 
None of these constructions enables the most illuminated cell to be 
determined, although to obtain this indication it is possible to 
investigate the distribution of luminous energy in the beam emitted by the 
source 30 and to observe what happens in the path. 
In the case of a laser, and likewise in the case of any source producing a 
parallel, cylindrical circular beam which is not specially corrected, this 
distribution is symmetrical about the axis of the beam and, in a plne 
passing through this axis, is substantially represented by a Gaussian 
centered on R.sub.3 and bounded by R.sub.1 and R.sub.2. Accordingly, a 
large part of the luminous energy of the beam is concentrated about the 
median ray R.sub.3 and the cell which receives this ray will be the cell 
which supplies the strongest signal. 
Accordingly, by following the progress of this ray in FIG. 3, it can be 
seen that it impinges on the cell 37 because it would have to cross the 
axis X.sub.3 X.sub.4 at the point of convergence of the reflected beam 
which, as shown in the FIG., is situated behind the cells because it is 
the image of the point P.sub.4 formed by the lens 33 and the mirror 32. In 
this case, therefore, the signal S.sub.2 emitted by the cell 37 is 
stronger than the signal S.sub.1 emitted by the cell 36. 
When the focussing point P.sub.2 of the incident beam is situated above the 
surface of the object 34, the point of convergence of the reflected beam 
is still situated on the axis X.sub.3 X.sub.4, but on this occasion in 
front of the cells, and the ray R.sub.3 thus cross this axis to impinge on 
the cell 36 which thus emits a signal S.sub.1 which on this occasion is 
stronger than the signal S.sub.2. 
The subtractor 39 performs the difference between the signals S.sub.1 and 
S.sub.2 and delivers an error signal to the motor 40 which contains proper 
amplification means and causes the mountiing 35 supporting the lens 33 to 
advance or move back along the axis X.sub.1 X.sub.2 until the point 
P.sub.2 is situated exactly on the surface of the object 34. At this 
moment, the image of P.sub.2 is formed exactly on the cells 36 and 37 and 
the ray R.sub.3 impinges on these two cells at a point situated on their 
connecting line. Thus, the signals S.sub.1 and S.sub.2 are now equal and 
their difference disappears when the deviation between the surface of the 
object 34 and the focussing point P.sub.2 disappears. 
The focussing arrangement thus formed is simple and does not necessitate 
any delicate adjustments. 
The following documents have been quoted during the French prosecution: 
French application No. Fr 2 122 590 (Leitz) 
French application No. FR 2 222 666 (Thomson-Brandt).