Heat mode recording element

A heat mode recording element is disclosed comprising a metal recording layer, preferably a bismuth layer, and a layer containing a roughening agent underneath the metal layer. An image is formed by exposure with intense laser light and the presence of the roughening agent reduces or eliminates interference patterns. In an alternative embodiment a roughened support is used instead of a layer containing a roughening agent.

FIELD OF THE INVENTION
 The present invention relates to an improved heat mode recording element
 containing a thin metal recording layer.
 BACKGROUND OF THE INVENTION
 Recording materials have been disclosed on which records are made thermally
 by the use of intense radiation like laser beams having a high energy
 density. In such thermal recording or heat mode recording materials
 information is recorded by creating differences in reflection and/or in
 transmission optical density on the recording layer. The recording layer
 has high optical density and absorbs radiation beams which impinge
 thereon. The conversion of radiation into heat brings about a local
 temperature rise, causing a thermal change such as evaporation or ablation
 to take place in the recording layer. As a result, the irradiated parts of
 the recording layer are totally or partially removed, and a difference in
 optical density is formed between the irradiated parts and the
 unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and
 4,291,119 and British Pat. No. 2,026,346)
 The recording layer of such heat mode recording materials is usually made
 of metals, dyes, or polymers. Recording materials like this are described
 in "Electron, Ion and Laser Beam Technology", by M. L. Levene et al.; The
 Proceedings of the Eleventh Symposium (1969); "Electronics" (Mar. 18,
 1968), P. 50; "The Bell System Technical Journal", by D. Maydan, Vol. 50
 (1971), P. 1761; and "Science", by C. O. Carlson, Vol. 154 (1966), P.
 1550.
 Recording on such thermal recording materials is usually accomplished by
 converting the information to be recorded into electrical time series
 signals and scanning the recording material with a laser beam which is
 modulated in accordance with the signals. This method is advantageous in
 that recording images can be obtained on real time (i.e. instantaneously).
 Recording materials of this type are called "direct read after write"
 (DRAW) materials. DRAW recording materials can be used as a medium for
 recording an imagewise modulated laser beam to produce a human readable or
 machine readable record. Human readable records are e.g. micro-images that
 can be read on enlargement and projection. An example of a machine
 readable DRAW recording material is the optical disc. To date for the
 production of optical discs tellurium and its alloys have been used most
 widely to form highly reflective thin metal films wherein heating with
 laser beam locally reduces reflectivity by pit formation (ref. e.g. the
 periodical `Physik in unserer Zeit`, 15. Jahrg. 1984/Nr. 5, 129-130 the
 article "Optische Datenspeicher" by Jochen Fricke). Tellurium is toxic and
 has poor archival properties because of its sensitivity to oxygen and
 humidity. Other metals suited for use in DRAW heat-mode recording are
 given in U.S. Pat. No. 4,499,178 and U.S. Pat. No. 4,388,400. To avoid the
 toxicity problem other relatively low melting metals such as bismuth have
 been introduced in the production of a heat-mode recording layer. By
 exposing such a recording element very shortly by pulses of a high-power
 laser the radiation is converted into heat on striking the bismuth layer
 surface. As a result the writing spot ablates or melts a small amount of
 the bismuth layer. On melting the layer contracts on the heated spot by
 surface tension thus forming small cavitations or holes. As a result light
 can pass through these cavitations and the density is lowered to a certain
 Dmin value depending on the laser energy irradiated.
 Heat mode recording materials usually do not require development and fixing
 processes and do not require darkroom operations because of their
 insensitivity to room light. Therefore they constitute a valuable
 alternative to conventional photosensitive materials based on silver
 halide emulsions, e.g. for phototype-setting or image-setting
 applications. As is generally known silver halide materials have the
 advantage of high potential intrinsic sensitivity and excellent image
 quality. On the other hand they show the drawback of requiring several wet
 processing steps employing chemical ingredients which are suspect from an
 ecological point of view. For instance the commonly used developing agent
 hydroquinone is allergenic and the biodegradation of disposed phenidone is
 too slow. As a consequence it is undesirable that depleted solutions of
 this kind would be discharged into the public sewerage; they have to be
 collected and destroyed by combustion, a cumbersome and expensive process.
 However, recording elements based on a thin metal layer show the drawback
 that the thin metal film may reflect more than 50% of the laser radiation,
 wasting the energy of the laser radiation. Accordingly, such material may
 require a substantial amount of energy for recording. Therefore, a high
 output laser light source is required if records are to be made by
 high-speed scanning. Methods to reduce reflectance are proposed in the
 Japanese Unexamined Patent Publications Nos. 40479/71 and 74632/76.
 However the proposed solutions have other drawbacks. Moreover, due to the
 high specular reflectance interference patterns arise with periods
 depending on the thickness of the protective cover usually present to
 protect the scratch-sensitive metal layer. As a consequence of these
 interference phenomena the finished image has an uneven and splodgy
 appearance.
 It is an object of the present invention to provide an improved heat mode
 recording element based on a thin metal layer which shows reduced or no
 interference patterns on laser recording.
 It is a further object of the present invention to provide a method for the
 formation of a heat mode image which has no uneven or splodgy appearance.
 SUMMARY OF THE INVENTION
 The objects of the present invention are realized by providing a heat mode
 recording element comprising, in order:
 (a) a support,
 (b) a layer containing a roughening agent,
 (c) a metal recording layer,
 (d) a protective element.
 In a preferred embodiment the metal layer is a vacuum-deposited thin
 bismuth layer having a thickness preferably comprised between 0.1 and 0.6
 .mu.m. The average particle size of the roughening agent preferably ranges
 between 0.3 and 2.0 .mu.m, most preferably around 1.0 .mu.m. A preferred
 roughening agent is composed of polymethylmethacrylate beads.
 The layer containing the roughening agent can be the subbing layer of the
 support or can be an extra layer between the subbing layer and the metal
 layer.
 The protective element preferably comprises a cover sheet and an adhesive
 layer.
 DETAILED DESCRIPTION OF THE INVENTION
 The different elements constituting the heat mode recording material of the
 present invention will now be explained in more detail.
 Although the support of the heat mode element can in principle be an opaque
 paper base preference is given to a transparent organic resin support.
 Useful transparent organic resin supports include e.g. cellulose nitrate
 film. cellulose acetate film, polyvinylacetal film, polystyrene film,
 polyethylene terephthalate film, polycarbonate film, polyvinylchloride
 film or poly-Alpha-olefin films such as polyethylene or polypropylene
 film. The thickness of such organic resin film is preferably comprised
 between 0.07 and 0.35 mm. In a most preferred embodiment of the present
 invention the support is a polyethylene terephthalate layer provided with
 a subbing layer.
 The layer containing the roughening agent can be the subbing layer itself
 applied to the support or can be an extra layer between the subbing layer
 and the metal layer.
 In principle layer (b) can contain no binder at all but preferably it
 contains a binder. Tis layer (b) can be coated in principle from an
 organic solvent or from an aqueous medium depending on the chemical nature
 of the binder. Organic solvent-soluble binders include e.g. polymers
 derived from .alpha.,.beta.-ethylenically unsaturated compounds such as
 e.g. polymethyl methacrylate, polyvinyl chloride, a vinylidene
 chloride-vinyl chloride copolymer, polyvinyl acetate, a vinyl
 acetate-vinyl chloride copolymer, a vinylidene chloride-acrylonitrile
 copolymer, a styrene-acrylonitrile copolymer, chlorinated polyethylene,
 chlorinated polypropylene, a polyester, a polyamide, polyvinylbutyral etc.
 Several organic solvents can be used for dissolving and coating these
 polymers. On the other hand water-soluble binders coatable from an aqueous
 medium can be used, e.g. gelatin, polyvinyl alcohol, polyvinyl
 pyrrolidone, carboxymethyl cellulose, methyl cellulose, ethyl cellulose,
 gum arabic, casein, different kinds of water-soluble latices, etc.
 In a preferred embodiment the roughening agent is incorporated in the
 subbing layer applied to the polyester support, in other words this
 subbing layer constitutes layer (b). This subbing layer can be applied
 before or after stretching of the polyester film support. The polyester
 film support is preferably biaxially stretched at an elevated temperature
 of e.g. 70-120.degree. C., reducing its thickness by about 1/2 to 1/9 or
 more and increasing its area 2 to 9 times. The stretching may be
 accomplished in two stages, transversal and longitudinal in either order
 or simultaneously. The subbing layer is preferably applied by aqueous
 coating between the longitudinal and transversal stretch, in a thickness
 of 0.1 to 5 .mu.m. In case of a bismuth recording layer the subbing layer
 preferably contains, as described in European Patent Application EP 0 464
 906, a homopolymer or copolymer of a monomer comprising covalently bound
 chlorine. Examples of said homopolymers or copolymers suitable for use in
 the subbing layer are e.g. polyvinyl chloride, polyvinylidene chloride, a
 copolymer of vinylidene chloride, an acrylic ester and itaconic acid, a
 copolymer of vinyl chloride and vinylidene chloride, a copolymer of vinyl
 chloride and vinyl acetate, a copolymer of butylacrylate, vinyl acetate
 and vinyl chloride or vinylidene chloride, a copolymer of vinyl chloride,
 vinylidene chloride and itaconic acid, a copolymer of vinyl chloride,
 vinyl acetate and vinyl alcohol etc.. Polymers that are water dispersable
 are preferred since they allow aqueous coating of the subbing layer which
 is ecologically advantageous.
 Said homopolymer or copolymer may be prepared by various polymerization
 methods of the constituting monomers. For example, the polymerization may
 be conducted in aqueous dispersion containing a catalyst and activator,
 e.g., sodium persulphate and meta sodium bisulphite, and an emulsifying
 and/or dispersing agent. Alternatively, the homopolymers or copolymers
 used with the present invention may be prepared by polymerization of the
 monomeric components in the bulk without added diluent, or the monomers
 may be reacted in appropriate organic solvent reaction media.
 The roughening agent incorporated in layer (b)--for many actual substances
 this term will be equivalent to the more familiar terms "matting agent" or
 "spacing agent", but the term is chosen for its functional aspect--must
 fulfil several requirements for the successful practice of the present
 invention. Chemical nature, concentration and particle distribution of the
 roughening agent must be chosen in such a way that a certain degree of
 unevenness can be introduced in the metal recording layer. It is shown
 that this unevenness can reduce the occurrence of interference patterns
 because the reflectance gets more diffuse. It will be clear that the
 roughening agent must be closely packed in the layer. It will also be
 easily understood that the thickness of layer (b), the average particle
 size and the coverage of the roughening agent must be tuned to each other
 in such a way that a sufficient number of the roughening particles must
 protrude above the interface layer (b)/metal layer in order to induce
 local deformation spots into this metal layer. When the average particle
 size is too low the roughening agent will not be able to introduce
 unevenness in the metal layer. When the average particle size is too great
 too high a coverage will be required which would make layer (b) too thick.
 So it is clear that an optimal particle size should be chosen for the
 roughening agent and that this optimum will depend on the mechanical
 strength of the metal layer and therefore on its thickness. For the
 preferred embodiment of a bismuth layer with a thickness of about 0.3
 .mu.m the average particle size of the roughening agent preferably ranges
 from 0.3 to 2.0 .mu.m, and is most preferably about 1.0 .mu.m. In this
 case the coverage of the roughening agent preferably ranges from 0.05 to
 1.0 g/m.sup.2, and is most preferably about 0.6 g/m.sup.2.
 It will also be clear that the optimal amount/m.sup.2 of the binder will be
 dependent on the average particle size of the roughening agent.
 The degree of roughness of layer (b) is best characterized by the so-called
 R.sub.a value. This so-called average roughness value is defined as the
 arithmic average value of the absolute amounts of all the measured
 distances of the roughness profile from the middle line within the
 measured interval. Layer (b) preferably has a R.sub.a value of at least
 0.2 .mu.m.
 The roughening agent can be chosen from a wide variety of chemical classes
 and commercial products provided the particles chosen show an excellent
 mechanical and thermal stability. Preferred roughening agents include
 following:
 the spherical polymeric beads disclosed in U.S. Pat. No. 4,861,818;
 the alkali-soluble beads of U.S. Pat. No. 4,906,560 and EP 0 584 407;
 the insoluble polymeric beads disclosed in EP 0 466 982;
 polymethylmethacrylate beads;
 copolymers of methacrylic acid with methyl- or ethylmethacrylate;
 TOSPEARL siloxane particles (e.g. types T105, T108, T103, T120), marketed
 by Toshiba Co;
 SEAHOSTAR polysiloxane--silica particles (e.g. type KE-P50), marketed by
 Nippon Shokubai Co;
 ROPAQUE particles, being polymeric hollow spherical core/sheat beads,
 marketed by Rohm and Haas Co, and described e.g. is U.S. Pat. Nos.
 4,427,836, 4,468,498 and 4,469,825;
 ABD PULVER, marketed by BASF AG;
 CHEMIPEARL, spherical poymeric particles, marketed by Misui Petrochemical
 Industries, Ltd.
 In principle, a thin intermediate layer can be applied between layer (b)
 and the metal recording layer for reasons of protection against physical
 damage. In this case the thin intermediate layer is coated together with
 layer (b) by slide hopper coating. It can contain the same kinds of binder
 as layer (b) at a coverage of lower than 1 g/m.sup.2 in order not to lose
 the roughening effect. However in a preferred embodiment there is no such
 an intermediate layer and the metal recording layer is positioned
 immediately on top of layer (b) in order to get the full effect of the
 unevenness introduced by the roughening agent.
 Possible metals for the recording layers in this invention include Mg, Sc,
 Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Ru, Rh, Pd, Ir,
 Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Si, Ge, Sn, As, Sb, Bi, Se, Te. These
 metals can be used alone or as a mixture or alloy of at least two metals
 thereof. Due to their low melting point Mg, Zn, In, Sn, Bi and Te are
 preferred. The most preferred metal for the practice of this invention is
 Bi.
 The metal recording layer may be applied on top of the layer containing the
 roughening agent by vapor deposition, sputtering, ion plating, chemical
 vapor deposition, electrolytic plating, or electroless plating. In the
 preferred case of Bi the recording layer is preferably provided by vapor
 deposition in vacuo. A method and an apparatus for such a deposition are
 disclosed in EP 0 384 041.
 The thickness of this Bi layer is preferably comprised between 0.1 and 0.6
 .mu.m. When this thickness is too low the recorded images do not have
 sufficient density. When on the other hand the thickness is too high the
 sensitivity tends to decrease and the minimal density, i.e. the density
 after laser recording on the exposed areas tends to be higher.
 Since the metal layer is very sensitive to mechanical damage a protective
 element must be provided on top of the metal layer. In a preferred
 embodiment this protective element comprises a transparent organic resin,
 acting as cover sheet, and an adhesive layer. A method for applying such a
 protective element by lamination in the same vacuum environment as wherein
 the deposition of the metal layer took place is disclosed in EP 0 384 041,
 cited above. The cover sheet can be chosen from the group of polymeric
 resins usable for the support of the heat mode element. In a preferred
 embodiment the cover sheet is also polyethylene terephthalate but
 preferably substantially thinner than the polyethylene terephthalate of
 the support.
 For the adherence of the hard protective outermost resin layer to the heat
 mode recording layer preferably a layer of a pressure-sensitive adhesive
 resin can be used. Examples of such resins are described in U.S. Pat. No.
 4,033,770 for use in the production of adhesive transfers (decalcomanias)
 by the silver complex diffusion transfer process, in the Canadian Patent
 728,607 and in the U.S. Pat. No. 3,131,106.
 Pressure-sensitive adhesives are usually composed of (a) thermoplastic
 polymer(s) having some elasticity and tackiness at room temperature (about
 20.degree. C.), which is controlled optionally with a plasticizer and/or
 tackifying resin. A thermoplastic polymer is completely plastic if there
 is no recovery on removal of stress and completely elastic if recovery is
 instantaneous and complete.
 Particularly suitable pressure-sensitive adhesives are selected from the
 group of polyterpene resins, low density polyethylene, a
 copoly(ethylene/vinyl acetate), a poly(C.sub.1 -C.sub.16)alkyl acrylate, a
 mixture of poly(C.sub.1 -C.sub.16)alkyl acrylate with polyvinyl acetate,
 and copoly(vinylacetate-acrylate) being tacky at 20.degree. C.
 In the production of a pressure-adhesive layer an intrinsically non-tacky
 polymer may be tackified by the adding of a tackifying substance, e.g.
 plasticizer or other tackifying resin.
 Examples of suitable tackifying resins are the terpene tackifying resins
 described in the periodical "Adhesives Age", Vol. 31, No. 12, November
 1988, p. 28-29.
 According to another embodiment the protective element is laminated or
 adhered to the heat-mode recording layer by means of a heat-sensitive also
 called heat-activatable adhesive layer or thermoadhesive layer, examples
 of which are described also in U.S. Pat. No. 4,033,770. In such embodiment
 the laminating material consisting of adhesive layer and abrasion
 resistant protective layer and/or the recording web material to be
 protected by lamination are heated in their contacting area to a
 temperature beyond the softening point of the adhesive. Heat may be
 supplied by electrical energy to at least one of the rollers between which
 the laminate is formed or it may be supplied by means of infra-red
 radiation. The laminating may proceed likewise by heat generated by
 high-frequency micro-waves as described e.g. in published EP-A 0 278 818
 directed to a method for applying a plastic covering layer to documents.
 A survey of pressure and/or heat-sensitive adhesives is given by J. Shields
 in "Adhesives Handbook", 3rd. ed. (1984). Butterworths--London, Boston,
 and by Ernest W. Flick in "Handbook of Adhesive Raw Materials"(1982).
 Noyens Publications, Park Ridge, N.J.--USA.
 The adhesive layer may be heat-curable or ultra-violet radiation curable.
 For heat-curable organic resins and curing agents therefore reference is
 made e.g. to the above mentioned "Handbook of Adhesive Raw Materials", and
 for UV curable resin layers reference is made e.g. to "UV Curing: Science
 and Technology"--Technology Marketing Corporation, 642 Westover
 Road--Stanford--Conn.--USA--06902 (1979). However, in heat mode recording
 with a meltable metal layer preference is given to an easily deformable
 adhesive layer so that it does not form a hindrance for the formation of
 small metal globules in the areas of the recording layer struck by high
 intensity radiation energy laser energy. The easy deformability of the
 adhesive interlayer is in favour of recording sensitivity.
 For several applications of a heat mode DRAW material such as the one of
 the present invention the dimensional stability is of utmost importance.
 Fields of application where the requirements for dimensional stability are
 very stringent are e.g. those where the heat moded image serves as an
 intermediate for the exposure of a lithographic printing plate, or as a
 master mask for the production of microelectronic integrated circuits or
 printed circuit boards (PCB). To improve the dimensional stablility one or
 more barrier layers can be applied onto the heat mode recording element
 retarding the uptake of water vapour as disclosed in European Patent
 Application Appl. No. 93201366, filed May 12, 1993. In a preferred
 embodiment this barrier layer is a vapour-deposited glass layer
 substantially composed of SiO.sub.x, x ranging from 1.2. to 1.8. Such a
 barrier layer can be applied to one of or to both outermost sides of the
 complete finished heat mode element of the present invention, or to one of
 or to both sides of the support of the recording element before the
 element is further produced.
 For the formation of a heat mode image using the element of the present
 invention any laser can be used which provides enough energy needed for
 the production of sufficient heat for this particular process of image
 formation. In a preferred embodiment a powerful infra-red laser is used.
 Most preferably a Nd-YLF laser is used emitting at 1053 nm.

The present invention will be illustrated now by the following example
 without however being limited thereto.
 EXAMPLE
 Preparation of Heat Mode Recording Elements
 The following substrates for the deposition of a bismuth layer were
 prepared:
 reference 1 (R-1): this substrate consisted of a polyethylene terphthalate
 support sheet subbed with a layer containing 0.16 g/m.sup.2 of a copolymer
 consisting of 88 mole % of vinylidene chloride, 10 mole % of
 methylacrylate and 2 mole % of itaconic acid, serving as a binder, and
 also containing 0.04 g/m.sup.2 of SiO.sub.2 with an average particle size
 of 0.1 .mu.m. A backing layer was also present containing, as antistatic
 element 5.2 mg/m.sup.2 of an epoxysilane hydrolyzed in polysulphonic acid,
 and 5 mg/m.sup.2 of SiO.sub.2 with an average particle size of 0.1 .mu.m.
 This reference 1 element was taken from current manufacturing by
 Agfa-Gevaert N.V.;
 reference 2 (R-2): an aqueous coating solution was prepared containing the
 same copolymer consisting of 88 mole % of vinylidene chloride, 10 mole %
 of methylacrylate and 2 mole % of itaconic acid, serving as a binder, and
 two conventional commercial wetting agents. This solution was coated on
 top of a subbed polyethylene terephthalate substrate corresponding to
 reference 1. After drying this extra layer contained 0.45 g/m.sup.2 of the
 copolymer;
 invention 1 (I-1): this substrate was similar to reference 2 with the
 exception that the extra layer contained only 0.16 g/m.sup.2 of the
 copolymeric binder and 0.09 g/m.sup.2 of roughening agent ROPAQUE OP62 LO,
 having an average particle size of 0.5 .mu.m, purchased from Rohm and Haas
 Co.
 invention 2 (I-2): this substrate was similar to reference 2 with the
 exception that the extra layer further contained a roughening agent
 consisting of polymethylmethacrylate beads, having an average particle
 size diameter of 1.0 .mu.m, at a coverage of 0.59 g/m.sup.2.
 To these four substrates a bismuth layer of 0.3 .mu.m thickness was applied
 by vacuum-deposition (vacuum of 10.sup.-2 Pa) in a Leybold apparatus,
 after a weak corona discharge of 0.05 Ampere. To the bismuth layer was
 laminated in vacuo a protective element consisting of a 8 .mu.m thick
 adhesive layer containing copoly(butylacrylate-vinylacetate), and of a
 cover sheet being a 12 .mu.m thick polyethylene terephthalate foil.
 Image Formation and Evaluation of Image Quality
 The four recording elements were exposed by means of a high-power internal
 drum laser recorder with following characteristics:
 laser type: Nd-YLF laser;
 wavelength: 1053 nm;
 spot diameter (1/e.sup.2): 18 .mu.m;
 pitch: 10.58 .mu.m;
 velocity of the rotating scanning mirror: 1663 rpm
 drum radius: 188.6 mm;
 The elements were exposed through the protective laminate side. Full areas
 and separate scan lines (1 on/10 off) were exposed at different laser
 powers ranging between 480 mW and 1330 mW. The obtained image quality was
 evaluated as follows.
 (a) Dmax and Dmin
 The densities of exposed and unexposed full areas were measured with a
 MACKBETH TD904 densitometer equipped with a UV-filter and a measuring spot
 of 3 mm. The results (mean values of different measurements, expressed in
 thousands) are summarized in table 1.
 TABLE 1
 Dmin
 480 700 900 1110 1260 1330
 element Dmax mW mW mW mW mW mW
 R-1 362 290 133 53 37 30 29
 R-2 390 345 98 46 35 31 32
 I-1 380 332 142 49 34 32 31
 I-2 289 233 108 49 34 32 31
 From table 1 it is clear that at least a laser power output, measured on
 the recording element plane, of 1110 mW is necessary to get a maximal
 density differentiation between exposed and unexposed areas. At this and
 above this power the presence or absence of a roughening agent has little
 influence on Dmin.
 (b) Macroscopic Evaluation of Homogeneity
 The macroscopic homogeneity was defined as the minimal laser power at which
 the full areas and lines showed no interference patterns or interference
 fringes any more. These values are summarized in table 2:
 TABLE 2
 homogeneity
 element full areas lines
 R-1 &gt;1330 &gt;1330
 R-2 &gt;1330 &gt;1330
 I-1 1200-1260 &gt;1330
 I-2 1110-1200 1110-1200
 The interference phenomena disappeared at a lower laser power the recording
 elements contained a roughening agent. The best result was obtained with
 roughening agent polymethylmethacrylate having an average grain size of
 1.0 .mu.m.
 (c) Microscopic Evaluation of Homogeneity
 The recorded full areas and lines were enlarged 100 fold by means of a
 Nikon microscope and photographed so that individual scan lines became
 visible.
 An arbitrary qualification ranging from 0 to 5 was assigned to the physical
 quality of the recorded full areas at 1200 mW power and 1330 power. This
 qualification range had following meaning:
 1: inhomogeneous, very intense interference spots;
 2: inhomogeneous, rather intense interference spots;
 3: rather homogeneous, still slight interference;
 4: practically completely homogeneous; sometimes very slight interference;
 5: very homogeneous; no interference.
 The quality results are summarized in table 3 :
 TABLE 3
 element quality at 1200 mw quality at 1330 mW
 R-1 1 2
 R-2 2 3
 I-1 4 5
 I-2 5 5
 The table clearly illustrates the superior results obtained with the
 elements according to the present invention.
 The recorded lines showed local variations in width due to local variations
 of laser power as a consequence of interference. Table 4 summarizes the
 minimal and maximal values of the line width (in .mu.m) obtained with
 laser powers varying between 1110 and 1330 mW.
 TABLE 4
 elements
 R-1 R-2 I-1 I-2
 Power min-max min-max min-max min-max
 1010 0-8 0-8 7-10 7-9
 1110 0-8 5-11 7-10 8-9
 1200 3-10 7-11 7-11 9.5-10
 1260 5-11 7-11 9-12 10-11
 1300 7-11.5 10-12 11-13 11-11.5
 1330 7-12 10-12 11-13 11-12
 It is clear from the table that the difference between minimal and maximal
 line width is smaller with the elements according to the invention.