GB2371562A

GB2371562A - Coated reflector for a lamp

Info

Publication number: GB2371562A
Application number: GB0102102A
Authority: GB
Inventors: Ian Bradshaw; Derek Hurst; Keith Parker
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 2001-01-26
Filing date: 2001-01-26
Publication date: 2002-07-31
Also published as: GB0102102D0

Description

COATED REFLECTOR FOR A LAMP This invention relates to a coated reflector for a lamp.

Current aluminium coated reflector lamps suffer a decline in reflectivity performance due to environmental and thermally induced coating degradation.

This leads to a significant increase in the transmission of IR and visible radiation through the back of the reflector. Minimizing back-transmitted infrared radiation is critical in applications where the lamp is in close proximity to heat sensitive materials.

Initial reflectivity of plain aluminium coated reflectors is of the order of 93% in the visible region, however this performance deteriorates rapidly down to below 90% within 100 hours of deposition.

I The present invention seeks to provide a reflector for long life lamp in which thermally sensitive components such as, seals, foils, electronics, ballasts, fixtures, transformers, power converters and so on, are protected from the IR generated by the lamp.

The invention also seeks to provide a reflector for a lamp in which the reflective metallic coating is protected from oxidation and thermal degradation.

In one embodiment of the invention it is sought, in addition to the above, to produce a coated reflector with a superior reflectivity performance compared with a plain aluminium coating.

According to the invention, there is provided a reflector for use in a display lamp comprising a substrate, for example glass, a visibly opaque coating layer of plain aluminium on the substrate and a protective dielectric layer on the aluminium layer.

Preferably there are at least two dielectric layers. There may be in excess of ten dielectric layers, for example, twenty one dielectric layers.

The layers may comprise alternate layers of high refractive index and low refractive index. By way of example the layers of high refractive index may be

selected from the group consisting of SiC, ZnS, Tir, Ta205, Nb2O5, Si3N4 Y203 and lrO2 while the layers of low refractive index may be selected from the group consisting of MgF2, Si2O3 and Al2O3. However this list is not exhaustive In this embodiment the layers comprise alternate layers of ZnS and Si203.

The plain aluminium coating is preferably 99.9% pure.

The invention will now be described in greater detail, by way of example, with reference to the drawing, the single figure of which is a sectional view through one embodiment of the invention

I In this drawing, is shown a representative section of the material used in fabricating the lamp. It will of course be appreciated that the actual reflector of the lamp has deeply contoured surfaces with high aspect ratio as prescribed by the design geometry of the optics. The section is not to scale. Coating layers are shown relatively large in comparison with the substrate.

In this embodiment, a glass substrate 3 of a suitable lamp glass is coated with a layer 5 of visibly opaque plain aluminium (99.9% pure), then one or more protective dielectric layers 7,9 are applied over the aluminium layer 5. In the embodiment shown, there are ten layers but the range available is from one layer to any desired practical number. Preferred numbers of layers are 2 and 21.

It will be appreciated that while the particular embodiment has been described using a glass substrate, other substrates such as plastics, non-ferrous metal or ferrous metal can be used It has been found that the deterioration due to de-lamination and chemical reaction (reduction and/or oxidation) of a plain aluminium coated reflector can be significantly reduced by the addition of a protective dielectric coating. Reflectivity of an optimized"Enhanced"coating can be has high as 99%, with minimum deterioration compared to uncoated plain Aluminium; thus this significantly improves the life performance of the coating compared to plain aluminium coating.

Several different manufacturing processes which include deposition techniques that are specifically designed for high aspect ratio substrates (where achieving uniformity of coating thickness is paramount) are available for the deposition of the aluminium and alternating layers of dielectric material. These include : physical vapour deposition (PVD), chemical vapour deposition (CVD), plasma impulse chemical vapour deposition (PICVD), ion assisted deposition (lAD), sputtering, ion implant, hybrids incorporating R. F. or microwave systems, dipcoating or spray coating. A preferred embodiment is one which includes a dichroic coating that has been tuned to optimise the reflective visible radiation.

The coating structure design depends upon the specific materials, their refractive index, layer thickness and number of layers. The design constraints on a dielectric interference filter are-such that the refractive indices in a bi-material stack must be widely disparate. Examples of materials that are be appropriate for use in such interference filters are listed in table 1. The list of high and low refractive index materials is not exhaustive and other combinations of high and low refractive index materials are available; these include TiO2 & Si203. Table 1: Materials suitable for multilayer dielectric interference filters

High Index Low Index SiC 2. 6 MgF2 1. 35 ZnS 2.2 SiO2 1. 45 TiO2 2.1-2.7 Al2O3 1. 85 Ta2O5 1.50 Nb2O5 2.35 Si3O5 1.46 - 1.48 Si3N4 2.0 Y203 1. 9 Zero2 2.0-2. 15 The durability of the dielectric coating on a substrate is dependent upon both the thermal cycling conditions (which may induce micro-cracks due to high tensile stress) and environmental conditions (such as oxidation and/or erosion due to water-with and without dissolved salts). Observed signs of degradation of dielectric coatings includes: * Reduced reflectively, due to oxidation of the dielectric material . Colour shift in the appearance of the reflected visible radiation, due to erosion of the surface layers and a reduction in the physical thickness # De-laminated surface appearance due to stress cracking; this can lead to both visible and IR radiation being transmitted through the substrate.

One embodiment of the invention includes a reflector with plain aluminium base coat and an over-coat of a dielectric material; such that the over-coat protects the Aluminum coating from degradation, however the over-coating (in this example) does not improve the reflectivity of the total coating system. By way of an example of such a coating system see table 2 below. Table 2 - ai-material protective stack

Layer No. Material Refractive Index Physical Thickness I nm Substrate 1.46 massive 1 Al 0. 82-5. 99i < 1000 (opaque) 2 Si203 1. 46 94 3 ZnS 2. 35 59 Air 1.00 massive The preferred embodiment includes a reflector with plain aluminium base coat and an over-coat of a dielectric material ; such that the over-coat both protects the Aluminum c. oating from degradation and also improves the visible reflectivity of the total coating system. Such a coated reflector would have a superior reflectivity performance compared with a plain aluminium coating. The coating is tuned to visible light, for example a wavelength of between 380 and 760 nanometres. By way of an example of such an enhanced coating system see table 3 below. Table 3 - Enhanced coating system which includes Aluminium and 21 layers of dielectric material

Layer No. Material Refractive Physical Index thickness (nm) Substrate 1.46 massive 1 i 0. 82- 5. 99i < 1000 (opaque) 2 ZnS 2. 35 79 3 Si2O3 1.46 128 4 ZnS 2. 35 79 5 Si2O 1. 46 128 6 ZnS 2. 35 68 7 Si2O3 1.46 110 8 ZnS 2.35 68 9 Si2O3 1.46 110 10 ZnS 2.35 59 11 Si2O3 1.46 95 12 ZnS 2. 35 59 13 2O3 1.46 95 14 ZnS 2. 35 53 15 Si! 1. 46 86 16 ZnS 2.35 53 17 Si2O3 1.4686 18 ZnS 2. 35 48 19 Si203 1. 46 77 20 ZnS 2. 35 48 21 O3 1.46 77 22 ZnS 2. 35 24 Air 1.00 massive

Claims

CLAIMS :

1. A reflector for use in a display lamp comprising a substrate, a visibly opaque coating layer of plain aluminium on the substrate and a protective dielectric layer on the surface of the aluminium layer.

2. A reflector as claimed in claim 1, wherein the substrate is high aspect ratio glass.

3. A reflector as claimed in claim 1, wherein the substrate is plastics, nonferrous metal or ferrous metal.

4. A reflector as claimed in claim 1 or 2 or 3, wherein there are at least two dielectric layers :

5. A reflector, as claimed in claim 4, wherein there are in excess of ten dielectric layers.

6. A reflector as claimed in claim 5, wherein there are twenty one dielectric layers.

7. A reflector as claimed in claim 4,5 or 6, wherein the layers comprise alternate layers of high refractive index and low refractive index.

8. A reflector as claimed in claim 7, wherein the layers of high refractive index are selected from the group consisting of SiC, ZnS, Tri02. Ta2O5 Nb2O5 Si3N4, Y2O3 and ZrO2,

9. A reflector as claimed in claim 7 or 8, wherein the layers of low refractive index are selected from the group consisting of MgF2, SO and AO3

10. A reflector as claimed in claim 4,5 or 6, wherein the layers comprise alternate layers of ZnS and Si203.

11. A reflector as claimed in any preceding claim wherein the plain aluminium coating is 99.9% pure.

12. A reflector as claimed any preceding claim, wherein the dielectric coatings are tuned to visible light.

13. A reflector as claimed any preceding claim, wherein the dielectric coatings are tuned to wavelengths between 380 and 760 nanometres.

14 A coated reflector for a lamp substantially as described herein with reference to the drawing.