DE20121858U1 - Sign comprising light-scattering, colored plastic covering and illuminated by light emitting diodes - Google Patents

Abstract

A light source comprises monochromatic, light emitting diodes (LEDs). At the frequency of their energy maximum, their covering has a transmissivity (DIN 5036) of at least 35% and a reflectivity (DIN 5036) of at least 15%.

Description

Illuminated device

The invention relates to an illuminable device, consisting essentially of a light source comprising one or more light-emitting diodes and a light-scattering cover made of colored plastic associated with the light source.

State of the art

Illuminatable devices, e.g. for advertising boards, consisting essentially of a light source and a light-diffusing cover made of colored plastic associated with the light source are known in principle (see e.g. JP 61159440). As a rule, incandescent lamps or fluorescent tubes are used as light sources, which have good luminosity and emit a broad light spectrum. Due to the broad light spectrum, corresponding colored plastic covers appear in an unlit state, e.g. in daylight, in the same color impression that can also be perceived when backlit using the aforementioned light sources.

Compared to light sources such as incandescent lamps or fluorescent tubes, light-emitting diodes have a significantly lower luminosity. However, colored light-emitting diodes are still very easy to see in the dark because they emit an almost monochromatic light that is relatively intense in the respective wavelength range. Colored light-emitting diodes are available from several manufacturers, for example in red, green, blue and yellow.

Coloring and coloring processes for plastics such as polymethyl methacrylate are well known, for example from EP-A 130 576.

Task and solution

The aim was to provide an alternative to the known illuminable devices in which colored plastic covers are illuminated using light bulbs or fluorescent tubes. In particular, the device should provide approximately the same color impression in incident light, e.g. in daylight, as in transmitted light. The device should allow for smaller installation depths than the previously known devices and be characterized by lower power consumption.

The task is solved by a

Illuminable device, consisting essentially of a light source and a light-scattering cover made of coloured plastic associated with the light source, characterized in that

the light source consists of one or more light-emitting diodes (LEDs) which emit a coloured, essentially monochromatic light and the associated light-scattering cover has a transmission (DIN 5036) of at least 35% and a reflection (DIN 5036) of at least 15% at the wavelength of the relative energy maximum of the light-emitting diode.

The invention is based on adapting the transmission and remission of the light-scattering plastic cover to the monochromatic light of the LED used in such a way that almost the same color impression can be achieved in incident light as well as in transmitted light. Corresponding advertising or information boards appear almost the same optically both during the day and when backlit.

This adaptation enables the use of LEDs that emit colored or monochromatic light for the above-mentioned purpose. The illuminable devices according to the invention require smaller installation depths, since LEDs are smaller than corresponding incandescent lamps or fluorescent tubes. Complicated shapes can also be implemented more easily. Power consumption is lower, with almost the same visibility when backlit. Since LEDs can be operated with low voltages, the electrical safety of the devices according to the invention is also considered to be higher and easier to ensure. Maintenance effort is also lower, since LEDs generally need to be replaced less often.

The invention is explained by the following figures, without however being limited to the embodiments shown.

Transmission/remission spectrum of a colored light-scattering plastic disc (Green 1, example series 1) suitable as a cover for a device according to the invention that can be illuminated with a green LED.

T = transmission spectrum

R = remission spectrum

LED = relative energy curve of the green LED with relative energy maximum at about 520 nm

Representation of the standard color chart with an example for the coordination or determination of suitable color locations of the transmission and remission of plastic covers suitable for a specific colored LED. The suitable color locations are in the part of the resulting rectangle that lies within the standard color chart.

LED = color location of the LED

U = achromatic point (x/y = 0.33/0.33)

A = maximum color location distance (0.2 units) from the color location of the LED on the straight line through U and LED

B = maximum color location distance (0.05 units) on both sides at right angles to the line through U and LED.

T = color coordinate of the transmission of the cover
R = color coordinate of the reflection of the cover.

Implementation of the invention

The invention relates to an illuminable device, consisting essentially of a light source and a light-scattering cover made of colored plastic assigned to the light source.

The light source consists of one or more or a large number of light-emitting diodes (LEDs) that emit colored, essentially monochromatic light. If necessary, LEDs of different colors can also be used simultaneously.

The color of the LED depends on the wavelength of its relative energy maximum. This relative energy maximum can be determined spectrophotometrically, for example, and plotted in a wavelength spectrum. The light source can be placed in an Ulbricht sphere (see DIN 5036), for example, and the light emitted can be measured. The highest point (peak) of the curve indicates the wavelength of the relative energy maximum.

The number of LEDs depends on the size of the device, the luminosity of the LEDs used and the overall brightness desired for the device when illuminated. LEDs are available, for example, as modules of 4 LEDs each in a holder, of which a large number can be installed in the device if necessary.

Light-emitting diodes (LEDs),

Suitable LEDs include red, blue, yellow or green LEDs.

A red LED has a relative energy maximum in the range of about 610 to 640 nm.

The red LED (Osram LM03-B-A), for example, has a relative energy maximum at about 620 nm.

A blue LED has a relative energy maximum in the range of about 440 to

The blue LED (Osram LM03-B-B), for example, has an energy maximum at about 460

The blue LED (ESS Blue), for example, has an energy maximum at about 475 nm.

A yellow LED has a relative energy maximum in the range of about 570 to

The yellow LED (Osram LM03-B-Y), for example, has an energy maximum at about 590

A green LED has a relative energy maximum in the range of about 500 to

The green LED (Osram LM03-B-T), for example, has an energy maximum at about 520

Light-diffusing plastic cover

The associated light-scattering cover made of plastic has a transmission (DIN 5036, see parts 1 and 3) of at least 35%, preferably at least 38%, particularly preferably at least 41% and a reflection (DIN 5036, parts 1 and 3, reflection or remission) of at least 15%, preferably at least 20%, particularly preferably at least 30% at the wavelength of the relative energy maximum of the light-emitting diode.

In particular, the transmission of a light-scattering cover associated with a yellow LED can be at least 50%, preferably at least 60%. The corresponding reflection can be at least 25%, preferably at least 30%.

In particular, the transmission of a light-scattering cover associated with a red LED can be at least 40%, preferably at least 45%. The corresponding reflection can be at least 22%, preferably at least 45%.

In particular, the transmission of a light-scattering cover associated with a green LED can be at least 40%, preferably at least 42%. The corresponding reflection can be at least 18%, preferably at least 20%.

In particular, the transmission of a light-scattering cover associated with a blue LED can be at least 40%, preferably at least 42%. The corresponding reflection can be at least 20%, preferably at least 22%.

In the event that LEDs of different colors are used simultaneously to achieve mixed colors, e.g. yellow and green LEDs produce a yellow-green color impression, the associated light-scattering cover made of plastic should have the transmission and reflection values required above at least at the wavelength of the relative energy maximum of one of the light-emitting diodes used, e.g. the yellow or green LED.

The associated light-scattering cover consists of a plastic that is transparent in the uncolored state and without a scattering agent, or has a transmittance (DIN 5036, see parts 1 and 3 / D65) of at least 50%, preferably at least 70%, particularly preferably from 75 to 92%. With a scattering agent but without a colorant, the transmittance can advantageously be at least 40%, particularly preferably at least 50%.

Suitable plastics include polymethyl methacrylate plastic, impact-modified polymethyl methacrylate, polycarbonate plastic, polystyrene plastic, styrene-acrylonitrile plastic, polyethylene terephthalate plastic, glycol-modified polyethylene terephthalate plastic, polyvinyl chloride plastic, transparent polyolefin plastic, acrylonitrile-butadiene-styrene (ABS) plastic or mixtures (blends) of various thermoplastics.

Due to their high weather resistance, polymethyl methacrylate plastics made from cast or extruded polymethyl methacrylate, e.g. with a methyl methacrylate content of 85 to 100 wt.%, are preferred, particularly for outdoor applications. If necessary, up to 15 wt.% of suitable comonomers such as esters of methacrylic acid (e.g. ethyl methacrylate, butyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate),

Esters of acrylic acid (e.g. methyl acrylate, ethyl acrylate, butyl acrylate, hexyl acrylate, cyclohexyl acrylate) or styrene and styrene derivatives such as α-methylstyrene or p-methylstyrene are polymerized or may be contained in the polymer.

The light scattering capacity of the cover, measured according to DIN 5036, can preferably have a value of at least 0.5, particularly preferably of at least 0.6, in particular at least 0.7. The better the light scattering capacity, the smaller the distances between the LEDs and the cover and the associated installation depths of the device can be realized.

BaSO ₄ , polystyrene or light scattering beads made of cross-linked plastic can be used as light scattering agents.

BaSO ₄ or polystyrene are preferred and are preferably incorporated into the plastic in an amount of 1.5 to 2.5 wt.%.

Light scatterer
Quantity of 0

en made of a cross-linked plastic are preferably introduced into the plastic in a concentration of 1 to 10 wt.%.

The requirement for high transmission with high light scattering is a difficult requirement to achieve. A high scattering power is achieved by titanium dioxide. However, since this colorant reflects a large part of the light

ed, only low levels of light transmission are possible.

Colorless scattering pigments are more favorable, with a refractive index that differs from the refractive index of acrylic glass by up to 0.2. Suitable examples include calcium carbonate, magnesium carbonate, aluminum trihydroxide, magnesium hydroxide, barium sulfate, etc.

Polymers that are in the appropriate refractive index range can also be used. For example, polystyrene can be dissolved in the monomer methyl methacrylate, which then precipitates during polymerization and results in a material with good light scattering. Cross-linked polymer particles can also be added, e.g. polymer beads made of cross-linked polystyrene or cross-linked copolymers of methyl methacrylate with phenyl (meth)acrylate or benzyl (meth)acrylate.

Production of coloured light-diffusing plastic covers

Scattering agents and colorants can be added or incorporated into the plastic during production by polymerization in the polymerizable mixture or during thermoplastic processing in the melt state, e.g. by extrusion or injection molding, in a conventional manner. In addition to the sheet form, any profiles such as tubes, rods, etc. can also be manufactured.

In this way, for example, plastic plates can be obtained, for example with a thickness of 0.5 to 10, preferably 1 to 5 mm, which can be used as covers for illuminable devices according to the invention with rectangular boxes, frames or holders. Corresponding pieces can also be converted and adapted to practically any shape as required by cutting, milling, sawing or other processing.

contraption

The device can be designed so that the LEDs and the light-scattering cover are arranged at a distance of 3 to 12, preferably 4 to 10 cm. At this distance, good illumination is achieved.

·

If the distance is too small, the position of the LED becomes visible in the form of a bright spot. If the distance is too large, the brightness decreases too much.

The LEDs can be located in a box or frame, for example, which is covered by the light-diffusing cover. The cover can be provided with an information-bearing layer, e.g. a film, or can itself already have the form of information, e.g. in the form of letters or numbers.

Colorants

Organic colorants are preferred as colorants for the purposes of the invention, as they have high brilliance and luminosity in both incident and transmitted light. In order to protect the acrylic glass against the effects of light and weather, light stabilizers, UV absorbers, antioxidants, etc. can be added.

Colorants used in plastics include soluble dyes or organic pigments, but also less preferred insoluble inorganic color pigments. Examples include:

For yellow coloring: Pyrazolone Yellow and Perinone Orange or mixtures thereof.

For red colorations: mixtures of pyrazolone yellow and anthraquinone red or naphthol AS and DPP red or mixtures thereof.

For green coloring: mixtures of Cu phthalocyanine green and pyrazolone yellow.

For blue coloring: anthraquinone blue and ultramarine blue or mixtures thereof.

Color locations

The invention is based on the idea that the closer the color locations of the transmission and remission of the colored cover are to the color location of the LED, the better the color impression should match in incident light and in transmitted light. However, it has been shown that a color match with a given LED color location can only be achieved approximately. In general, deviations that lie on or near the straight line that runs through the achromatic point (x/y = 0.33/0.33) and the color location of the LED can be tolerated more easily than deviations that are the same size but further away from the described straight line.

It is desirable that the color locations are located as close to the edge of the color standard chart as possible, as this is where the color brilliance is at its highest. This is also due to the fact that the color locations of the LEDs are also located at or near the edge of the standard color chart due to the monochromatic light. It should be noted that the actually detectable (measured) color locations can also deviate from the theoretically expected color locations.

In many cases, the corresponding coloring cannot be achieved with a colorant alone. When mixing, care must be taken to ensure that the

Individual components should not be too far apart from each other on the color standard chart, as the mixed color may then have insufficient brilliance.

The color locations of the transmission and remission of the colored plastic cover in relation to the standard color chart should preferably lie in a range which, in relation to a straight line that runs through the achromatic point (x/y = 0.33/0.33) and the color location of the LED, is not more than 0.2 x/y units, preferably not more than 0.1 x/y units away from the color location of the LED in the direction of the straight line and not more than 0.05 x/y, preferably not more than 0.03 x/y units perpendicular to both sides of the straight line (see also Fig. 2/2).

Commercially available measuring devices are available to the specialist for measuring color locations.

Device for yellow (or yellow-green) lighting

The LEDs used can, for example, emit a yellow (or yellow-green) light and have a color location in the range of the coordinates x/y = 0.5/0.5 +/- 0.02.

In this case, the plastic of the cover can be colored with a mixture of 0.075 to 0.09, preferably 0.081 to 0.084 wt.% pyrazolone yellow and 0.002 to 0.004, preferably 0.0028 to 0.0032 wt.% perinone orange.

It is advantageous to combine this coloring with BaSO4 as a scattering agent in an amount of 1.9 to 2.1 wt.%.

Device for red lighting

The LEDs used can, for example, emit red light and have a color location in the range of coordinates x/y = 0.67/0.33 +/- 0.02.

In this case, the plastic of the cover can be colored with a mixture of 0.13 to 0.17, preferably 0.14 to 0.16 wt.% pyrazolone yellow and 0.01 to 0.03, preferably 0.17 to 0.23 wt.% anthraquinone red.

It is advantageous to combine this colouring with polystyrene as a scattering agent in an amount of 1.9 to 2.1 wt.%.

The plastic of the cover can also be colored with a mixture of 0.055 to 0.07, preferably 0.061 to 0.064 wt.% Naphtol AS (2-hydroxy-3-naphthoic acid anilide) and 0.005 to 0.015, preferably 0.008 to 0.012 wt.% DPP red (dipyrrolopyrrole red).

It is advantageous to combine this coloring with BaSCU as a scattering agent in an amount of 1.9 to 2.1 wt.%.

Device for green lighting

The LEDs used can, for example, emit a green light and have a color location in the range of coordinates x/y = 0.16/0.73 +/- 0.02.

In this case, the plastic of the cover can be treated with a mixture of

0.01 to 0.025, preferably 0.013 to 0.017 wt.% Cu phthalocyanine green and 0.025 to 0.045, preferably 0.028 to 0.032 wt.% pyrazolone yellow.

It is advantageous to combine this coloring with BaSO ₄ or polystyrene as a scattering agent in an amount of 1.9 to 2.1 wt.%.

Device for blue lighting

The LEDs used can, for example, emit blue light and have a color location in the range of coordinates x/y = 0.14/0.06 +/- 0.02.

In this case, the plastic of the cover can be colored with a mixture of 0.005 to 0.01, preferably 0.006 to 0.008 wt.% anthraquinone blue and 0.05 to 0.1, preferably 0.07 to 0.08 wt.% ultramarine blue.

It is advantageous to combine this coloring with BaSO ₄ as a scattering agent in an amount of 1.9 to 2.1 wt.%.

The plastic of the cover can also be colored with 0.007 to 0.013, preferably 0.009 to 0.011 wt.% anthraquinone blue.

It is advantageous to combine this colouring with polystyrene as a scattering agent in an amount of 1.9 to 2.1 wt.%.

4/2 · · · ■«· t * t

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Uses

In the device according to the invention, the described coloured plastic elements containing scattering agents are used as a cover and coloured LEDs are used as a light source.

EXAMPLES Examples Series 1: Red 1, Yellow 1, Blue 1, Green 1

0.5 parts of t-butyl perpivalate and 20 parts of polystyrene (e.g. from BASF) are dissolved in 1000 parts of methyl methacrylate.

For this purpose, the dyes are added according to Table 1, dissolved with intensive stirring, filled into a silicate glass chamber separated by a 3 mm thick cord and polymerized in a water bath at 45°C for about 16 hours. The final polymerization takes place in a tempering cabinet at 115 ⁰ C for about 4 hours.

Examples Series 2: Red 2, Yellow 2, Blue 2, Green 2

In 1000 parts of prepolymeric methyl methacrylate syrup (viscosity approx. 100OcP)

1 part 2,2'-azobis-(2,4-dimethylvaleronitrile) dissolved.

In this mixture you add a colour paste consisting of

3 parts of a soluble polymethyl methacrylate resin,

20 parts barium sulfate

and the colorants according to Table 2, which are dispersed in 30 parts of methyl methacrylate using a high-speed disperser (rotor/stator principle),

t « · &igr;

The mixture is stirred intensively, filled into a silicate glass chamber separated by a 3mm thick cord and polymerized in a water bath at 45°C for about 16 hours. The final polymerization takes place in a tempering cabinet at 115°C for about 4 hours.

Dyes Series 1

Color Cu-Phthalo
cyanine green Pyrazolone
yellow Anthraquinone
blue Perinon
orange Anthraquinone
violet Anthraquinone
red Rome — 0.1500 — — — 0.0200 Yellow — 0.0825 — 0.003 — — GrürM 0.0200 0.0400 — — — — Blue — — 0.0100 — — —

Information: in wt.%

Tab. 2: Colorants Series 2

L.J.No. Naphthol
AS DPP
Red Cu-Phtalo-
cyanine green Ultramarin
blue Pyrazolone
yellow Anthraquinone
blue Perinon
orange Red 2 0.0625 0.01 — — — — — Yellow 2 — — — — 0.0825 — 0.003 Green 2 — — 0.015 — 0.03 — — Blue 2 — — — 0.077 — 0.007 —

Information: in wt.%

Results:

In a white-painted metal box with an open top, measuring 90 x 470 mm and 100 mm high, 32 light-emitting diodes, e.g. from OSRAM (8 modules with 4 LEDs each), are mounted on the inside base (many manufacturers have standard LEDs that have a similar color tone to each other). The permissible operating current is set to between 320 - 400mA, depending on the type, using a power supply unit with an operating voltage of 10V.

The samples described above are placed on this box and assessed for colour. The incident light test (daylight effect) is carried out by illuminating with a 150 W daylight lamp (D65 according to DIN 6173, quality class 1, e.g.

• IM *«·

from Siemens) at a distance of approx. 60cm from above. The LEDs are switched off. The transmitted light test is carried out in a darkened room with the LEDs switched on according to the operating instructions above. The color measurements are carried out using the Chroma-Meter CS-100 color measuring device from Minolta. This device allows contactless measurements of light sources and object colors. The distance between sample and device is 1m. The luminance Y in Cd/m ² is also measured using this device.

The results of the color measurements and luminances are shown in Table 3. For comparison, Table 4 shows corresponding color measurements and luminances of commercially available covers made of polymethyl methacrylate with standard colors that are not specifically matched to the LEDs.

Table 3

Color coordinates x,y and luminance Y in Cd/m ² for LED backlighting with coloring according to the invention.

Color LEDXmax
in nm transmission
at LEDXmax [
at in Y
Cd/ ^m2 X y LEDXmax
in nm transmission
at LEDXmax reflection
at LEDXmax Y
in
Cd/ ^m2 X y Yellow 62% reflection
LEDXmax 141 0.527 0.467 590 28% 62% 55.1 0.582 0.417 Yellow 2 55% 26% 130 0.533 0.461 620 20% 50% 49.7 0.686 0.313 RoU 48% 32% 127 0.682 0.317 520 18% 18% 11.4 0.139 0.779 Red 2 42% 23% 120 0.682 0.317 460 30% 27% 4.31 0.139 0.037 Green 1 43% 50% 36.3 0.141 0.780 Green 2 41% 19% 36.4 0.139 0.777 Blue 43% 21% 6.56 0.138 0.045 Blue 2 43% 24% 6.31 0.138 0.041 590 24% 590 320 520 520 520 460 460 Table 4
Comparative measurements with commercially available plastic covers Color Yellow 370* Red 568 * Green 710* Blue 601*

*= Product names of the manufacturer, Röhm GmbH & Co. KG, D-64293 Darmstadt.

The results (Table 3) show that the colored acrylic glass produced using the above procedure achieves significantly higher luminance (brightness) with LED backlighting than the colorings that correspond to the state of the art (Table 4). At the same time, the light scattering is so good that even illumination is achieved at a distance of just 40 mm from the LED.

Table 5:

Measurements on comparison samples according to the examples of series 1 and 2, but without colorant but with the specified scattering agent in the specified quantities (polystyrene or barium sulfate) show a scattering power > 0.5 at a transmittance > 40%.

additive Scattering power Transmittance Polystyrene 0.65 56% Barium sulfate 0.80 50.5%

For comparison: With a corresponding white coloring with titanium dioxide, a very good scattering power of approx. 0.90 can be achieved. However, the transmission level is then only around 20 - 30%. As a result, these types appear very dark in transmitted light and are generally not suitable for the purposes of the invention.

If the colour coordinates according to Table 3 are entered into the standard colour chart (see e.g. DIN 5033 or corresponding standard literature), it is clear that the values (and thus the colour tones) are within the limits required by the invention.

close to the line of the wavelength of the same color tone (line between the achromatic point and the color location of the respective LED color). During the visual inspection, the good match of the color tone can be seen in incident and transmitted light.

The transmission curve in Fig. 1/2 for green LEDs shows that the maximum transmission and reflection of the color green 1 (series 1) correspond quite well with the wavelength of the relative energy maximum of the LED. The transmission values in these areas are significantly above the required 30%, the reflection values are above the required 15%.

Claims (18)

1. Illuminable device, consisting essentially of a light source and a light-scattering cover made of colored plastic associated with the light source, characterized in that the light source consists of one or more light-emitting diodes (LEDs) which emit a colored, essentially monochromatic light and the associated light-scattering cover has a transmission (DIN 5036) of at least 35% and a reflection (DIN 5036) of at least 15% at the wavelength of the relative energy maximum of the light-emitting diode. 2. Device according to claim 1, characterized in that the LEDs and the light-scattering cover are arranged at a distance of 3 to 12 cm from one another. 3. Device according to claim 1 or 2, characterized in that the light-scattering cover consists of a cast or extruded polymethyl methacrylate plastic. 4. Device according to one or more of claims 1 to 3, characterized in that the plastic of the cover has a light scattering power measured according to DIN 5036 of at least 0.5. 5. Device according to claim 4, characterized in that BaSO ₄ , polystyrene or light scattering beads made of a cross-linked plastic are contained as light scattering agents. 6. Device according to claim 5, characterized in that BaSO ₄ or polystyrene is contained as scattering agent in an amount of 1.5 to 2.5 wt.%. 7. Device according to one or more of claims 1 to 6, characterized in that the LEDs are located in a box or frame which is covered by the light-scattering cover. 8. Device according to one or more of claims 1 to 7, characterized in that the color locations of the transmission and the remission of the colored cover made of plastic, based on the standard color chart, lie in a range which, based on a straight line running through the achromatic point (x/y = 0.33/0.33) and the color location of the LED, is not more than 0.2 x/y units away from the color location of the LED in the direction of the straight line and not more than 0.05 x/y units at right angles to both sides of the straight line. 9. Device according to one or more of claims 1 to 8, characterized in that the LEDs emit yellow light and have a color location in the range of the coordinates x/y = 0.5/0.5 +/- 0.02. 10. Device according to claim 9, characterized in that the plastic of the cover is colored with 0.075 to 0.09 wt.% pyrazolone yellow and 0.002 to 0.004 wt.% perinone orange. 11. Device according to one or more of claims 1 to 8, characterized in that the LEDs emit red light and have a color location in the range of the coordinates x/y = 0.67/0.33 +/- 0.02. 12. Device according to claim 11, characterized in that the plastic of the cover is colored with 0.13 to 0.17 wt.% pyrazolone yellow and 0.01 to 0.03 wt.% anthraquinone red. 13. Device according to claim 11, characterized in that the plastic of the cover is colored with 0.055 to 0.07 wt.% Naphtol AS and 0.005 to 0.015 wt.% DPP red. 14. Device according to one or more of claims 1 to 8, characterized in that the LEDs emit green light and have a color location in the range of the coordinates x/y = 0.16/0.73 +/- 0.02. 15. Device according to claim 14, characterized in that the plastic of the cover is colored with 0.01 to 0.025 wt.% Cu-phthalocyanine green and 0.025 to 0.045 wt.% pyrazolone yellow. 16. Device according to one or more of claims 1 to 8, characterized in that the LEDs emit blue light and have a color location in the range of the coordinates x/y = 0.14/0.06 +/- 0.02. 17. Device according to claim 16, characterized in that the plastic of the cover is colored with 0.05 to 0.1 wt.% ultramarine blue and 0.005 to 0.01 wt.% anthraquinone blue. 18. Device according to claim 16, characterized in that the plastic of the cover is colored with 0.007 to 0.013 wt.% anthraquinone blue.