GB2023872A

GB2023872A - Two dimensional scanner with correcting element

Info

Publication number: GB2023872A
Application number: GB7920619A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1978-06-16
Filing date: 1979-06-13
Publication date: 1980-01-03
Also published as: IT7923545A0; NL7806504A; IT1121096B; GB2023872B; DE2920870A1; FR2428854A1; SE7905208L

Abstract

A device is described for two- dimensional scanning with the aid of a pivotable plane mirror 4 and a rotatable mirror drum 3. By including a correction element 5 between the plane mirror 4 and the image plane bo of an objective system 1 and by ensuring that the axis of rotation r of the plane mirror 4 passes substantially through the centre of curvature of the image plane bo, a satisfactory correction is obtained both for the image plane curvature as a result of the rotating mirror drum 3 and the image plane curvature as a result of the pivotable plane mirror 4. <IMAGE>

Description

SPECIFICATION Two-dimensional scanner The invention relates to a device for two-dimensional scanning, in particular in the infrared wave-length range, comprising an objective system, a plane mirror which is pivotable about an axis parallel to its mirror plane for scanning in a first direction, a rotatable drum which is provided with a plurality of mirror faces for scanning in a second direction perpendicular to the first direction, and a radiation-sensitive detector.

Such a scanning device, which is used in a so-called thermal image (or infrared) camera or in an infrared viewing device, is known for example from U.K. Patent Specification No. 1,287,280.

In this type of scanning device the pivotable plane mirror oscillates comparatively slowly for sequentially applying the various horizontal lines of a scene or object to the detector, i.a for the vertical scanning. Every horizontal line of the scene or object is scanned with the mirror drum, which rotates with a comparatively high speed. If the thermal image is reproduced on a television monitor, a horizontal line is scanned in 64 microsecs, whilst the plane mirror scans at 50 Hz. A picture element of the scene or object is always imaged on the detector, so that this detector is chosen to be small in area and may be termed a point-scanning detector.

In the said scanning device the problem occurs that the image plane of the objective system is plane or spherical, whilst the image plane curvature introduced by the pivotable or tilting plane mirror and the rotating mirror drum differ with respect to each other and to the image plane curvature of the objective system. Moreover, the image plane curvature introduced by the rotating mirror drum is opposed to the image plane curvature of the objective system. This gives rise to a defocusing at the edges of the field of view of the objective system.

The correction of the objective system for the two different image plane curvatures might have been considered. However, it is impossible to realise this with a rotation-symmetrical optical system. Another approach might have been to design the scanning device so that the two image plane curvatures are equal to each other and to correct the objective for the one image plane curvature. However, this would drastically affect the design of the scanning device, and limit the number of degrees of freedom in the design, prohibiting the use of a rotatable mirror, so that this design becomes less suitable for practical use.

It is the object of the present invention to provide a two-dimensional scanning device in which the image plane curvature resulting from the tilting mirror and that resulting from the rotating mirror drum are satisfactorily corrected by comparatively simple means, without reducing the number of degrees of freedom in the design of the device.

The invention provides a devicefortwo-dimensional scanning, in particularforthe infrared wavelength range, comprising an objective system, a plane mirror which is pivotable about an axis parallel to the mirror plane for scanning in a first direction, a rotatable drum which is provided with a plurality of mirror faces for scanning in a second direction perpendicular to the first direction, and a radiation sensitive detector, and a correction element included between the pivotable plane mirror and the image plane of the objective system, which correction element renders the image plane curvature introduced by the rotatable drum equal to the image plane curvature of the system constituted by the objective system and the correction element, the pivotal axis of the plane mirror approximately passing through the centre of curvature of the image plane of the objective system.

The correction element has effect in one direction solely along the optical axis of the device and has off-axis effect in a direction perpendicular thereto. It is in this last-mentioned direction, in the foregoing referred to as the second scanning direction, that the correction element influences the image plane curvature.

The image plane curvature in the first scanning direction is corrected by ensuring that the pivotal axis of the plane mirror passes through the centre of curvature of the image plane of the objective system.

The scanning device in accordance with the invention may be compact. The correction element may comprise a single positive lens of the same material, for example germanium, as the objective system.

However, preferably the correction element comprises a negative lens with a small refractive index. The negative lens may comprise a single lens element, but alternatively it may comprise two or more lens elements.

If the negative lens consists of a material whose dispersion is greater than that of the material of the objective system this lens may also be used for correcting the longitudinal chromatic aberrations of the objective system in addition to the image plane curvature.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 7 schematically represents the principle of a known two-dimensional scanning device, Figure 2 represents the image formation by the pivotable plane mirror used in this device, Figures 3 and 4 represent the image formation by the rotatable mirror drum used in the scanning device, Figure 5schematically represents the principle of a two-dimensional scanning device in accordance with the invention, Figures 6 and 7 show an embodiment of a device in accordance with the invention, Figure 8 shows an intermediate objective of special design for use in a device in accordance with the invention.

Figure 1 shows very schematically the construction used in a known two-dimensional scanning device.

The optical axis is shown as a straight line rather than folded, as in practical devices, by reflecting scanners.

In fact, the radiation path in this Figure extends from the left to the right. However, for a better understanding the reciprocal path of rays from the detector to the objective system will be considered.

In Figure 1 the objective system is represented by a single lens 1. However, the objective system may also be constituted by a doublet or by a mirror objective such as a Schmidt objective or a Bouwers-Maksutov objective. The location of the detector is represented by the point D. Since the rotatable drum is a mirror drum, the detector cannot be arranged inside the drum, and the radiation reflected by the drum must be focused onto the detector. For this purpose there is provided an intermediate objective 2. On the righthand side of Figure 1 there is furthermore shown a rectangular coordinate system XYZ. The Z-direction is the direction of the radiation from an object or scene to be scanned which passes through the objective system.

The scanning element used for scanning in the X-direction is represented by the block 3. This scanning element is a mirror drum, shown in more detail in Figures 3 and 4 which is rotated about an axis in the Y-direction. The block 4 represents the scanning element which performs scanning in the Y-direction. This element is a pivotable plane mirror, shown in Figures 2 and 6, which is tilted about a pivoting axis in the X-direction.

In the absence of the two scanning elements the detector would be imaged in point D'. If the two scanning elements are present, point D' will be re-imaged in point D" when the two scanning elements are in the zero position.

When the two scanning elements 3 and 4 are put into motion the image D' will move along the curve h with a radius of curvature Rh in the XZ-plane, as a result of the rotation of the element 3. The tilting movement of the scanning element 4 alone would cause the image D" to move along a curve V having a radius of curvature Rv in the YZ-plane. When the both scanning elements are operative the image D" consequently moves along a curved surface, or image plane, b with radii of curvature Rh and Rv. Rh is then unequal to Rv.

Figure 2 schematically illustrates the image formation bythetilting mirror along, i.e. element4 in Figure 1.

In this Figure D1 is the detector or an image thereof. The distance from D1 to the centre of rotation r of the mirror is Rv. If the mirror is in the zero position, D1 is imaged in D". A rotation of the mirror through an angle a results in an image D"2 of D1. The angle D" rid"2 is then 2a. It can readily be demonstrated that D"2 is situated on the circle through D1 and D"1 with the centre r. This is because D1D"1 and D1D"2 are bisected at right angles by rm and rm' respectively, so that Dud"1 and Dud"2 are chords of the circle. This circle is the circle V of radius Rvin Figure 1.

Figures 3 and 4 illustrate the principle of scanning by means of a mirror drum only. The mirror drum has a plurality of facets. One facet comprises two mirror faces 8 at right angles to each other. As can be seen in Figure 3 the incident radiation beam 7 is focused by the objective system 1 to a point which is situated inside the mirror drum 3. The facets reflect the radiation beam, after which this beam is imaged onto the detector D by the intermediate objective. Conversely, it is therefore correct to state that the intermediate objective images the detector onto the circumscribed circle of the mirror drum 3.

Figure 4 shows some facets of the mirror drum on an enlarged scale. The drum is rotated about an axis through the point M and at right angles to the plane of drawing. The uninterrupted lines represent the zero position of the drum. The dashed lines represent the position of the facets after the drum has rotated through an angle cp. After rotation through an angle equal to 360 /n, where n is the number of facets of the drum, the drum reaches the zero position again. It is assumed that the detector is imaged in the point D1. The image as a result of reflection on the faces E1 K and E2K may be represented as a mirror image of point D, relative to point K. Point D1 is a fixed point, because the detector is stationary.During rotation of the drum the point K describes an arc of a circle, as a result of which the image D'3, of point D1 describes the dashed arc with D1 as the extreme positions. It can be shown that the detector then scans a circular curve in the object space with a radius of curvature equal to the inner diameter (2 KM) of the mirror drum 3.

The image plane b0 of the objective system 1 is flat, or as is assumed in Figure 1, spherical with a radius of curvature R.

In the known scanning devices one of the image plane curvatures R and h is corrected and the other is not.

Alternatively the correction may be such that correction is provided for both image plane curvatures, but only to a very small extent.

In accordance with the invention suitable correction is provided both for the image plane curvature in the XZ plane and for that in the YZ plane. As is shown in Figure 5 this is achieved by including the correction element in the form of a lens 5 in the radiation path. The correction lens ensures that when D' is imaged on D" the image plane curve h changes into the image plane curve h' of opposite curvature and having a radius of curvature Rh'. The lens 5 only provides a correction in the XZ plane. In the YZ plane the correction lens has effect only along the z-axis since the image plane curve h lies entirely in the XZ plane, so that it produces a displacement of the image point D" along the z-axis in comparison with the situation of Figure 1. The radius of curvature Rv of the image field curvature in the YZ-plane is not affected by the lens 5. The correction element has a such a power that Rh' = Rv. The objective system may now be designed so that the radius of curvature R of its image plane curvature is equal to Rv and thus also equal to Rh', SO that the objective system is corrected for both image plane curvatures.

The correction lens 5 is disposed between the pivotable mirror and the image plane of the objective system near said image plane.

Figure 6 shows an embodiment of a scanning device in accordance with the invention in more detail. The objective system 1, which is represented as a single lens, has an image field b0 which curves towards the objective system and has a radius of curvature R. A beam which is incident parallel to the optical axis 00' is focused in point D", whilst a beam which is incident at an angle with the axis 00' is focused in point D"4. The pivotable mirror 4 is shown in two positions, namely the zero position, represented by the uninterrupted lines, and a position which deviates therefrom, represented by dashed lines. The axis about which the mirror pivoted passes through point rand is perpendicular to the plane of drawing.By selecting, in accordance with the invention, the distance r-D" so as to equal R, the image plane curvature resulting from the tilting mirror is adapted to that of the objective system.

In the absence of the correction lens 5 the mirror 4 would consecutively image all points situated on the arc (in the YZ plane of Figure 1 ) through points D" and D"4 in one point F', because the axis of rotation of the mirror 4 passes through the centre of curvature of the spherical image plane of the objective system. By situating the correction lens 5 in the radiation path the points of said arc are all imaged in point D'. The position of the centre of rotation r is not very critical. This point may also be situated at a few mm's distance from the centre of the sphere b0.

The points which are situated on the are h' (in the XZ plane of Figure 1) would be imaged on the curve h" in the absence of the correction lens. The correction lens 5 ensures that the points of the curve h' are imaged on the curve h through D'. The curve h satisfactorily coincides with the cylindrical image plane of the mirror drum 3.

In a practical embodiment the axis of rotation 9 of the mirror drum will extend at an angle y with the optical axis of the system constituted by the objective system 1, the tilting mirror 4 and the correction lens 5. Thus the detector need not be disposed in the path of the incident radiation as was the case in Figure 3.

The radiation reflected by the mirror drum is focused on to the detector D by an intermediate optical system or relay lens 2 as is shown in Figure 7. This Figure shows a scanning device whose objective system comprises two lens elements, whilst the correction lens is also constituted by two lens elements.

If the imaging system constituted by the objective system 1, the pivotable mirror 4, the mirror drum 3 and the relay lens 2 is corrected perfectly in respect of coma, astigmatism and spherical aberration, the following relationship is found for the image field curvature of this system.

where n1 is the refractive index and fi the focal length of the individual optical elements whilst R is the radius of the image plane curvature. It is evident that, if this formula is valid, for a negative total image plane curvature which is adapted to that of the mirror drum, a negative lens with a low refractive index is required for the correction lens. By manufacturing the lens from a material such as ZnS, or ZnSe, whose dispersion is great relative to that of germanium, the longitudinal chromatic aberrations of the objective system can also be corrected by means of the correction lens.

If the image need not be perfect in respect of coma, astigmatism and spherical aberration, the aforementioned formula need no longer be satisfied. The correction element may then consist of a single lens of the same material, for example germanium, as the objective system. The chromatic abberrations then cannot be eliminated by the correction lens.

If the correction element 5 is only used for correcting image plane curvatures, the longitudinal chromatic aberrations of the objective system can be corrected by including a negative lens element with a great dispersion in the intermediate objective 2. As is shown in Figure 8 the intermediate objective may then comprise two lens elements 11 and 12 of germanium and a lens element 13 of for example ZnS interposed between them. The lateral chromatic aberration of the objective system is of course not affected by the relay lens. Because of the small dispersion of germanium this chromatic aberration is not annoying.

Claims

1. A device for two-dimensional scanning, in particular for the infrared wavelength range, comprising an objective system, a plane mirror which is pivotable about an axis parallel to the mirror plane for scanning in a first direction, a rotatable drum which is provided with a plurality of mirror faces for scanning in a second direction perpendicular to the first direction, and a radiation sensitive detector and a correction element included between the pivotable plane mirror and the image plane of the objective system, which correction element renders the image plane curvature introduced by the rotatable drum equal to the image plane curvature of the system constituted by the objective system and the correction element, the pivotal axis of the plane mirror approximately passing through the centre of curvature of the image plane of the objective system.

2. A device as claimed in Claim 1, wherein the correction element is constituted by a negative lens with a small refractive index.

3. A device as claimed in Claim 2, wherein the correction lens is manufactured from a material having a dispersion which is greater than that of the material of the objective system.

4. A device as claimed in Claim 1, wherein an intermediate objective included between the mirror drum and the detector comprises a negative lens element of a material having a dispersion which is greater than that of the material of the objective system.

5. A device for two-dimensional scanning substantially as described with reference to Figure 6, Figure 7 or Figures 7 and 8 of the accompanying drawings.