PHOTOPOLYMER COMPRISING A NEW CLASS OF PHOTO INITATOR

Abstract

The present invention relates to photopolymer comprising a photopolymerizable component and a photo initiator system. Further aspects of the present invention are a holographic media which comprises such a photopolymer, a display which comprises such a holographic media and the use of such a holographic media to make chip cards, security documents, bank notes and/or holographic optical elements especially for displays.

Claims

1.-18. (canceled)

19. A photopolymer comprising a photopolymerizable component and a photo initiator system, wherein the photo initiator system comprises a compound according to formula (I) ##STR00035## in which R.sup.1 to R.sup.6 are independently of each other hydrogen, halogen, alkyl, cyano, carboxyl, alkanoyl, aroyl, alkoxy, aryl, alkoxycarbonyl, aminocarbonyl, which can be further substituted mono- or dialkylamino; A is together with X.sup.1 and X.sup.2 and the atoms connecting them independently of each other a five- or six-membered aromatic or quasiaromatic or partially hydrogenated heterocyclic ring which may each contain 1 to 4 heteroatoms and/or be benzo- or naphtho-fused and/or be substituted by nonionic moieties, in which case the chain attaches to the ring in position 2 or 4 relative to X.sup.1, X.sup.1 is nitrogen, or X.sup.1—R.sup.7 is O or S; X.sup.2 is O, S, N—R.sup.10, C(R.sup.11).sub.2 or CR.sup.12R.sup.13; R.sup.7 and R.sup.10 are independently of each other alkyl, alkenyl, cycloalkyl or aralkyl; R.sup.11 is hydrogen or alkyl, R.sup.12 and R.sup.13 are independently of each other C.sub.1- to C.sub.4-alkyl, C.sub.3- to C.sub.6-alkenyl, C.sub.4- to C.sub.7-cycloalkyl or C.sub.7- to C.sub.10-aralkyl or conjointly form a CH.sub.2—CH.sub.2—CH.sub.2—CH.sub.2— or CH.sub.2—CH.sub.2—CH.sub.2—CH.sub.2—CH.sub.2— bridge, Q is a monovalent anion; R.sup.8 and R.sup.9 are independently of each other substituents with a Hammett substituent constant σ.sub.m>0.3 and B is a connecting group containing 1 or 2 carbon atoms.

20. The photopolymer according to claim 19, wherein R.sup.8 and R.sup.9 are independently of each other substituents with a Hammett substituent constant σ.sub.m>0.34 and <0.90.

21. The photopolymer according to claim 19, wherein R.sup.8 and R.sup.9 are independently of each other alkoxycarbonyalkyl, halogen substituted alkyl, cyano substituted alkyl, acyl substituted alkyl, amido substituted alkyl, or R.sup.8 and R.sup.9 together form imido substituted alkyl.

22. The photopolymer according to claim 19, wherein R.sup.8 and R.sup.9 are independently of each other alkoxycarbonyethyl, alkoxycarbonymethyl, halogen substituted methyl, halogen substituted ethyl, cyano substituted methyl, cyano substituted ethyl, acyl substituted methyl, acyl substituted ethyl, amido substituted ethyl, amido substituted methyl, imido substituted methyl.

23. The photopolymer according to claim 19, wherein R.sup.7 and R.sup.10 are independently of each other C.sub.1- to C.sub.16-alkyl, C.sub.3- to C.sub.6-alkenyl, C.sub.5- to C.sub.7-cycloalkyl or C.sub.7- to C.sub.16-aralkyl.

24. The photopolymer according to claim 19, wherein R.sup.11 is hydrogen or C.sub.1- to C.sub.4-alkyl, and is methyl.

25. The photopolymer according to claim 19, wherein X.sup.1 is N.

26. The photopolymer according to claim 19, wherein R.sup.6 is methyl or hydrogen.

27. The photopolymer according to claim 19 comprising 0.01 to 5.00 weight-% of the compound according to formula (I).

28. The photopolymer according to claim 19, wherein the photo initiator system further comprises at least one co-initiator, selected from borate initiators, trichloromethyl initiators, aryloxide initiators, bisimidazole initiators, ferrocene initiators, aminoalkyl initiators, oxime initiator, thiol initiators, or peroxide intiators.

29. The photopolymer according to claim 19, wherein the photopolymer further comprises matrix polymers.

30. The photopolymer according to claim 29, wherein the matrix polymers are three dimensional cross-linked and preferably three dimensional cross-linked polyurethanes.

31. The photopolymer according to claim 19, wherein it further comprises monomeric fluorourethanes and preferably a monomeric fluorourethane according to formula (II) ##STR00036## in which n is ≧1 and n is ≦8 and R.sup.14, R.sup.15, R.sup.16 are hydrogen and/or, independently of one another, linear, branched, cyclic or heterocyclic organic rests which are unsubstituted or optionally also substituted by heteroatoms, at least one of the rests R.sup.14, R.sup.15, R.sup.16 being substituted by at least one fluorine atom.

32. The photopolymer according to claim 19, wherein the photopolymerizable component comprises a mono- and/or multifunctional urethane-(meth)-acrylate.

33. A holographic media wherein it comprises a photopolymer according to claim 19.

34. The holographic media according to claim 33, wherein at least one hologram is recorded into the holographic media.

35. A display wherein it comprises a holographic media according to claim 34.

36. A method comprising utilizing the holographic media according to claim 33 to make chip cards, security documents, bank notes and/or holographic optical elements.

Description

[0134] The invention will be described in more detail by the following examples.

Starting Materials:

[0135] Starting materials to synthesize C1-C13 were prepared according to procedures reported in the literature.

[0136] In the synthesis of C1, 4′-[N,N-bis(2-chloroethyl)amino]benzaldehyde was prepared according to Huang, Chibao; Qu, Junle; Qi, Jing; Yan, Meng; Xu, Gaixia, Organic Letters, 2011, vol. 13, 1462-1465.

[0137] In the synthesis of C2, N-[2-cyanoethyl)-N-(cyanomethyl)amino]benzaldehyde was prepared according to Liao, Yi; Robinson, Bruce H. Tetrahedron Letters, 2004, vol. 45, 1473-1475.

[0138] In the synthesis of C3, C4, C7-C13, N-[2-cyanoethyl)-4-[N,N-di(ethoxycarbonylmethyl)-amino]-benzaldehyde [1208-03-3] was prepared according to Kumari, Namita; Jha, Satadru; Bhattacharya, Santanu, Journal of Organic Chemistry, 2011, vol. 76, 8215-8222.

[0139] In the synthesis of C5, C6, 3-bromo-4-[N,N-di(ethoxycarbonylinethypamino]-benzaldehyde was prepared according to Venkateswarlu, Katta; Suneel, Kanaparthy; Das, Biswanath; Reddy, Kuravallapalli Nagabhusharia; Reddy, Thummala Sreenivasulu, Synthetic Communications, 2009, vol. 39, p. 215-219.

[0140] In the synthesis of C11, 1-ethyl-4-methyl pyridinium was prepared according to Kim, Min Ji; Shin, Seung Hoon; Kim, Young Jin; Cheong, Minserk; Lee, Je Seung; Kim, Hoon Sik, Journal of Physical Chemistry B, 2013, vol. 117, 14827-14834.

[0141] In the synthesis of C12, 3-ethyl-2-methyl-4,5-dihydrothiazolium was prepared according to Zimmermann, Thomas, Journal of Heterocyclic Chemistry, 1999, vol. 36, 813-818.

[0142] In the synthesis of C13, 1-ethyl-2-methyl pyridinium was prepared according to Kim, Min Ji; Shin, Seung Hoon; Kim, Young Jin; Cheong, Minserk; Lee, Je Seung; Kim, Hoon Sik, Journal of Physical Chemistry B, 201, vol. 117, 14827-14834.

[0143] The reagents and solvents used were acquired commercially. [0144] CGI-909 Tetrabutylammonium tris(3-chloro-4-methylphenyl)(hexyl)borate, [1147315-11-4] is a product produced by BASF SE, Basle, Switzerland. [0145] Desmorapid Z Dibutyltin dilaurate [77-58-7], product from Bayer MaterialScience AG, Leverkusen, Germany. [0146] Desmodur® N 3900 Product from Bayer MaterialScience AG, Leverkusen, Germany, hexane diisocyanate-based polyisocyanate, iminooxadiazinedione content at least 30%, NCO content: 23.5%. [0147] Fomrez UL 28 Urethanization catalyst, commercial product of Momentive Performance Chemicals, Wilton, Conn., USA.

Test Methods:

Isocyanate Content (NCO Value)

[0148] The isocyanate contents reported were determined according to DIN EN ISO 11909.

Preparation of Dyes:

Synthesis of C1

[0149] 1.24 g of N,N-bis(2-chloroethyl)amino-benzaldehyde and 0.87 g of 1,3,3-trimethyl-2-methylene indoline were mixed in a flask containing 3 mL of acetic anhydride and 9 mL of acetic acid and heated at 90° C. for 6 h. The reaction mixture was poured into 50 mL of water, stirred for 30 min and filtered. A solution of 1.72 g of sodium tetraphenyl borate in 10 mL of water was added to the filtered solution and the solid which was precipitated was filtered and collected. Dried at 50° C. Reddish orange powder.

[0150] Yield 2.43 g (67%) λ.sub.max 512 nm (AN).

Synthesis of C2

[0151] 0.85 g of N-(2-cyanoethyl),N-(cyanomethyl)amino-benzaldehyde and 0.69 g of 1,3,3-trimethyl-2-methylene indoline were mixed in a flask containing 3 mL of acetic anhydride and 9 mL of acetic acid and heated at 90 C for 6 h. The reaction mixture was poured into 50 mL of water, stirred for 30 min and filtered. A solution of 1.36 g of sodium tetraphenyl borate in 10 mL of water was added to the filtered solution and the solid which was precipitated was collected by filtration. Dried at 50° C. Reddish orange powder.

[0152] Yield 1.91 g (70%) λ.sub.max 476 nm (AN).

Synthesis of C3

[0153] 2.93 g of 4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 1.73 g of 1,3,3-tris methyl-2-methylene indoline were mixed in a flask containing 6 mL of acetic anhydride and 18 mL of acetic acid and heated at 80° C. for 3 h. The reaction mixture was poured into 50 mL of water, stirred for 30 min and filtered. A solution of 3.42 g of sodium tetraphenyl borate in 25 mL of methanol was added to the filtered solution and the solid which was precipitated was collected by filtration. Orange powder.

[0154] Yield 4.76 g (62%) λ.sub.max 511 nm (AcOEt).

Synthesis of C4

[0155] 1.40 g of 4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 0.83 g of 1,3,3-trimethyl-2-methylene indoline were mixed in a flask containing 6 mL of acetic anhydride and 18 mL of acetic acid and heated at 80° C. for 3 h. The reaction mixture was poured into 100 m1, of water, stirred for 30 min and filtered. A solution of 1.71 g of sodium bis(2-ethylhexyl) sulfosuccinate in 100 mL of ethyl acetate was added to the filtered solution and the mixture was extracted with 100 mL of ethyl acetate. The ethyl acetate solution was separated, dried with magnesium sulfate and evaporated to give red oil.

[0156] Yield 3.1 g (92%) λ.sub.max 503 nm (AcOEt).

Synthesis of C5

[0157] 0.70 g of 3-bromo-4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 0.32 g of 1,3,3-trimethyl-2-methylene indoline were mixed in a flask containing 6 mL of acetic anhydride and 18 mL of acetic acid and heated at 80° C. for 3 h. The reaction mixture was poured into 50 m1, of water, stirred for 30 min and filtered. A solution of 0.64 g of sodium tetraphenyl borate in 25 mL of methanol was added to the filtered solution and the solid which was precipitated was collected by filtration. Orange powder.

[0158] Yield 1.29 g (81%) λ.sub.max 469 nm (AcOEt).

Synthesis of C6

[0159] Using 1.70 g of 3-bromo-4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 0.87 g of 1,3,3-trimethyl-2-methylene indoline as starting materials. Same procedure as synthetic example C3. Red oil.

[0160] Yield 2.8 g (88%) λ.sub.max 456 run (AcOEt).

Synthesis of C7

[0161] Using 2.0 g of 4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde, 1.41 g of 5-chloro-1,3,3-trimethyl-2-methylene indoline and 2.33 g of sodium tetraphenyl borate as starting materials. Same procedure as synthetic example C4. Reddish orange powder.

[0162] Yield 3.8 g (70%) λ.sub.max 428 nm (AcOEt).

Synthesis of C8

[0163] Using 2.0 g of 4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde, 1.41 g of 5-chloro-1,3,3-trimethyl-2-methylene indoline and 2.33 g of sodium tetraphenyl borate as starting materials. Same procedure as synthetic example C4. Reddish orange powder.

[0164] Yield 3.8 g (70%) λ.sub.max 428 nm (AcOEt).

Synthesis of C9

[0165] 2.1 g of [N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 1.5 g of 2-(1,3,3-tri-methylindolin-2-ylidene)acetonitrile were mixed in a flask containing 1.2 g of phosphoryl chloride and 20 mL of toluene and heated at 70° C. for 3 h. The reaction mixture was poured into 50 mL of water, stirred for 30 min and filtered. A solution of 2.59 g of sodium tetraphenyl borate in 25 mL of methanol was added to the filtered solution and extracted with ethyl acetate and the extract was evaporated. After purification by column chromatography (silica gel, cyclohexane/ethyl acetate V:V=1:2 as eluent) gave reddish oil.

[0166] Yield 0.7 g (23%) λ.sub.max 520 nm (AcOEt).

Synthesis of C10

[0167] 2.0 g of [N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 2.2 g of 3-ethyl-2-methyl benzthiazolium ethylsulfate were mixed in a flask containing 20 mL of pyridine and heated at 1.10° C. for 3 h. The reaction mixture was poured into 50 mL of water, stirred for 30 min and filtered. A solution of 2.46 g of sodium tetraphenyl borate in 25 mL of methanol was added to the filtered solution and the solid which was precipitated was collected by filtration and recrystallized from ethanol. Orange powder.

[0168] Yield 3.40 g (64%) λ.sub.max 496 nm (AcOEt)

Synthesis of C11

[0169] 1.39 g of 4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 1.30 g of 1-ethyl-4-methyl pyridinium ethylsulfate were mixed in a flask containing 0.39 g of ammonium acetate, 0.30 g of acetic acid and 50 mL of acetonitrile and heated at 100° C. for overnight. The reaction mixture was poured into 50 mL of water, stirred for 30 min and filtered. A solution of 2.46 g of sodium tetraphenyl borate in 25 mL of methanol was added to the filtered solution and the solid which was precipitated was collected by filtration. The solid was refluxed with 100 mL of methanol for 30 min and filtrated. The filtered solution was evaporated to yield of product as orange powder.

[0170] Yield 1.2 g, λ.sub.max 450 nm (AN).

Synthesis of C12

[0171] 1.39 g of 4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 1.30 g of 3-ethyl-2-methyl thiazolium ethylsulfate were mixed in a flask containing 0.77 g of ammonium acetate, 0.60 g of acetic acid and 50 mL of acetonitrile and heated at 100° C. for 3 days. The reaction mixture was poured into 50 mL of water, stirred for 30 min and filtered. A solution of 2.63 g of sodium tetraphenyl borate in 25 mL of methanol was added to the filtered solution and the solid which was precipitated was collected by filtration. The solid was washed with 100 mL of methanol and filtrated. The filtered solution was evaporated to yield of product as orange oil.

[0172] Yield 0.9 g, λ.sub.max 444 nm (AN).

Synthesis of C13

[0173] 0.70 g of 4-[N,N-di(ethoxycarbonylmethyl)amino]-benzaldehyde and 0.66 g of 1-ethyl-2-methyl pyridinium ethylsulfate were mixed in a flask containing 0.50 g of piperidine and 50 mL of acetonitrile and heated at 100° C. for overnight. The reaction mixture was poured into 50 mL of water, stirred for 30 min. A solution of 0.85 g of sodium tetraphenyl borate in 20 mL of methanol was added and filtered. The filtered solution was extracted with 300 mL of ethyl acetate and evaporated to yield of product as orange oil.

[0174] Yield 0.89 g (50%), λ.sub.max 422 nm (AN).

Reference Compounds RC1 and RC2

[0175] The preparation of RC1 is described in EP 10190324.3, Example 18. The preparation of RC2 is described in EP 10190324.3, Example 17.

[0176] The spectroscopic properties of the compounds C1-C13 and of the reference compounds RC1, RC2 are compiled in table 1. The solvents use were acetonitrile (AN) and ethyl acetate (AcOEt), respectively. The suitable laser wavelength given are examples for commercially well available lasers.

TABLE-US-00001 TABLE 1 Examples of Compounds Absorp- Suitable laser tion λ.sub.max wavelength NO. Dye cation Anion (Solvent) (nm) C1 [00005] [00006] 512 (AN) 532 C2 [00007] [00008] 476 (AN) 532, 473, 455 C3 [00009] [00010] 511 (AcOEt) 532 C4 [00011] [00012] 503 (AcOEt) 532 C5 [00013] [00014] 469 (AcOEt) 532, 473, 455 C6 [00015] [00016] 456 (AcOEt) 532, 473, 455 C7 [00017] [00018] 528 (AcOEt) 532 C8 [00019] [00020] 511 (AcOEt) 532 C9 [00021] [00022] 520 (AcOEt) 532 C10 [00023] [00024] 496 (AcOEt) 532, 473, 455 C11 [00025] [00026] 450 (AN) 473, 455 C12 [00027] [00028] 444 (AN) 473, 455 C13 [00029] [00030] 442 (AN) 473, 455 Reference compound RC1 [00031] [00032] 548 (AN) 532 RC2 [00033] [00034] 527 (AN) 532

Preparation a Photopolymer Compounds

Preparation of Polyol 1

[0177] In a 1 L flask, 0.18 g of tin octoate, 374.8 g of ε-caprolactone and 374.8 g of a difunctional polytetrahydrofuran polyetherpolyol (equivalent weight 500 g/mol of OH) were initially charged and heated up to 120° C. and maintained at that temperature until the solids content (proportion of nonvolatile constituents) was 99.5% by weight or higher. This was followed by cooling to obtain the product as a waxy solid.

Preparation of Acrylate 1: (phosphorus thioyltris(oxy-4,1-phenyleneiminocarbonyloxy-ethane-2,1-diyl)triacrylate)

[0178] In a 500 mL round-bottom flask, 0.1 g of 2,6-di-tert-butyl-4-methylphenol, 0.05 g of dibutyltin dilaurate (Desmorapid® Z, Bayer MaterialScience AG, Leverkusen, Germany) and also and 213.07 g of a 27% solution of tris(p-isocyanatophenyl) thiophosphate in ethyl acetate (Desmodur® RFE, product from Bayer MaterialScience AG, Leverkusen, Germany) were initially charged and heated to 60° C. Thereafter, 42.37 g of 2-hydroxyethyl acrylate were added dropwise and the mixture was further maintained at 60° C. until the isocyanate content had dropped below 0.1%. This was followed by cooling and complete removal of the ethyl acetate under reduced pressure to obtain the product as a partly crystalline solid.

Preparation of Acrylate 2: 2-({[3-(methylsulphanyl)phenyl]carbamoyl}oxy)ethyl prop-2-enoate)

[0179] In a 100 mL round-bottom flask, 0.02 g of 2,6-di-tert-butyl-4-methylphenol, 0.01 g of Desmorapid® Z, 11.7 g of 3-(methylthio)phenyl isocyanate were initially charged and heated to 60° C. Thereafter, 8.2 g of 2-hydroxyethyl acrylate were added dropwise and the mixture was further maintained at 60° C. until the isocyanate content had dropped below 0.1%, This was followed by cooling to obtain the product as a pale yellow liquid.

Preparation of Additive 1: (Bis(2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl) 2,2,4-tri-methylhexane-1,6-diyl biscarbamate)

[0180] In a round-bottom flask, 0.02 g of Desmorapid Z and 3.6 g of 2,4,4-trimethylhexanes 1,6-diisocyanate were initially charged and heated to 70° C. This was followed by the dropwise addition of 11.39 g of 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptan-1-ol and the mixture was further maintained at 70° C. until the isocyanate content had dropped below 0.1%. This was followed by cooling to obtain the product as a colorless oil.

Preparation of Holographic Media

Example Medium 1 (M1-M10) and Reference (RM1-RM2)

[0181] 3.38 g of polyol component 1 were mixed with 2.00 g of acrylate 1, 2.00 g of acrylate 2, 1.50 g of additive 1, 0.10 g of CGI 909 (product from BASF SE, Basle, Switzerland), 0.018 g of dye from Table 1 and 0.35 g of ethyl acetate at 40° C. to obtain a clear solution. The solution was then cooled down to 30° C., 0.65 g of Desmodur® N3900 (commercial product from Bayer MaterialScience AG, Leverkusen, Germany, hexane diisocyanate-based polyisocyanate, portion on iminooxadiazinedione at least 30%, NCO content: 23.5%) was added before renewed mixing. Finally, 0.01 g of Fomrez UL 28 (urethanization catalyst, commercial product of Momentive Performance Chemicals, Wilton, Conn., USA) was added and again briefly mixed in. The mixed photopolymer formulation was applied on 36 μm thick polyethylene terephthalate film. The coated film was dried for 5.8 minutes at 80° C. and finally covered with a 40 μm polyethylene film. The achieved photopolymer layer thickness was around 14 μm.

Holographic Testing:

Measurement of the Holographic Properties of Diffraction Efficiency DE and Refractive Index Contrast an of the Holographic Media by Means of Twin-Beam Interference in a Reflection Arrangement.

[0182] A holographic test setup as shown in FIG. 1 was used to measure the diffraction efficiency (DE) of the media. The beam of a DPSS laser (emission wavelength 532 nm) was converted to a parallel homogeneous beam with the aid of the spatial filter (SF) and together with the collimation lens (CL). The final cross sections of the signal and reference beam are fixed by the iris diaphragms (1). The diameter of the iris diaphragm opening is 0.4 cm. The polarization-dependent beam splitters (PBS) split the laser beam into two coherent beams of identical polarization. By means of the λ/2 plates, the power of the reference beam was set to 0.87 mW and the power of the signal beam to 1.13 mW. The powers were determined using the semiconductor detectors (D) with the sample removed. The angle of incidence (α.sub.0) of the reference beam is −21.8°; the angle of incidence (β.sub.0) of the signal beam is 41.8°. The angles are measured proceeding from the sample normal to the beam direction. According to FIG. 2, therefore, α.sub.0 has a negative sign and β.sub.0 a positive sign. At the location of the sample (medium), the interference field of the two overlapping beams produced a pattern of light and dark strips parallel to the angle bisectors of the two beams incident on the sample (reflection hologram). The strip spacing Λ, also called grating period, in the medium is 188 nm (the refractive index of the medium assumed to be ˜4.504).

[0183] FIG. 1 shows the geometry of a holographic media tester (HMT) at λ=532 nm (DPSS laser): M=mirror, S=shutter, SF=spatial filter, CL=collimator lens, λ/2=λ/2 plate, PBS=polarization-sensitive beam splitter, D=detector, I=iris diaphragm, α.sub.0=−21.8°, β.sub.0=41.8° are the angles of incidence of the coherent beams measured outside the sample (outside the medium). RD=reference direction of the turntable.

[0184] Holograms were recorded in the medium in the following manner: [0185] Both shutters (S) are opened for the exposure time t. [0186] Thereafter, with the shutters (S) closed, the medium is allowed 5 minutes for the diffusion of the as yet unpolymerized writing monomers.

[0187] The holograms recorded were then reconstructed in the following manner. The shutter of the signal beam remained closed. The shutter of the reference beam was opened. The iris diaphragm of the reference beam was closed to a diameter of <1 mm. This ensured that the beam was always completely within the previously recorded hologram for all angles of rotation (Ω) of the medium. The turntable, under computer control, swept over the angle range from Ω.sub.min to Ω.sub.max with an angle step width of 0.05°. Ω is measured from the sample normal to the reference direction of the turntable. The reference direction of the turntable is obtained when the angles of incidence of the reference beam and of the signal beam have the same absolute value on recording of the hologram, i.e. α.sub.0=−31.8° and β.sub.0=31.8°. In that case, Ω.sub.recording=0°. When α.sub.0=−21.8° and β.sub.0=41.8°, Ω.sub.recording is therefore 10°. In general, for the interference field in the course of recording of the hologram:

α.sub.0=θ.sub.0+Ω.sub.recording.

θ.sub.0 is the semiangle in the laboratory system outside the medium and, in the course of recording of the hologram:

[00001]${\theta }_0=\frac{{\alpha }_0-{\beta }_0}{2}.$

Thus, in this case, θ.sub.0=−31.8°. At each setting for the angle of rotation Ω, the powers of the beam transmitted in the zeroth order were measured by means of the corresponding detector D, and the powers of the beam diffracted in the first order by means of the detector D. The diffraction efficiency was calculated at each setting of angle Ω as the quotient of:

[00002]$\eta =\frac{P_D}{P_D+P_T}$

P.sub.D is the power in the detector for the diffracted beam and P.sub.T is the power in the detector for the transmitted beam.

[0188] By means of the process described above, the Bragg curve, which describes the diffraction efficiency η as a function of the angle of rotation Ω for the recorded hologram, was measured and saved on a computer. In addition, the intensity transmitted into the zeroth order was also recorded against the angle of rotation Ω and saved on a computer.

[0189] The maximum diffraction efficiency (DE=η.sub.max) of the hologram, i.e. the peak value thereof, was determined at Ω.sub.reconstruction. In some cases, it was necessary for this purpose to change the position of the detector for the diffracted beam in order to determine this maximum value.

[0190] The refractive index contrast Δn and the thickness d of the photopolymer layer were now determined by means of coupled wave theory (see: 1-1. Kogelnik, The Bell System Technical Journal, Volume 48, November 1969, Number 9 page 2909—page 2947) from the measured Bragg curve and the variation of the transmitted intensity with angle. In this context, it should be noted that, because of the shrinkage in thickness which occurs as a result of the photopolymerization, the strip spacing Δ′ of the hologram and the orientation of the strips (slant) can differ from the strip spacing Δ of the interference pattern and the orientation thereof. Accordingly, the angle α.sub.0′ and the corresponding angle of the turntable Ω.sub.reconstruction at which maximum diffraction efficiency is achieved will also differ from α.sub.0 and from the corresponding Ω.sub.recording. This alters the Bragg condition. This alteration is taken into account in the evaluation process. The evaluation process is described hereinafter:

[0191] All geometric parameters which relate to the recorded hologram and not to the interference pattern are shown as parameters with primes.

[0192] For the Bragg curve η(Ω) of a reflection hologram, according to Kogelnik:

[00003]$\eta =\{\begin{array}{c}\frac{1}{1-\frac{1-{\left(\xi /v\right)}^2}{{\sin }^2\left(\sqrt{{\xi }^2-v^2}\right)}},\mathrm{for}.Math..Math.v^2-{\xi }^2<0\\ \frac{1}{1+\frac{1-{\left(\xi /v\right)}^2}{{\sinh }^2\left(\sqrt{v^2-{\xi }^2}\right)}},\mathrm{for}.Math..Math.v^2-{\xi }^2\ge 0\end{array}.Math..Math.\mathrm{with}.Math.\text{:}.Math..Math.v=\frac{\pi .Math.\Delta .Math..Math.n.Math.d^{\prime }}{\lambda .Math.\sqrt{.Math.c_x.Math.c_r.Math.}}.Math..Math.\xi =-\frac{d^{\prime }}{2.Math.c_s}.Math.\mathrm{DP}.Math..Math.c_s=\cos \left({\vartheta }^{\prime }\right)-\cos \left({\psi }^{\prime }\right).Math.\frac{\lambda }{n.Math.{\Lambda }^{\prime }}.Math..Math.c_r=\cos \left({\vartheta }^{\prime }\right).Math..Math.\mathrm{DP}=\frac{\pi }{{\Lambda }^{\prime }}.Math.\left(2.Math.\cos \left({\psi }^{\prime }-{\vartheta }^{\prime }\right)-\frac{\lambda }{n.Math.{\Lambda }^{\prime }}\right).Math..Math.{\psi }^{\prime }=\frac{{\beta }^{\prime }+{\alpha }^{\prime }}{2}.Math..Math.{\Lambda }^{\prime }=\frac{\lambda }{2.Math.n.Math.\cos \left({\psi }^{\prime }-{\alpha }^{\prime }\right)}$

[0193] In the reconstruction of the hologram, as explained analogously above:

′.sub.0=η.sub.0+Ω

sin(′.sub.0)=n.Math.sin(′)

[0194] Under the Bragg condition, the “dephasing” DP=0. And it follows correspondingly that:

α′.sub.0=θ.sub.0+Ω.sub.reconstruction

sin(α′.sub.0)=n.Math.sin(α′)

[0195] The as yet unknown angle β′ can be determined from the comparison of the Bragg condition of the interference field in the course of recording of the hologram and the Bragg condition in the course of reconstruction of the hologram, assuming that only shrinkage in thickness takes place. It then follows that:

[00004]$\sin \left({\beta }^{\prime }\right)=\frac{1}{n}.Math.\left[\sin \left({\alpha }_0\right)+\sin \left({\beta }_0\right)-\sin \left({\theta }_0+{\Omega }_{\mathrm{reconstruction}}\right)\right]$

v is the grating thickness, ξ is the detuning parameter and ψ′ is the orientation (slant) of the refractive index grating which has been recorded. α′ and β′ correspond to the angles α.sub.0 and β.sub.0 of the interference field in the course of recording of the hologram, except measured in the medium and applying to the grating of the hologram (after shrinkage in thickness). n is the mean refractive index of the photopolymer and was set to 1.504. λ is the wavelength of the laser light in the vacuum.

[0196] The maximum diffraction efficiency (DE=η.sub.max), when ξ=0, is then calculated to be:

[00005]$\mathrm{DE}={\tanh }^2\left(v\right)={\tanh }^2\left(\frac{\pi .Math.\Delta .Math..Math.n.Math.d^{\prime }}{\lambda .Math.\sqrt{\cos \left({\alpha }^{\prime }\right).Math.\cos \left({\alpha }^{\prime }-2.Math..Math.\psi \right)}}\right)$

[0197] FIG. 2 shows the measured transmitted power P.sub.T (right-hand y-axis) plotted as a solid line against the angle detuning ΔΩ; the measured diffraction efficiency η (left-hand y-axis) plotted as filled circles against the angle detuning ΔΩ (to the extent allowed by the finite size of the detector), and the fitting to the Kogelnik theory as a broken line (left-hand y-axis).

[0198] The measured data for the diffraction efficiency, the theoretical Bragg curve and the transmitted intensity are, as shown in FIG. 2, plotted against the centered angle of rotation ΔΩ≡Ω.sub.reconstruction=Ω=α′.sub.0−′.sub.0, also called angle detuning.

[0199] Since DE is known, the shape of the theoretical Bragg curve, according to Kogelnik, is determined only by the thickness d′ of the photopolymer layer. An is corrected via DE for a given thickness d′ such that measurement and theory for DE are always in agreement. d′ is adjusted until the angle positions of the first secondary minima of the theoretical Bragg curve correspond to the angle positions of the first secondary maxima of the transmitted intensity, and there is additionally agreement in the full width at half maximum (FWHM) for the theoretical Bragg curve and for the transmitted intensity.

[0200] Since the direction in which a reflection hologram also rotates when reconstructed by means of an Ω scan, but the detector for the diffracted light can cover only a finite angle range, the Bragg curve of broad holograms (small d′) is not fully covered in an Ω scan, but rather only the central region, given suitable detector positioning. Therefore, the shape of the transmitted intensity, which is complementary to the Bragg curve, is additionally employed for adjustment of the layer thickness d′.

[0201] FIG. 2 shows the plot of the Bragg curve η according to the coupled wave theory (broken line), the measured diffraction efficiency (filled circles) and the transmitted power (black solid line) against the angle detuning ΔΩ.

[0202] For a formulation, this procedure was repeated, possibly several times, for different exposure times t on different media, in order to find the mean energy dose of the incident laser beam in the course of recording of the hologram at which DE reaches the saturation value. The mean energy dose E is calculated as follows from the powers of the two component beams assigned to the angles α.sub.0 and β.sub.0 (reference beam where P.sub.r=0.87 mW and signal beam where P.sub.s=1.13 mW), the exposure time t and the diameter of the iris diaphragm (0.4 cm):

[00006]$E\left(\mathrm{mJ}/{\mathrm{cm}}^2\right)=\frac{2.Math.\left[P_r+P_s\right].Math.t\left(s\right)}{\pi .Math.{0.4}^2.Math.{\mathrm{cm}}^2}$

[0203] The powers of the component beams were adjusted such that the same power density is attained in the medium at the angles α.sub.0 and β.sub.0 used.

[0204] In an alternative setup according to FIG. 1 a DPSS laser with an emission wavelength λ of 473 nm could be used. In this case α.sub.0=−21.8° and β.sub.0=41.8° are same as if using the emission wavelength λ=532 nm but the reference beam power was set to P.sub.r=1.31 mW and signal beam power was set to P.sub.s=1.69 mW.

[0205] The media obtained as described were subsequently tested for their holographic properties in the manner described above using a measuring arrangement as FIG. 1. The following measurements were obtained for Δn at dose E [mJ/cm.sup.2]:

TABLE-US-00002 Laser used to record Dose Dye Hologram (nm) DE Δn (mJ/cm.sup.2) Example test no M1 C1 532 0.99 0.030 31.8 M2 C3 532 0.97 0.033 31.8 M3 C4 532 0.97 0.031 31.8 M4 C5 532 0.98 0.027 31.8 M5 C5 473 0.93 0.031 95.5 M6 C6 532 0.95 0.031 31.8 M7 C6 473 1.00 0.036 23.9 M8 C7 532 0.98 0.033 31.8 M9 C8 532 0.95 0.027 31.8 M10 C10 532 0.96 0.030 31.8 Reference RM1 RC1 532 0.93 0.018 31.8 RM2 RC2 532 0.97 0.025 31.8

[0206] The above experimental data shows that the inventive photopolymers possess a higher sensitivity to light compared to known holographic media, i.e. they have a higher DE and Δn if the same dose as for the references was used during holographic recording.