FIELD OF THE INVENTION

The invention relates to a relief optical device for creating non-spectral colour images and it will be applicable as a security feature to protect goods and documents against falsification and counterfeiting.

BACKGROUND OF THE INVENTION

There are relief optical devices a, consisting of holograms and diffraction patterns which create colour images owing to the diffraction and interference phenomena. The iridescent colours of such images vary within the whole visible spectral range upon changing the observation and/or the incidence beam angle. The spectral colours are saturated rainbow colours, with one, predominating wavelength in the spectrum, e.g. red (630nm), or green (550nm), or blue (450nm) etc., and a spectral image is the image, formed by means of one spectral colour. The non-spectral colours are obtained as a result of mixing of two or more spectral colours, whereby an unsaturated colour, e.g. brown, violet, crimson etc. is obtained as a consequence thereof, and a non-spectral image is the image, tinted by the non-spectral colours. Visually these kind of images strongly distinguish themselves from the printed images, obtained by means of pigments and ink, while the complexity of technology also determines their application for the protection against counterfeiting.lt is known from the prior art Relief optical devices with non-periodic scattering structures , which can display images in nonspectral colours. IV

A disadvantage of the indicated devices is their low diffraction efficiency, which in some cases amounts only to 6,4%.

Another disadvantage is that the non-spectral colours are formed only in zero order - the direction of reflection.

DISCLOSURE OF THE INVENTION

The goal of the present invention is to create a relief optical device for displaying bright non-spectral images. The task is solved by the proposed relief optical device for creating nonspectral colour images, including a set of areas of unspecified shape, filled with relief periodic diffraction patterns, having two levels of depth, whereby according to the invention the period of the patterns in these areas varies from an area to an area in random order within the range from 0,6um to 5 μιη, preferably with a step less than 0,050um, the angle of inclination of the pattern grooves varies from an area to an area in random order from 0 to 180 degrees with a step less than 0,7 degrees, the size of the areas is within the range from 5μπι to SOum and the pattern relief depth, which preferably varies within the limits of 0,1 um - 0,8um, determines the colour hue of the non-spectral colours.

In accordance with one embodiment of the invention, the relief periodic diffraction patterns have more than two discrete levels of depth, which forms a complicated pattern's groove profile shape.

According to another embodiment of the invention, the relief of the diffraction patterns is formed at the boundary of a metal and a transparent medium, preferably air or polymer material.

In accordance with a further embodiment of the invention, the relief of the diffraction patterns is formed at the boundary of two transparent media, having a different refractive indexes.

The proposed relief optical device, object of the present invention, creates bright images of non-spectral colours.

EXPLANATION OF THE ATTACHED FIGURES

An exemplary embodiment of the present invention is shown in the attached figures explaining it, but not limited to, where:

Figure 1 depicts a schematic diagram of the relief optical device;

Figure 2: The dependence J _q (m/2) (Bessel function) of the intensity of the light, diffracted on a phase sinusoidal shape grating into particular order (q), on the phase modulation value (m); Figure 3: Spectral dependence of the zero diffraction order for a binary grating at different depths;

Figure 4 : Example of mixing red and blue colours in order to obtain magenta colour in the first order of diffraction from patterns with a period of 0,8um and l,16uin;

Figure 5: Example of mixing the complimentary colours: cyan (470nm) colour and yellow (570nm) colour in order to obtain white colour in the first order of diflraction;

Figure 6: Examples of patterns with different groove profiles;

Figure 7: Spectral dependence of a pattern with different groove profiles;

Figure 8: Example of obtaining rose colour in the zero order by means of changing the dimensional ratio between exposed and non-exposed parts of the pattern;

Figure 9: Relief of a reflective optical device, formed on a polymer material and a metal coating laid on;

Figure 10: Relief of a transparent optical device, formed on a polymer material and a dielectric coating laid on.

EXEMPLARY EMBODIMENT OF THE INVENTION

According to the enclosed Figure 1, the proposed relief optical device for creating non-spectral colour images 1, includes a set of relief phase diffraction patterns 2 with two levels of depth, which are calculated and recorded for each particular case. The relief phase diffraction patterns 2 are filled with areas 3 of unspecified shape. Thereby the period T of the diffraction patterns 2 in these areas 3 varies from an area to an area in random order within the range from 0,6um to 5,0um, preferably with a step less than 0,050um, the angle of inclination of the grooves φ of the patterns 2 varies from an area to an area in random order from 0 to 180 degrees with a step less than 0,7 degrees, and the size of the areas 3 is within the range from 5 to 50um. The theoretical diffraction efficiency of the phase pattern depends on the groove profile, reaching through sinusoidal groove profile 33,8 % /2/, and in case of rectangular profile - 41 % /3/. In this way the periodic structures allow to obtain much more brighter non-spectral colours, than in the case of using non-periodic structures.

The diffraction patterns have several diffraction orders. Zero order: the direction of the light distribution coincides with the beam passed over, or with the reflected beam. Within the zero order the colour is formed on account of substracting of several spectral components. Within the remaining diffraction orders (1,2,3....) the non-spectral colour is formed due to the superposition of light from different patterns. Such parameters, like pattern space period, relief profile and groove width-to-period ratio are used in the process control of the spectral pattern features.

Practically, a pattern with a period from 0,2um to 2,0um is used to master the optical diffraction devices. The scalar diffraction theory is inapplicable to the analysis of such structures. Therefore, their analysis is done by means of the numerical solution of the Maxwell's equation. Almost all charts in this work are obtained by methods of such kind.

It is known, that the efficiency of the relief phase structures depends on the magnitude of the phase modulation, which is determined by the formula:

According to the diffraction theory, the intensity of the light, which diffracts from a phase sinusoidal pattern to the particular order, is proportional to the square of the Bessel function of the relevant phase modulation . This dependence is shown schematically in Figure 2. If the phase modulation corresponds to the maximum of the diffraction efficiency in the q-th order for a definite wavelength of light, the phase modulation will not be optimal for the other wavelengths and the light with mis wavelength will be redistributed to other diffraction orders. Thereby, the diffraction pattern can operate as a spectral filter.

A reflective metal pattern is considered for all examples below, unless otherwise indicated.

In case of changing the relief depth h of a binary pattern, its spectral characteristics also vary. Figure 3 depicts an example of a zero order spectral dependence of diffraction for a pattern with a period 0,8um at a different modulation depth. At a depth h = 0,25um there is a reddish colour in the zero order, at h = 0,35 um - a greenish colour, at h = 0,45 um - a magenta colour.

In order to obtain non-spectral colours in the non-zero diffraction order, patterns of such kind shall be used, which are optimized for the diffraction of the relevant spectral components. Figure 4 illustrates an example of mixing red and blue colours, in order to obtain a magenta colour in the first diffraction order of a pattern with a period of 0,80um and l,16um.

If complimentary colours, e.g. blue and yellow colours, are mixed in this way, a resulting white colour can be obtained (Figure 5).

Moreover, the spectral characteristics of the pattern also depends on the groove profile d. Figure 6 shows patterns with different profile of the groove d: binary 4, sinusoidal 5 and stepped 6, and Figure 7 - the relevant spectral characteristics of the zero diffraction order 7, 8 and 9. It can be seen, that in the case of binary profile, the zero order is coloured in red, in the case of sinusoidal profile, the colour will be dark magenta, and in the case of stepped profile - magenta.

If another colour shade is required to be obtained, the ratio between the exposed and the non-exposed part of the pattern can be changed. For example, in order to obtain rose colour, a pattern with a depth hi =0,25um and a ratio between the groove d and the period T equal to 0,6 (Figure 8) has to be recorded. In order to increase the image observation angle on azimuth, a set of patterns with a different inclination angle of the grooves φ is necessary to be applied. The average diameter of the human eye pupil is about 3 mm. People usually look at a hologram at a distance of 250 mm. Therefore, the step of changing the angle of inclination φ of the pattern shall not be larger than 0,7 degrees. Otherwise, a sampling of the image will be seen - it will "flicker" upon changing the observation angle.

PREPARATION AND APPLICATION

Example 1

Preparation of a non-spectral image in the first diffraction order.

The relief optical device consists of raster text images of crimson colour and white colour background. The pixel size is selected to be lOum, because this size provides graphics at high resolution and pixels are not visible to the naked eye. A topology, consisting of periodic patterns, is exposed by means of electron-beam lithography on a silicon wafer, coated with resist with thickness of lum. Patterns with a period of 0,7um and l,0um are exposed in the magenta areas, and patterns with 0,75um and 0,90um are exposed in the white areas. The inclination angle of the pattern varies from a pixel to a pixel in random order. The range of changes is ± 10° with a step of 0,3°. Relief diffraction patterns are obtained after resist developing. Blue colour and red colour are mixed upon diffraction of patterns with periods 0,7um and l,0um in the direction of the first diffraction order. A magenya colour is obtained as a result thereof, and a white colour is obtained upon mixing light blue colour and yellow colour of patterns with periods 0,75pm and 0,90 um.

After recording and resist developing, a thin conductive silver layer with thickness of 0,050um is deposited on its surface by means of vacuum thermal evaporation. Subsequently, a nickel copy is produced by use of electroplating. As a result thereof, the microrelief with the resist is copied onto the metal. The obtained nickel copy is used for stamping on a polymer material, e.g. polycarbonate, to reproduce the optical device on the polymer material. The optical device, obtained in this way, is covered by an aluminum layer 10 with thickness of 0,1 um, in order to increase its reflectivity and the brightness of the device (Figure 9) . In that case, the device can only be seen upon reflection. Should the occasion arise to preserve the transparency of the device, a dielectric material with high refractive index - zinc sulfide 11 is deposited, in order to increase the relief reflectivity (Figure 10). After applying adhesive and cutting to final shape, the optical device is applied on the object and it is used as a sign for its authenticity.

Example 2

Preparation of a non-spectral image in a zero diffraction order.

Hie relief optical device consists of raster text images of red colour and yellow colour background. The pixel size is selected to be lOum, because this size provides graphics at high resolution and pixels are not visible to the naked eye. A topology, consisting of periodic patterns with a period, which vary from a pixel to a pixel in random order, is exposed by means of electron-beam lithography on a silicon wafer, coated with resist of lum thickness. The range of period changes is from lum to 3um with a step of 0.05um. The inclination angle of the pattern varies from a pixel to a pixel in random order. The range of angle changes is ± 10° with a step of 0,3°. After resist developing a microrelief is obtained, which height depends on the exposure dose. Meanwhile, the dose is selected in such a way, that the relief height is 0,25um in the red areas, and 0,35um in the yellow areas. A red/yellow image will be observed at a close to the zero order of the diffraction .

After recording and resist developing, the process continues as already described in Example 1.

Literature:

1. US2009/0179418 Al; 2. Christopher Palmer. Erwin Loewen, Editor (first edition). Diffraction Grating Handbook. Fifth edition, THERMO Richardson Grating Laboratory);

3. (Wolfgang Singer, Michael Totzeck, Herbert Gross. Handbook of Optical Systems, Physical Image Formation, John Wiley & Sons, 2006);

4. (Josef W. Goodman. Introduction to Fourier Optics, Second Edition. The McGraw-Hill Companies, Ihc, 1996).