LAMP WITH A BLACK OR ALMOST BLACK CASING MADE OF A POLYMER The invention refers to a lamp with apparently black or almost black casing that is made of a polymer. More precisely, the invention refers to a lamp, whose polymer casing is black or almost black for the light from the visible spectral range. It is a small lamp with high luminosity, to be used e. g. in a vehicle. A lamp is a device whereby a luminous flux from a bulb being a light source, for example, is either distributed when it is used for illumination or beamed in a headlamp. Besides the light source, the lamp also comprises a casing and an output optics surrounding the said light source. In the case discussed, the casing and the bezel are not translucent for the light from the visible spectral range. Namely, for the needs of the optical output system and also due to aesthetical reasons, the lamp is often provided with a bezel.

The human eye has a different sensitivity to the light of different wavelengths. The interval of wavelengths ranging from 380 nm to 700 nm is considered as the visual spectral range. The human eye, however, is at least slightly sensitive also to the light with a wavelength even up to 880 nm.

A usual bulb with a filament radiates more than 50 % of the luminous flux in the near (from 750 nm to 1,300 nm) and middle (from 1,300 nm to 3,000 nm) infrared spectral range.

A halogen bulb, however, radiates the major part of the luminous flux in the wavelength range from 700 nm to 1,100 nm and only 7 % in the wavelength range below 700 nm, i. e. in the visible spectral range.

The usefully exploited part of the whole luminous flux radiated by the light source leaves the lamp through its output optics. The remaining portion of the said whole luminous flux, however, partly traverses the lamp casing as a transmitted luminous flux, is partly absorbed in the casing wall and partly reflected from the inner surface of the casing. Especially the transmitted luminous flux in the visible spectral range is disturbing and not desired. Just on the contrary, however, a stronger transmitted luminous flux in the infrared range is even desired since it takes away the radiated infrared light that has not left through the output optics. Afterwards the heat from the absorbed luminous flux is dissipated either by infrared radiation from the hot lamp casing or by convection.

Optical properties of the lamp as determined by the application should not be spoilt by reflections of light in the visible spectral range taking place within the lamp casing.

Therefore, the casing and the bezel should be black or almost black for the light from the visible spectral range. Thus, the light from the visual spectral range is neither transmitted nor reflected by the casing or the bezel of the lamp.

All light from the visible spectral range in the luminous flux that has impinged upon the casing or the bezel is absorbed in the black or almost black casing or bezel, while the light from the infrared spectral range in the luminous flux that has impinged upon the said casing or bezel can, generally speaking, also be absorbed only partially, since the casing or the bezel that is black or almost black for visible light is not necessarily black for the infrared light.

The luminous flux from the visible and also from the infrared spectral range absorbed in the casing and the bezel strongly heats the lamp, the more so the wider the wavelength interval of light, in which the casing and the bezel absorb all light.

The known casing and bezel of a modern lamp that are black or almost black, in order to achieve their black colour, are made of a polymer with added pigments on the basis of soot or metal oxides. And the said pigments absorb the luminous flux in the visible spectral range as well as in the whole infrared spectral range. Therefore, the temperature of a modern small headlamp exceeds 200 °C.

In connection with what has just been said, there should be mentioned coloured polymer materials based on polymethylmetaacrylate or polycarbonate, absorbing all light in the visible spectral range and transmitting the light in the infrared spectral range. They are used, for example, for output windows of various remote controls using infrared beams. This, however, only represents a general state of the art since only experts in fields far away from the field of the illuminating engineering for vehicles are familiar with these materials. Namely, the materials mentioned are not used for lamp casings anywhere because they cannot withstand temperatures appearing in headlamps in spite of the self-cooling effect of these materials.

Technical solutions are known where special polymers are used for the casing and the bezel of the lamp in order to solve problems caused by the high temperature of the lamp.

Thus special rare polymers ("hi-end"polymers) are used to manufacture the lamp casing and bezel.

An engineer polymer, polyetherimide, that withstands a temperature exceeding 200 °C, is already used for the manufacture of the headlamp casing and bezel. However, such high temperatures result in the evaporation of adhesive components in the joint between the casing and the output glass; the vapours of these components also condense on the optical surfaces of the lamp and make them blurry. Further, under the influence of high temperatures the polymer reflector, decorative bezel or polymer cover glass might become deformed although they are made of the material mentioned. Moreover, a deformation of the body, onto which the headlamp is fixed, might occur. Finally, the lamps with casings made of such material are expensive.

Numerous technical solutions solving particular problems arising due to high lamp temperature are known as well. Only some of them are cited below.

In the patent GB 2079434 A a preferably metal light screen between the light source and the hot point on the output glass of the lamp is described. The said metal light screen spoils the appearance of the lamp. In the patent FR 268776 Al, however, as such light screen a filter is proposed, whereby the infrared light should be stopped. For this purpose, the filter has to be cooled.

In the patent GB 2025597 A the mounting of a metal plate above the bulb is proposed in order to prevent the appearance of the hot point on the casing, however, problems of corrosion of this plate as well as higher assembling costs arise. The patent DE 19 72 198 Al proposes the installation of a metal plate, whose one end is fastened to the metal body of the vehicle. Such a body, however, is not always close to the lamp and the problem of sealing outlet of the plate from the casing appears. In the patent FR 2718826 Al even a cooling of the lamp by windshield washing liquid is proposed, which is an extremely expensive solution and sensitive to hydraulic system defects, such as a pump defect, tube leakage and similar.

The problems described are also encountered at a bulb base embodiment as described in the patent EP 0 193 631 Al, the said embodiment being a solid metal body in contact with the metal part of the car body.

The above technical solutions solve problems of modern lamps in the first or the se- cond presented way. These technical solutions represent a general state of the art in the field of the invention and are of no particular relevance for the technical solution as proposed by this invention. Consequently, the technical problem of the invention is to propose such a lamp of the kind described, whose temperature should remain as low as possible also under the influence of the luminous flux radiated by a light source placed within the lamp.

The said technical problem is solved as recited in the claims.

The need for a solution of the described technical problem already existed for a long time. The evidence thereof are numerous efforts in various directions to solve the pro- blem. In an unexpected manner, the presented teaching of the invention opens a basically new and important way of solving the known technical problem. It can be expected that the presented subject matter of the invention will be accepted in the market immediately and to an increasing degree.

The invention will be now disclosed in detail on the basis of the description of two variant embodiments.

Awareness of the eye spectral sensitivity curve on the one hand and of the radiation spectrum of the mentioned two kinds of bulbs on the other hand results in the idea that the strong heating of the lamp casing and bezel should be reduced by making the casing and bezel of a polymer material that absorbs all light in the visible spectral range up to a cut-off wavelength A, co, lying in a border range between the visible and the near infrared spectral range, and that only partially absorbs, hence also transmits or reflects, the light in the near infrared and middle infrared spectral range from the mentioned cut-off wavelength Xc,, on.

Here it should be considered that the human eye is at least slightly sensitive also to the light, having a wavelength even around 880 nm. Experiments have shown that a material with the cut-off wavelength of ScO = 800 nm is suitable for a casing or bezel, i. e. a material that absorbs all light with ? < 800 nm and only partially absorbs the light with XcO > 800 nm.

When, however, the lamp casing is enclosed in a cavity of a vehicle, as for example in a bumper, from which only the output optics of the lamp protrudes, a trickling of the reddish light through the casing is not visible and a material with X,,, = 700 nm can be used for the lamp casing.

Also the reflectance of the mentioned polymer material for the light in the near and middle infrared spectral range from the mentioned cut-off wavelength kc, on contributes to the fact that at least a part of the bulb luminous flux in these spectral ranges directed to the casing or bezel is reflected thereby and that sooner or later a part of the reflected luminous flux leaves through the output optics of the lamp.

Any of the following polymers can be used, in accordance with the invention, as a translucent polymer for the manufacture of the casing or bezel: polycarbonate Lexan (General Electric) or Makrolon (Bayer) 'modified polycarbonate Makrolon 1260-translucent (Bayer) polycarbonate co-polymer Apec 9354 or Apec 9339/5 (Bayer) polyamide 6 Durethan B (Bayer).

Two embodiments of the first variant of the invention The lamp of the invention uses a casing and a bezel, both made of a coloured polymer with the said cut-off wavelength being eo 700 nm.

Any of the above-mentioned translucent polymers is coloured, in accordance with the invention, by means of perinone with colour index C. I. 564120, solvent red 135, in a concentration from 0.05 mass % to 1 mass % and antraquinone with colour index C. I. 61565, solvent green 3, in a concentration from 0.05 mass % to 1 mass %.

In a preferred embodiment of the first variant of the invention, the same colorants are used and the concentration of perinone ranges from 0.08 mass % to 0.25 mass % and the concentration of antraquinone ranges from 0. 08 mass % to 0.25 mass %.

Thereby is achieved that in the infrared spectral range from 700 nm to 3,000 nm the casing absorbs less than 60 % and transmits at least 25 % of the incident luminous flux.

Two embodiments of the second variant of the invention The lamp of the invention uses a casing and a bezel, both made of a coloured polymer with the said cut-off wavelength being A. co = 800 nm.

Any of the above-mentioned translucent polymers is coloured, in accordance with the invention, by means of perinone with colour index C. I. 564120, solvent red 135, in a concentration from 0.05 mass % to 1 mass % and phthalocyanine with colour index C. I. 74260, pigment green 7, in a concentration ranging from 0.05 mass % to 1 mass % and antraquinone with colour index C. I. 615290, solvent blue 97, in a concentration from 0.05 mass % to 1 mass %.

In a preferred embodiment of the second variant of the invention, the same colorants are used and the concentration of perinone ranges from 0.1 mass % to 0.2 mass %, the concentration of phthalocyanine ranges from 0,1 mass % to 0,2 mass %, and the concentration of antraquinone ranges from 0.1 mass % to 0.15 mass %.

At the lower limits of the colorant concentration a sufficient absorption of visible light in the car headlamp casing is ensured by a thickness of the walls of the polymer cast part exceeding 3 mm. The upper limits of the colorant concentration are empirical limits where the absorption in the near infrared spectral range becomes too strong and starts to adversely influence the strength of the polymer.

In the headlamp according to the invention, which is provided with a casing and bezel made of a polymer coloured in a proposed manner, the temperature of the casing is lower by up to 70 K and the temperature at its other spots is lower by up to 40 K.

By the significant decrease in the temperature of the casing and bezel during the operation of the lamp according to the invention with respect to the temperatures of known lamps the use of a less expensive polymer is made possible. The manufacturing experience gained by tests shows that the lamp casing and bezel according to the invention should have an absorption in the near and middle infrared range lower than 60 %, and at the same time the casing transmittance at least 25 % is desirable.

The decrease in the casing temperature also results in a decreased evaporation of adhesive components as well as in a decreased subsequent condensation of the vapours of the adhesive on the optical surfaces of the lamp.