A LIGHT DIFFUSIVE COATING AND ITS FORMATION AND A LAMP HAVING THE COATING

Abstract: A light diffusive coating having an excellent light diffusion effect, a method of forming the coating and a halogen lamp with the coating are described. The light diffusive coating includes bubbles within a continuous coating consisting of a metal oxide formed on a base. The method of forming the light diffusive coating is made by the inclusion of bubbles formed by the evaporization of a high boiling-point organic solvent within the metal oxide coating formed by the application of a combination of an organo-­ metallic compound with a high boiling-point organic solvent to a base surface, and the subsequent decomposition of the organo-metallic compound on baking. The halogen lamp has a porous light diffusive coating formed on the coating established on the surface of the outer vessel which allows visible light to pass through but reflects infra-red light, so that strain, caused by the difference in thermal expansion coefficient of the structural material of the outer vessel and the structural material of the light diffusive layer occuring when the outer vessel is at a high temperature, is absorbed by the porous structure, and prevents peeling.

Description: This invention relates to a method of producing light diffusive coatings on a base surface of e.g. lamps such as halogen lamps, high pressure discharge lamp bulbs, optical filters, and window glass, giving diffusive characteristics, and to its use in providing lamps which are more efficient and give uniform illumination. In order to give uniform distribution of illumination over the illuminated surface, halogen lamps for photocopiers, for example, have several filaments connected in series through conductors, and the whole assembly is enclosed in a straight tube type outer vessel made of silica glass so that it lies along the axis of the tube. However, because this halogen lamp includes a large amount of infra-red radiation in its emissions, there is a danger that the item being photocopied will be damaged by the heat. For this reason, the lamp surface generally undergoes a honing treatment to provide a diffusive surface. This method, however, has the disadvantage of being very laborious. In addition, there are methods of forming a light diffusive coating by applying a fine diffusive powder such as silica to the outside of the lamp by electrostatic coating and etching. These types of coating are generally weak in resisting mechanical stresses, wear easily, and in some cases the light diffusive effect falls off with a long period of use. Dependent on the type of fine light diffusive powder, there are cases where the coating has a poor affinity for the base, e.g. quartz glass, and readily peels off. Taking account of these points, the applicants have previously developed the technology of forming a coating on the outer surface of the transparent outer vessel which allows visible light to pass through, but reflects infra-red light, and by providing a light diffusive coating on top of the infra-red reflective coating, the visible light which has passed through the infra-red reflective coating is dispersed by the light diffusive coating, giving a uniform distribution of illumination on the illuminated surface. This was proposed in Japanese Patent Application Number Sho 58-95001(95001/1983). However, when this technology is applied to lamps, such as halogen lamps for example, because the temperature of the lamp's outer vessel is extremely high, with some light diffusive coatings, there is the danger that the light diffusive coating may peel off due to the effect of repeated switching on and off over a long period. This means that it is essential to select a light diffusive coating which is stable regardless of the rise and fall in temperature and which is mechanically strong. In patent specification CH-A-473 738 there is disclosed a method of coating a vitreous substrate with a light dispersive coating derived from a pyrogenic inorganic oxide suspended in a film forming ester solution. Titanium dioxide can be used in suspension. Additives may be employed to form a microporous coating. US-A-4,459,506 is concerned with lamps having an anti-fragility coating of hollow microspheres which are impact-absorbing. The present invention seeks to provide a method of making a light diffusive coating which can be readily applied to large-scale production, is mechanically robust and resistant to wear, can be applied without problem to the inside or outside of the base surface of lamps, etc., regardless of their material, and which provides excellent diffusion. The invention, moreover, does not adversely affect the effectiveness of Application Number Showa 58-95001 by adding to the limits required by the light diffusive coating proposed in the application, but, instead, aims to produce a lamp with no danger of the diffusive coating peeling off, even with repeated switching on and off over a long period. The present invention also provides a lamp having the light diffusive coating. The light diffusive coating provides light diffusion by the inclusion of bubbles and/or pits within a continuous coating which is formed on a base. The present invention provides a method of forming a light diffusive coating characterised in that a mixture containing an organo-metallic compound and a high boiling-point organic solvent is applied to a base and dried to form a coating which contains dispersed droplets of said high boiling point solvent, the coating is then baked whereby said droplets evaporate and expand into bubbles and/or pits within a metallic oxide coating which is formed by decomposition of the organo-metallic compound. The bubbles and/or pits are formed by the vaporization of a high boiling-point organic solvent within the metal oxide coating, by the application of a combination of an organo-metallic compound with a high-boiling organic solvent to a base surface, and subsequent decomposition of the organo-metallic compound on baking. The present invention also includes a halogen lamp which has a light diffusive coating formed as described above on the coating established on the surface of the outer vessel which allows visible light to pass through but reflects infra-red light, so that strain, caused by the difference in thermal expansion coefficient of the structural material of the outer vessel and the structural material of the light diffusive layer occurring when the outer vessel is at a high temperature, is absorbed by the structure, and prevents peeling. Additional advantages and features of the present invention will become apparent to persons skilled in the art from a study of the following description and of the accompanying drawings given by way of example, in which: Figure 1 shows a section of a halogen lamp in accordance with one embodiment of the present invention; Figure 2 shows an enlarged schematic section of section II marked on Figure 1; Figure 3 shows an electron micrograph of the outer surface of the light diffusive coating (part 3) of Figure 2; Figure 4 shows a schematic section of the coating before baking in the method of forming the light diffusive coating; Figure 5 shows a schematic section of a multilayered light diffusive coating; and Figure 6 is a graph showing the effectiveness of the light diffusive coating in this embodiment. Throughout the drawings, like reference numerals and letters are used to designate like or equivalent elements for the sake of simplicity of explanation. Referring now to Figure 1, there is shown a halogen lamp for a photocopier as an example of a lamp according to the present invention. This halogen lamp has had a coating 2, which allows visible light to pass through but reflects infra-red light, formed on the outside surface of a straight tube type outer vessel 1 made from a transparent, heat-resistant glass such as quartz glass. A porous light diffusive coating 3 has been formed on top of the reflective coating 2. Pressure sealed seal sections 4 have been constructed at each end of the outer vessel 1, and molybdenum foil conductors 5 have been embedded into each of these seal sections 4. One end of each of these pieces of molybdenum foil 5 is in contact with internal conductors 6 which are stretched inside the outer vessel 1. Between these internal conductors 6 several filaments 7 are connected serially in position along what is virtually the centre line, via non light-emitting sections formed by low resistance conductors 8. The serially connected filaments 7, and low resistance conductors 8 are held in position on the centre line of the outer vessel 1 by means of anchors 9. The other ends of the foil conductors 5 are connected to the caps 10 attached to each end of the outer vessel 1 through external conductors which are not shown in the drawing. The required halogen is sealed inside the outer vessel 1 along with an inert gas such as argon. Referring now to Fig. 2, which is an enlargement of section II in Fig.1, there is shown in detail the infra-red reflecting coating 2 described above which typically has a multilayered coating, consisting of alternate high refractive index layers 2a (left-slanting hatching) made from titanium oxide; TiO₂ etc., and low refractive index layers 2b (right-slanting hatching), made from silica; SiO₂, etc. Due to the effect of optical interference, this allows visible light to pass through readily, but reflects infra-red light very well. The layers 2a, 2b are exaggerated in the drawing; an appropriate thickness is 0.2 - 0.3 µm(micron meter). Various types of the light diffusive coating 3 described above may be considered, but the drawings show a typical example. As is made clear by Fig. 2, and by the electron micrograph in Fig. 3, the light diffusive coating 3 has numerous bubbles 32 inside the continuous coating material 31, made from metal oxide, and numerous circular pits 33 which are a variety of these bubbles, are formed on the surface of the coating 31. The transmitted light is dispersed by these bubbles 32 and pits 33, and the light diffusive layer 3 appears cloudy to the naked eye. The light diffusive layer 3 is formed with a thickness of 3000 - 8000 A(angstrom), or 0.3 - 0.8 µm. Examples of suitable metal oxides are titanium oxide; TiO₂, silica; SiO₂, alumina; Al₂O₃, zirconia; ZrO₂, zinc oxide; ZnO, tantalum oxide; Ta₂O₅, tin oxide; SnO₂ and indium oxide; In₂o₃. Even though the filaments 7 of the halogen lamp are arranged with spaces between them, because their light is emitted after dispersion by the light diffusive layer 3, there is no irregular illumination of the illuminated surface. Moreover, even though there is a large difference between the thermal expansion coefficients of the titanium oxide which makes up the light diffusive coating 3 and the reflective coating 2 and quartz glass forming the outer vessel 1, because the reflective coating 2 and the light diffusive coating 3 are together extremely thin, and the outer light diffusive coating 3 has the bubbles 32 and the pits 33 within its material 31, mechanical strain caused by the difference in thermal expansion coefficients is alleviated, and the light diffusive coating 3 does not peel off even with repeated switching on and off over a long period. Furthermore, because the light diffusive coating 3 is extremely thin, the level of light loss, at no more than 3 - 4 %, is excellent. In addition, because the light diffusive coating 3 material is continuous, it is mechanically strong, there is no danger of wear, and there are no changes in the light diffusive characteristics over a long period of use. A suitable manufacturing method for this halogen lamp will now be outlined. Firstly, a straight tube type halogen lamp with the filaments 7 sealed inside is manufactured by normal methods. A titanium solution is then prepared by dissolving an organic titanium salt, with tetraisopropyl titanate as the main constituent, in an organic solvent with an acetic acid ester (e.g. ethyl acetate) as the main constituent. This solution has 2 - 10 % titanium by weight, and its viscosity is adjusted to approximately 1.0 cps (0.01 gcm⁻¹s⁻¹). The sealed halogen lamp described above then has its first high refractive index layer 2a-1, consisting of titanium oxide, formed on the outer surface of the outer vessel 1 by dipping it into this titanium solution, withdrawing it at a speed of approximately 30 cm/minute, drying and baking it. A silica solution is also prepared by dissolving an organic silicon salt, with ethyl silicate as its main constituent, in an organic solvent with an acetic acid ester (e.g. ethyl acetate) as the main constituent. This solution has 2 - 10 % silicon by weight, and its viscosity is adjusted to approximately 1.0 cps (0,01 gcm⁻¹s⁻¹). The halogen lamp with the first high refractive index layer 2a-1 described above then has its first low refractive index layer 2b-1, consisting of silica, formed by dipping it into this silica solution, withdrawing it at a speed of approximately 35 cm/minute, drying and baking it. Further high refractive index layers 2a-2... and low refractive index layers 2b-2... are formed alternately in the same way up to a total of about 10 layers, completing the construction of the infra-red reflective coating. 5 - 10 % of a high boiling-point organic solvent such as di-2-ethylhexyl phthalate or di-octyl phthalate (abbreviated to DOP, hereinafter) is then added to the solution of an organic titanium compound in a low boiling-point organic solvent in the example described above, and a low boiling point organic solvent is added to this to bring the density down to a suitable level. The halogen lamp with the infra-red coating 2 described above is then dipped in this solution, withdrawn at a speed of 30 - 50 cm/minute and dried. DOP described above is a colourless oily liquid which boils at 231 °C and 5 mm Hg (6.7.10² Pa), and mutually dissolves with the low boiling-point organic solvent to a certain extent. In the drying process, only the low boiling-point organic solvent evaporates. After drying, as can be seen in Fig. 4, the gel-state coating 34 is clouded with dispersed microscopic droplets of DOP. The halogen lamp with the gel-state coating 34 is baked in air for approximately 5 minutes at approximately 600 C. When this is done, the evaporation of the droplets of DOP dispersed throughout the gel-state coating 34 and the decomposition of the organic titanium compound occur more or less simultaneously. Because of this, the droplets of DOP evaporate in almost the same position, expand and become bubbles 32. DOP vapour in these bubbles 32 is gradually substituted by air. The droplets of DOP near to the surface of the gel-state coating 34 burst due to their expansion, link up with the outside air and become the pits 33. Finally a light diffusive layer 3 containing bubbles 32 and pits 33 within a continuous titanium oxide coating is formed, as shown in Fig. 3. As is clear from this formation process, the bubbles 32 and pits 33 in the present invention are basically the same thing. In this way, in addition to appearing to improve the formation of the organic titanium compound coating, DOP forms droplets within the gel-state coating 34 which grow into bubbles 32. Because of this, the bubbling is effective even though the gel-state coating 34 is extremely thin. Also, DOP has a high boiling point, close to the decompositon temperature of the organic titanium compound, so that even when heat decomposition of the organic titanium compound begins, sufficient vapour pressure is maintained within the bubbles 32. This prevents the bubbles 32 from being crushed or the pits 33 being levelled out before the decomposition and solidification of the organic titanium compound. Accordingly, the coating 34 hardens with the bubbles 32 and pits 33 still included and in good shape. If too little DOP is added, the bubbles 32 and pits 33 are too small, and the light diffusive effect is reduced. Conversely, if too much DOP is added, large irregularities in the sizes of the bubbles 32 and pits 33, cause local irregularities in diffusion, and large pits visible to the naked eye may be formed. If the coating 34 is made thicker by adjusting the viscosity of the coating material, large bubbles and pits are produced, but on the other hand, the pits 33 become shallower, and light dispersion deteriorates. In contrast to this, if the coating 34 is made thinner, then since the bubbles 32 and pits 33 remain small, it is possible to form a coating with several layers of this thin light diffusive coating. According to experimentation, when a light diffusive coating 3 with a thickness of 0.5 - 1 µm formed by several layers of thin light diffusive coating 31 was observed with an electron microscope, it was found to have 20,000 - 60,000 bubbles 32 and pits 33 per 1 mm², and despite its thinness, it gave excellent light diffusion, and moreover there were no mechanical or optical problems with the infra-red reflective coating 2. The high boiling-point organic solvent is not restricted to DOP, but may be any with similar physical properties which does not produce decomposition residues on baking. Light diffusive coatings were made by the same method as above, by adding varying proportions of various high boiling-point organic solvents to the organic titanium compound. The light diffusion effect of the coatings was investigated, and the results are shown in Table 1 in next page. Here, DBP, DOP and DAP are respectively abbreviations for di-butyl phthalate, di-octyl phthalate, and di-allyl phthalate. From Table 1 it can be seen that when a light diffusive coating 3 is formed using an organic titanium compound, then 5 - 20 % of DBP or DOP should be added. In addition, if low light diffusion is satisfactory, then 10 - 20 % of DAP may also be used. Light diffusive coatings were made by the same method as above, by adding varying proportions of various high boiling-point organic solvents to the organic silicate. The light diffusion of the coating was investigated, and the results are shown in Table 2 in next page. Solvent 5 % conc. 10 % conc. 20 % conc. DBP Good diffusion Good diffusion Good diffusion DOP Good diffusion Good diffusion Good diffusion DAP Transparent Weak diffusion Weak diffusion From Table 2 it can be seen that when a light diffusive coating is formed using an organic silicate then 5 - 20 % of DBP or DOP should be added. If formed under the same conditions then the titanium oxide diffusive coating gives better light diffusion than the silica light diffusive coating. This is thought to be due to the difference in their refractive indices. Furthermore, the dispersive effect of the light diffusive coating can be increased by applying several layers of a thin light diffusive coating, about 0.1 µm, to the same base. In this case, the first thin light diffusive coating is formed on the base by the method described above, and the second and subsequent thin light diffusive coatings are formed on top. In this case, as shown In Fig. 5, the pits 33A, 33B which are produced when the lower layers 31A, 31B are formed are covered up by the upper layers, but the bubbles 32A, 32B remain as they are. The nett result of this is equivalent to a thick light diffusive coating with an extremely large number of bubbles, providing excellent diffusion. When the light diffusive coating is formed in several layers, the method used may be that of application and baking repeated for each layer, or that of applying several layers and baking afterwards. The light diffusion effect may be further improved by the production of 0.001 - 10 µm microcrystals by baking the continuous coating 31 in air at 1,000 C for 10 minutes after the baking and hardening process described in the embodiment above. If this is done, the titanium layers in the previously formed infra-red reflective coating also crystalize, but this is acceptable. In addition, light diffusive coatings of different metal oxides may be formed by using organic compounds of other metals in the same way as in the method described above. The light diffusion effect of the light diffusive coating 3 may also be strengthened by combination with the pre-existing technology of dispersing particles of other metal compounds within the light diffusive coating material 31. Special properties may also be produced by adding metal compounds with special properties to these light diffusive coating materials. For example, by introducing powders with good thermal conductivity such as metal powders and aluminium oxide powder, the thermal radiation is increased. Also, if colouring powders such as cobalt blue are introduced, a coloured light diffusive coating which colours emitted light is produced. In these cases, the additive may or may not have a light diffusive effect, but if such an effect is required, the particle size of the additive should probably be 0.001 µm or larger. The operation of the halogen lamp in the embodiment shown in Fig. 1 and Fig. 2 will now be explained. When electricity is passed between the caps 10 the filaments 7 emit light. Of the light emitted by the filaments 7, the infra-red light is reflected by the infra-red reflective coating 2 with most of it returning to the filaments 7, heating them, and helping to improve their light-emitting efficiency. The visible light which has passed through the infra-red reflective coating 2 is dispersed by the light diffusive coating 3 and emitted as diffused light. Consequently, with this lamp, the distribution of light on the illuminated surface is more uniform, and an optical image of the filaments 7 does not arise, even if it is used in combination with a reflector. Since there are many bubbles 32 and pits, (variant bubbles) 33 in the light diffusive coating 3 of this halogen lamp, if the outer vessel 1 of the lamp is heated to a high temperature, the strains caused by the difference in thermal expansion coefficients of the outer tube and the titanium oxide of the light diffusive coating are absorbed by the bubbles 32 and pits 33. Because of this light diffusive coating 3 resists peeling off, even with repeated switching on and off over a long period. In addition, the light diffusive coating in this embodiment is extremely thin, 0.5 - 1 µm, which helps to reduce the strains, and is another reason why peeling does not occur. The light distribution along the axis of a photocopier lamp rated at 150 V, 250 W, in the embodiment described above, before and after formation of the light diffusive coating 3 were compared. The results are shown in Fig. 6. In Fig. 6, the X axis takes the centre of the lamp as 0 and measures the distance along the axis in mm. The Y axis gives the relative value of illumination, taking the centre of the straight tube type halogen lamp as 100. Curve A (solid line) shows the light distribution of the lamp after formation of the coating 3, and curve B (dotted line) shows the light distribution of the lamp before formation of the coating 3. From this graph it can be seen that the light distribution has been made uniform by formation of the coating 3. Also, when the brightness (total luminous intensity) of the lamp before and after coating was compared, it fell by no more than 2 - 3 % with the formation of the coating. This low light loss is a characteristic of this light diffusive coating 3. The present invention can also be applied to halogen lamps such as those using T-shaped tube type outer vessels or G-shaped tube type outer vessels. In a T-shaped type vessel, the filaments should be arranged almost concentrically, and in a G-shaped type vessel they should be arranged close to the centre of the rounded section. In addition, this invention may be used to form layers of coatings which let through visible light but reflect infra-red light and porous light diffusive coatings in the way described above on the outer surface of both blocked and unblocked transparent outer vessels. Moreover, it is possible to produce normal tube type halogen lamps with neither infra-red reflective coating or light diffusive coating on the centre section. The present invention is also applicable to halogen lamps for cars by forming layers of infra-red reflective coating and light diffusive coating on the outer surface of a T-shaped tube type outer glass vessel and placing a single filament in the centre of this vessel. Since, even if used in combination with a reflector, no optical image of the filament is formed on the illuminated surface, no irregular illumination is produced by the lamp. The present invention may also be applied to ordinary lamps. The outer vessel may be either spherical or pear-shaped, and the filament should be placed at the centre of the sphere or hemisphere section. In addition the present invention may be applied by forming layers of infra-red reflective coating and light diffusive coating on the outer surface of an unblocked inner tube, which then has a filament placed in its centre, and the assembly is then sealed in a cylindrical outer tube. In this case the inner tube touches the outer tube. In the embodiment described above, a light diffusive coating 3 was formed on the outside surface of an outer tube, but the invention is not limited to this application. It may also be formed on any surface, such as the inside surface of a halogen lamp, on the inside or outside surfaces of ordinary lamps and infrared lamps, etc., using soft glass such as soda lime glass or hard glass such as borosilicate glass, on the inside or outside surfaces of the outer vessel of high pressure discharge lamps, or on plate glass such as optical filters or window glass. Here, the surfaces on which the coating may be formed are given the generic name "base". Since the light diffusive coating in the present invention produces light diffusion by means of bubbles included in a continuous coating consisting of a metal oxide formed on a base, in spite of being extremely thin, it gives excellent light diffusion, reduces mechanical strain due to differences with the thermal expansion coefficients of the base, and can withstand repeated heating and cooling shocks over a long period, is mechanically robust, there is no reduction in light diffusion even over a long period of use, and light loss is extremely small. The method of forming the light diffusive coating in the present invention is by applying an organo-metallic compound combined with a high boiling-point solvent to a base and baking so that the organo-metallic compound decomposes to form a metallic oxide coating. Because of this, after application, the high boiling-point organic solvent forms tiny droplets which disperse throughout the coating, and then vaporize in the same position to form bubbles. This method allows the very simple formation of a light diffusive coating which is extremely thin and gives excellent light diffusion. Furthermore, since the present invention involves the establishment of a coating which allows visible light to pass through but reflects infra-red light on the outside surface of the transparent outer vessel of a halogen lamp, and the establishment of a porous light diffusive coating on top of this reflective coating, there is little infra-red radiation, efficiency is high, the light distribution on the illuminated surface is uniform, and there is no danger of the appearance of an optical image of the filament, even if used in combination with a reflector. Moreover, there is the advantage that there is no danger of the coating peeling off with repeated switching on and off over a long period, even though the outer vessel may be heated at high temperatures.

Claims: A method of forming a light diffusive coating characterised in that a mixture containing an organo-metallic compound and a high boiling-point organic solvent is applied to a base and dried to form a coating which contains dispersed droplets of said high boiling point solvent, the coating is then baked whereby said droplets evaporate and expand into bubbles and/or pits within a metallic oxide coating which is formed by decomposition of the organo-metallic compound. A method according to claim 1 characterised in that the high boiling point organic solvent is selected from di-octyl phthalate, di-butyl phthalate and di-allyl phthalate. A method according to claim 1 or 2 characterised in that the metal of said light diffusive coating is titanium. A method as claimed in any preceding claim wherein multiple coatings are produced by repeating the application, drying and baking steps, or by applying several coatings and baking afterwards. A method as claimed in any preceding claim wherein the base to be coated already has an infra-red reflecting coating. A method as claimed in any preceding claim wherein pits are formed in the outer surface of the applied coating. A lamp having a transparent glass outer vessel, light diffusive coating on the outer surface of said outer vessel, and a filament sealed inside said outer vessel, characterised in that the coating has been obtained by a method as claimed in any preceding claim. A halogen lamp having a transparent glass outer vessel, a coating which allows visible light to pass through, but reflects infra-red light, formed on the outer surface of said outer vessel, and a filament placed in the central section of said outer vessel, characterised in that it has a light diffusive coating produced by a method as claimed in any one of claims 1 to 6.

Inventors: KARINO, TOSHIO  C/O PATENT DIVISION, KAWAKATSU, AKIRA C/O PATENT DIVISION, YUGE, YOOJI C/O PATENT DIVISION, Karino, Toshio c/o Patent Division
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: C03C17
IPC codes: H01K1, C23C16, H01J61, C03C17, G02B5