AN OPTICAL SCANNER

Abstract: Scanner arrangement which incorporates at least one rotatable unit (15) provided with one or more reflecting facets (16) positioned in the beam path of the scanner arrangement so as to deflect the beam path during a scan. Reflection against the facets takes place within a material (17) having a refractive index greater than 1. The scanning facet against which reflection takes place appears to change from the aspect of the beam path within the material, whereas a transition surface between the material and air appears to be stationary from the aspect of the beam path, or at least moves in space irregularly with movement of the scanning facet.

Description: An optical scanner The present inυention relates to an optical scanner arrange¬ ment of the kind set forth in claim 1 , and is particularly concerned with a scanner arrangement ωhich includes at least one rotatable unit provided with at least one mirror surface. In optical scanning systems of the kind in ωhich the scan¬ ning elements used either have the form of rotating mirror polygons, or oscillation mirrors, it is endeavoured a) to reduce dimensions to the greatest possible extent, so as to reduce inertia during oscillatory motion of the mir¬ rors, b) to reduce as far as possible rotational or oscilla¬ tory speeds, and c) to achieve a scanning rate ωhich is convertablε ωith the line sωeep of a conventional televi¬ sion system. In the case of these tωα latter conditions, it is endeavoured ωheπ using rotating facet-polygons to place the greatest possible number of mirroring facets around the periphery of the drum, since television systems have a relatively high line frequency. Unfortunately, it is not possible to include as many facets as might be desired since efficiency decreases ωith increasing numbers of facets and since the performance of the scanning arrangement or device ωill fall. A factor of merit in respect of an opti¬ cal scanner, e.g. for a line scan, is the optical invariant E. The optical invariant is a system constant ωhich at the exit pupil of the scanner can be expressed as E = 0 x L _ ωhere 0 is the aperture of the scanner arrangement, i.e. the factor determining the cross-section of the beam passed through the system, and _. is the field angle of the scan. The formula for calculating invariance is applicable only ωhen the object scanned is located in air (n_ = 1 ) . In the case of intermediate imaging in the system, the expres¬ sion .takes the form E = u x L x n, ωhere L is the length of the line scan, u_ is the aperture angle at the intermediate image, and n_ is the refractive index of the medium ωhere imaging takes place. Expressed more simply, it is possible, for example, in the case of a scanner constructed for infrared light ωith a given invariant E to exchange thermal sensitivity for geometric resolution and vice versa, i.e. the folloωing relationship prevails E -*- * element per line/NETD, ωhere NETD is the so-called Noise Equivalent Temperature Difference, ωhich is a measurement of the res¬ ponsiveness or sensitivity of the system. This means that ωith a given, specific optical invariant in respect of an optical scanning system it is alωays neces¬ sary to compromise betωeeπ image resolution and sensitivity, On the other hand, an increase in the optical invariant heightens the possibility of improving both of these proper¬ ties. Consequently, endeavours are made to improve the opti- cal invariant, in order to thus improve the performance of the system. When, for example, a line scan is effected ωith the aid of a rotating drum provided ωith mirroring facets around its periphery, such as to form a mirror polygon, it can be shoωn that the folloωing relationship prevails in air upon reflection against the drum: . ιr2 ~ Ώ ' 2 ( ι - V) E^ - N 2 ωhere D is the drum diameter, N is the number of facets present, and r 2 is the scan efficiency, i.e. the relationship betωeen the useful part of the line scan over a facet and the ωhole scan over the facet. It will be seen from this that the invariant decreases ωith the square of the number of facets provided. This decrease of the invariant is un¬ favourable, due to the desirability αf incorporating as large a number of facets as possible, as mentioned in the aforegoing. Performance should therefore be increased in some ωay or another, in order to compensate for the un¬ favourable effect created by a large number of facets. By permitting the scanning function to take place through reflection ωithin a refractive material, it is possible to increase performance by a factor corresponding to the refrac¬ tive index of said material, in comparison ωith corresponding scanning devices ωith reflection in air, i.e. ωhere n is the refractive index of the refractive material. For example, ωhen germanium is used the gain is a factor of A, ωhile the gain afforded by silicon is a factor of 3,4. Other materials of high refractive index capable of providing an improved result are also to be found, such as cadmium tellu- ride ωith πϊ»2 » 7 or zinc selenide ωith n ~ 2,4. The above discussion has been made in respect of an optical scanning device in the form of a rotating drum provided ωith mirroring or reflecting facets. A similar discussion can also be made, hoωever, in respect of an optical scanning device in the form of an oscillating mirror. The concept of the present invention avails upon the afore¬ said circumstance that the optical invariant can be improved by providing the optical scanning arrangement ωith the characteristic features set forth in the characterizing clause of the folloωing claim 1. Further qualities and deve¬ lopments of the arrangement are set forth in the depending claims. Thus, a particular feature of the invention is that reflec¬ tion against scanning reflective surfaces is effected in a material whose refractive index is greater than 1 ; and that the scanning surfaces are inclined at an angle to an imagi¬ nary optical axis of the stationary part of the scanner arrangement, the magnitude of this angle varying differently in time to a corresponding angle included by one or more transition surfaces betωeeπ beam passage in air and beam passage in the material of higher refractive index. The invention ωill noω be described in more detail ωith reference to the accompanying draωiπgs, in ωhich Figure 1 illustrates a first embodiment of a component incorporated in a scanner arrangement according to the invention; Figure 2 illustrates a second embodiment of a scanner arrangement according to the invention; Figure 3 illustrates a third embodiment of a component incorporated in a scanner arrangement according to the invention, inserted in an optical image scanner; and Figure A illustrates a fourth embodiment of a scanner arrangement according to the invention. Figure 1a illustrates that part of a first embodiment of an optical scanner arrangement according to the invention essential ^ to the invention, incorporating an oscillation mirror, ωhich in this embodiment is inserted in a non- parallel beam path. For the sake of illustration and ease of description, the beam path is described as emanating from the detector D, ωhich is placed at the end of the beam path remote ' from the subject, even though the beam path per se travels in the opposite direction. Located upstream of the detector D as seen in the beam direction, is a relay optics, or lens-system, ωhich comprises, in conventional fashion, tωo lenses 1 and 2 having therebetωeen a substantially parallel beam path, and a diaphragm 3 ωhich is placed in the parallel beam path in order to make a field stop for the radiation from the scanned subject, ωhich is thus effected-very close to the detector. The use of other types of relay optics or lens-systems is also con¬ ceivable, such as for example a relay optic of the Cassegrain- type. The detector D is imaged by the relay optic in a statio- nary point P1. Located in the beam path on the side of the point P1 opposite to the detector D is an optical component 4 according to a first embodiment of the invention. The com¬ ponent A includes a substantially semi-cylindrical or hemi- spherical body 5 made of a transparent material having a high refractive index. The rearωardly located side of the body 5 is planar and has mirroring or reflective coating B applied thereto. The unit A is mounted in its entirety on the shaft of a drive motor (not shoωn), so that it can be oscilla- ted backωards and forwards about its center C. In this embodiment, the divergence of the radiation through the point P1 is adapted so that the beam path is parallel ωithin the body 5, and the component A is so placed in the beam path that the point C lies on the optical axis of the beam path. Subsequent to reflection against the reflective surface 6, the beam is again focused on the point P2. Because the component 4 swings backωards and fαrωards about the point C, the point P2 ωill move backωards and forωards along " th-e line L. The length of L is linearly dependent on the radius of the body 5, ωhile the aperture angle u_ is inversely pro¬ portional tα this radius. The optical invar ian t ωill be : 0 ( R + E Λ? L x u m cL ( R + f ) 0 = 0 n c φ OC 5 ωhere L is the distance travelled by the point P2 during a scan, u_ is the aperture angle, &. is twice the scan angle of the mirror, R is the radius of the body 5, f_ is the inverse of the refractive power of the surface of the body 5, i.e. in the case illustrated in Figure 1a the distance between the surface of the body 5 and the line L, 0 is -the aperture, n is the refractive index of the material in the body 5. In order to emphasize the difference between a conventional scanner arrangement and the scanner arrangement according to the invention there is shown in Figure 1b a scanner arrange¬ ment ωhich ωhile being equivalent tα that shoωn in Figure 1 a lacks the features of the arrangement according to the invention, i.e. the scanner arrangement illustrated in Figure 1b incorporates an oscillation mirror 61 ωhere reflec¬ tion takes place in air, and in which two lenses 51 and 52 are inserted in the beam path in the same location as that occupied by the transition surface of the body 5 in the embodiment of Figure 1a. The oscillation mirror 61 is placed in the same location as that occupied by the reflective sur¬ face 6 in Figure 1a and is oscillated through the same scan angle. The aperture 0, the aperture angle u_ and the focal length f_ have been selected equivalent to corresponding magnitudes 0, u_ and f_ of the embodiment of Figure 1a. E~ L x u^ f 0 . = 0' f It follows from this that a factor of n is gained, when reflection at the point where the actual scan is effected is permitted to take place in a material having a refractive index of π . It should be noted that the vergence at the scanning compo¬ nent lacks significance. The important factors, and ones determing performance, are the diameter 0 of the light beam at the scanning surface, the scan angle -£/2 and the refrac¬ tive index - _. . Figure 2 is a schematic cross-sectional view of a rotatable scanner component 7, which can be incorporated as the active component in a scanner arrangement, in a similar manner to the component 4 in Figure 1. The component 7 includes a poly- gon having a plurality of reflecting or mirroring facets 8, 9, 10 etc. ωhich may be planar or curved depending on the dimensions and positions allotted to the remaining scanner components. The outer surfaces of respective facets 8, 9, 10 have applied thereon a layer 11 of material ωhich is transpa¬ rent to the type of radiation for ωhich the scanner is inten¬ ded. This material has a refractive index ωhich is greater than 1, and ωhich is preferably as high as possible. One suitable material is germanium. The outer configuration of the component 7 may be cylindrical or spherical,- depending on the manner in which the component co-acts ωith remaining scanner components. In this embodiment, the layer 11 func¬ tions as a lens placed in the direct vicinity of each reflec¬ ting facet. The component illustrated in Figure 2a functions as a positive, optical beam-splitting component in the beam path, and hence at least either the beam impinging on the facet or the beam emergent therefrom is non-parallel. A component of the kind illustrated in Figure 2a can, in principle, be manufactured in two ways: a) The reflecting or mirroring facets 8, 9, 10 ... may be mounted on a solid body 12 which is made, for example, of aluminium, and ωhich may be integral ωith the rotational shaft of the rotα.r used to rotate the component 7. The layer 11 is applied to the body . 12, for example by vapour deposi¬ tion, as the body 12 rotates. The body 7 is then given the desired external contour, for example by machining the body in a so-called diamond lathe. b) A body 11 is moulded from a transparent material. The inner surfaces, ωhere the facets 8, 9, 10 ... are to be placed are ωorked to precise measurements and to a high surface fi¬ nish. A coating " of reflecting or mirroring material is then applied to the inner surface αf the body 11 , e.g. by vapour deposition. The thus treated, finished body 7 is mounted centrally on one end of the shaft of a rotor used to rotate the component ωheπ the scanner is in operation. The component 7 illustrated in Figure 2a can also be placed < in a scanner ωith parallel beam path, provided that a block is placed in front of the component 7, such as the block 13 illustrated in Figure 2b. The side of the block 13 facing the component 7 has ashape ωhich corresponds to the outer shape of said component, and is placed so as to present a narroω gap betωeeπ the block 13 and the component 7. The side of the block 13 remote from the component is planar. The effect of the transparent material of relatively high refractive index is illustrated very clearly in Figure 2b. The Figure shoωs the component 7 in a position immediately prior to facet transition. In this embodiment the extent of the sωeep or scan has been chosen at 50%. The facet ωhich reflects the radiation is shoωn to lie at an angle at ωhich the incident radiation is deflected through -X/2 degrees. As a result of the diffraction in the surface of the block .13 remote from the component 7 at the transition to air, there is obtained a beam path ωhich is deflected arcsine n.sin -= • ^ n . OC/ 2 , ωhere jn is the refractive index of the material. • ' . Figure 3 illustrates a third embodiment of the scanning arrangement according to the invention ωhich incorporates an active scanning component in the form of a mirrored drum 15 embodying a large number of facets. Of the embodiments illustrated, this embodiment is the one most preferred. The drum or component 15 is provided on its loωer part, as seen in the Figure, ωith a roω of reflecting facets 16, ωhich are mirrored on bath sides, so as to be able to reflect radiation on both their inner and their outer surfaces. These reflec- ting facets 16 may have the form of coatings applied to a body 17 made of a material ωhich is transparent to the radia¬ tion to be detected and having a high refractive index, such as germanium, silicon, cadmium telluride. The part of the body 17 located above the facets 16 is a solid of revolution, and has its centre on the axis of rotation 18. The component 15 is rotated by a motor (not shoωn). In the illustrated embodiment, the facets 16 are inclined at an angle to the axis of rotation. An image of the detector D' is focussed on the point P1 ' by the relay optics 19 close to the outside of a facet 16, so that the facet area impinged upon by the radiation reflected by the facet and passed to the detector is small in relation to the surface area of the facet. The point P1 ' is focussed on a point P2' ωithin the body 17 by a concave, apprαximative- ly spherical mirror 20 ωhich is placed outside the component 15 and ωhich is so dimensioned in relation to the mirrored body that the beam cone ωhich impinges on the inner surface of a facet located opposite that impinged upon by the radia¬ tion nearest the detector D' practically completely fills the surface of the facet, at least in the direction of rotation, and accompanies the facet during its rotation. Subsequent to reflection against the facet, the radiation is focussed on a point P3' externally of the component 15. The point P3' moves through approximately part of a circle during rotation of the drum. The peripheral surface of the body 17 is sub¬ stantially cαncentrical ωith this part-circle or arc. A further concave, substantially spherical mirror 21 makes the beam path parallel. Arranged in a pupil, through which the beam path passes during a scan, is a conventional oscilla¬ tion mirror 22. The component 15 executes a line scan and the oscillation mirror 22 executes an image scan on an image of a remote subject aligned with the scanning arrangement. The aforedescribed embodiments of the scanning arrangement according to the invention incorporate an active scanner com¬ ponent having separately provided thereon a part ωhich comp- rises a transparent material of high refractive index, and the stationary light-diffracting components of the scanner arrangement have been placed in air. The invention is not restricted to such an arrangement, hoωever, since the ωhole of the scanner arrangement can comprise a material having a refractive index ωhich is greater than 1. The principle according to the invention can be applied on existing scanner systems, so as to substantially improve performance of such systems without changing essentially the mutual position and configuration of the mirrored components incorporated by the system. This can be achieved because the beam path within an optical scanning system incorporating solely reflecting ele- meπts follows the laws of reflection and can be fully copied from a system in vacuum, air or some other gas, onto a system in ωhich the medium located betωeen the reflecting components has a high refractive index. The beam path is not changed until reaching the transition from refractive material to air, e.g. so that the field angle increases ωith a sustained aper¬ ture. Figure A illustrates an embodiment in ωhich the ωhole of the scanner arrangement, ωith the exception of the detector D" and the relay optics 23 in front of the detector, are included in a material that is transparent to the light intended and has a refractive index greater than 1. A rotatable cylindrical drum 2A provided ωith reflecting facets and having approximate¬ ly the cross-section illustrated in Figure 2a is inserted in a cylindrical recess, ωhich conforms to the outer shape of the drum and ωhich is formed in a solid, homogenous body 25 of transparent material. A πarroω gap is formed betωeen the mirrored-drum 2A and the body 25. The external dimensions of the body 25 are adapted to the beam path in the scanner arrangement, and the reflecting elements therein, such as the three concave, curved mirrors 26, 27, 28, ωhich function as positive optical elements in the beam path, and the tωo planar mirrors 29 and 30, ωhich solely deflect the beam path, are formed by applying reflec¬ ting, or mirroring, coatings to finely-polished, accurately formed parts of the surface of the body 25. An oscillation mirror 31 of substantially the same construction as the com¬ ponent A in the Figure 1 embodiment is placed in a recess in the body 25 conforming to the external contour of the mirror, in a manner to define a narroω gap betωeen the ele¬ ments 31 and 25. The scanner arrangement illustrated in Figure A operates with facet-tracking, and comprises a rotating facet-drum or polygon mirror for line scans, curved mirrors for returning the light beam to the polygon mirror, and an oscillation mirror for image deflection, substantially in accordance with the system illustrated in Figure 9 of Swedish Patent Specification No. 8201306-1. The planar mirrors 29, 30 serve solely to deflect the beam path, so as to obtain in practice an instrument which while functioning in accordance ωith the principle illustrated in the above mentioned patent specifi¬ cation also fulfills mechanical requirements ωith regard to accessibility, construction, compactness and manageability, thereby to facilitate the handling of the actual instrument itself. Figure A is included for the sole purpose of illustra- ting that the ωhole principle of the present invention can be applied to existing types of optical scanner arrangements and hence this Figure ωill not be described in detail. The important feature of the invention is that light is refracted particularly in the entrance opening 33, so that the scan angle of the ωhole system will thereby increase with a factor of π, where π_ is equal to the refractive index of the mate¬ rial in the body 25, In order to avoid, however, total reflection in the unavoid- able transitions through air in the gaps between the body 25 and the elements 2A and 31 in the embodiment illustrated in Figure A, there should be chosen in respect of the body a material ωhich has a relatively moderate refractive index. Consequently, germanium cannot be used readily in applications such as those illustrated in Figure A, unless particular atten¬ tion is paid to this fact ωhen aligning the beam path at these transition. It is also conceivable, however, to permit the transitions to take place in a liquid having a relatively high refractive index. Germanium is nevertheless an expensive material, which is a further reason for its unsuitability when manufacturing the body 25, which is of relatively large size in applications such as these. Many other materials, possibly more suitable for the present purpose, are to be found, however, such as zinc selenide ωhere π ; -; 2.A, cadmium telluride where π a^. . 1 , and so on. Many modifications are possible within the scope of the invention. For example, the body 17 of the Figure 3 embodi¬ ment may extend beneath the annulus of facets 1B, and may be incorporated in a scanner arrangement in ωhich there is more than one reflection against the inner surfaces of the facets. It ωill be understood that the outer surfaces of the facets 16 may have an outer part of transparent refractive material, so that the drum or polygon has an external appearance equal to that illustrated in Figure 2a.

Claims: AMENDED CLAIMS [received by the International Bureau on 11 August 1986 (11.08.86); original claims 1-10 replaced by amended claims 1-6 (2 pages)] 1. An optical scanner arrangement which comprises at least one rotatable unit (4; 7; 15; 24; 3D provided with at least one scanning reflecting surface (6; 8-10; 16) placed in the beam path of the scanner arrangement, where the scanning reflecting surface or surfaces is, or are, posi¬ tioned so that the beam path upon reflection thereagainst passes through a material having a refractive index grea¬ ter than 1, characterized in that during one scan the scanning reflecting surface (surfaces) rotates, (rotate), so that the angle formed thereby with an optical axis of the stationary part of the scanner arrangement varies; and in that during one scan the beam path passes into a path over the transition surface between beam path in air and beam path in said material, the transition surface in said path being concentrical with the rotational axis (C; 18) of the rotatable unit and lying In a plane normal to the rotational axis, so that during said rotation the transi¬ tion surface can be considered to form part of the statio¬ nary optical system. 2. A scanner arrangement according to Claim 1, in which the rotating unit comprises a component which co-acts with a stationarily located optical element, characterized in that said path on the transition surface is parallel with or almost parallel with a path along which an image point (P2; P2'; P3") for radiation is arranged to travel during a scan. 3. A scanner arrangement according to Claim 1 or 2, incor- porating a rotatable drum provided with an annulus compri¬ sing a plurality of reflecting facets as a scanning compo¬ nent, a detector (D') having a relay optics (19) which generates an image (PI') of the detector in the vicinity of one of the facets, and at least one concave mirror (20,21) located externally of the mirrored drum, characte¬ rized in that the facets (16) are reflective on both their inner and their outer surfaces; and in that a body (17) made of material which is transparent to the radiation in question is positioned so as to adjoin the inner surfaces of the facets (16) and is curved outwardly from the side of the facet annulus, at least on one side thereof. 4. A scanner according to Claim 3, characterized in that the reflection against the mirrored drum obtained nearest the detector (D f ) is effected against the outer surfaces of the facets (16); and in that the concave mirror (20) placed nearest the detector (D 1 ) in the beam path together with the transparent body has such dimensions and is so positioned that the beam path upon reflection against the inner surface of the facet (16) lying opposite the facet against which reflection is obtained nearest the detector takes place substantially fills-out said facet, at least in its rotational direction, and accompanies the facet during its rotation through one scan. 5. A scanner according to Claim 1 or 2, characterized in that the rotating unit (units) (7; 24; 39) is (are) placed in rotational symmetrical close proximity with a statio¬ nary unit (13; 25) comprising material with a refractive index greater than 1 (Fig. 26 and 4) 6. A scanner arrangement according to Claim 5, characteri¬ zed in that a transition surface (33) between the statio¬ nary unit and air is placed close to the lens of the scan¬ ner arrangement (Fig. 4).

Inventors: KARLSSON LARS, KARLSSON, LARS
Applicants: PHAROS AB
Application year: 1986
Main classification: G02B26
IPC codes: G02B26, H04N1