EP0118962A1

EP0118962A1 - Electric lamp having a coloured lamp envelope

Info

Publication number: EP0118962A1
Application number: EP84200339A
Authority: EP
Inventors: Christiaan Prozee; Bauke Jacob Roelevink
Original assignee: Philips Gloeilampenfabrieken NV; Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1983-03-14
Filing date: 1984-03-09
Publication date: 1984-09-19
Also published as: CA1214195A; JPS59177851A

Abstract

An electric lamp according to the invention has a lamp envelope (1) which at its inner surface is coated with two electrostatically applied powder layers. One layer (2a) comprises a cadmium pigment and a light-scattering powder and the other inner layer (2b) comprises mainly light-scattering powder. In order to obtain comparable optical properties, in the lamp according to the invention a considerably smaller quantity of pigment is required than in conventional lamps having a single coating layer.

Description

The invention relates to an electric lamp having a glass lamp envelope which is sealed in a vacuum-tight manner and in which is arranged a light source connected to current-supply conductors extending through the wall of the lamp envelope to the exterior, said lamp envelope having on its inner surface an electrostatically applied coloured coating which comprises a light-scattering powder and at least one cadmium pigment. An incandescent lamp of this kind is known from United States Patent Specification 3 32₀,46₀.
Such a lamp can be used in an environment in which it has to be avoided that light attracts insects, as a festive illumination lamp, a disc illumination lamp and the like.
Electrostatically applied powder layers have the advantage as compared to layers formed from a powder suspension that during application no solvents and binders are introduced into the lamp envelopeo In fact the powder is dusted in a dry state in a lamp envelope whose wall is given a positve potential with respect to the powder_o Under the influence thereof the powder adheres to the wall.
An electrostatically applied powder layer has characteristic properties which distinguish the layer from a layer formed from a powder suspension. The layer has a very low packing density, which is even fifty times lower than that of a layer formed from a suspension of the same powder mixture. At its surface the layer has a very high degree of roughness as compared with a smooth surface of a layer obtained from a suspension. A noticeable difference is further that, when an electrostatically coated lamp envelope is observed along a tangent to the lamp envelope, it is clearly visible that the wall of the lamp envelope has a certain thickness. On the contrary, with a lamp envelope coated by means of a suspension, the wall thickness of the lamp envelope of the usual type, observed in the same manner, is not perceptible.
The requirement is imposed on coloured lamps that they have the same colour both in operation and out of operation, that is to say with transmitted and incident light, respectively, while the coating scatters the light generated by the lamp so that the light source is not visible and the wall of the lamp envelope is illuminated uniformly. It has been found that especially when deep colours are desired, cadmium compounds have to be used as pigments for several colours, such as red, yellow, and colours formed therewith, such as orange. These pigments have a large colouring force, as a result of which they colour the lamp intensely despite a mixing with light-scattering powder. Cadmium compounds moreover have a high thermal stability. A disadvantage of cadmium compounds, however, is their toxicity and this is the reason why it has to be avoided that at the end of the life of the lamps large quantities of these compounds show up in the environment.
The invention has for its object to provide a coloured lamp in which, whilst maintaining the colour properties of the lamp, the content of cadmium compounds is low, that is to say lower than in conventional coloured lamps.
According 10 the invention, in a lamp of the kind described in the opening paragraph this is achieved in that the coating comprises a layer on said surface of a mixture of light-scattering powder and pigment powder and an overlying layer of mainly light-scattering powder.
In a lamp according to the United States Specification mentioned in the opening paragraph, the coating comprises a single layer of a homogeneous powder mixture. This layer has to be comparatively thick in order to scatter to a sufficient extent the emitted light and to illuminate the lamp envelope uniformly. Further, the pigment content of the layer has to be comparatively high in order to provide for the lamp envelope the same colour with incident light and with transmitted light. The mixture used according to the aforementioned United States Patent Specification comprises equal parts by weight of silicon dioxide and cadmium sulphide.
In the lamp according to the invention, a thin outer layer having a high pigment content is sufficient. The thickness of this layer and the content of pigment depend upon the colour saturation required for the lamp. In general, the layer will have an average thickness of 50 to 70 _/um and the pigment content will be 35 to 45 % by weight. The inner layer mainly comprises light-scattering powder and does not, or substantially does not contain pigment. The average thickness of the inner layer is connected with the desired extent of uniformity with which the lamp envelope is illuminated by the light source. The inner layer will generally have an average thickness of approximately 75 to 130 _/um.
As compared with lamps having the same power, the same colour and the same uniformity of illumination of the lamp envelope with a conventional electrostatically applied coating, the lamp according to the invention contains a considerably smaller quantity of pigment, i.e. an up to 70% smaller quantity.
As the light-scattering constituent of the coating, use is mostly made of silicon dioxide or a mixture of various silicon dioxides. This constituent generally has a primary particle size lying mainly between 10 and 30 nm.
Examples of cadmium compounds that can be used as pigment in the coating are cadmium sulphide (yellow), cadmium sulphoselenide (red), mixture of these two substances (orange), cadmium sulphide/ultramarine blue (sodium polysulphide silico-aluminate) (green). The pigments have a primary particle size lying mainly between 100 and 5000 nm.
Powder mixtures can be prepared in dry state, for example, in a "fluidizing mixer". In this case, the desired resistivity of the mixtures can be adjusted by using both a hydrophobic light-scattering material (having, for example, a resistivity of 10¹⁴ Ω) and a hydrophylic light-scattering material (having, for example, a resistivity of 10⁷Ω), such as silicon dioxides.
In general, in the lamp according to the invention, a filament, which may be accommodated in an inner envelope, will be utilized as a light source. However, alternatives are high-pressure gas discharges, such as high-pressure sodium and high-pressure mercury vapour discharges.

Examples.

Lamps which during operation at 220 V consumed a power of 15, 25, 40 and 60 W, respectively, were manufactured with an A-60 lamp envelope, that is to say a lamp envelope having a spherically curved part, whose maximum diameter is 60 mm, and a neck-shaped part. The lamp envelopes were invariably coated at their inner surface with an outer electrostatically applied layer and an inner electrostatically applied layer. The outer layer had the following composition:

Pigment 40 % by weight; hydrophobic Si0₂ 20 % by weight; hydrophylic SiO₂ 40 % by weight. The inner layer consisted of equal parts by weight of hydrophobic and hydroplaylic SiO₂. The average thickness and the weight of the outer and the inner layers were: 50 _/um, 40 mg, and 120 _/um, 34 mg, respectively.
The lamps were manufactured in different colours with cadmiun sulphide, cadmium sulphoselenide-coprecipitate, cadmium sulphide/cadmiumsulphoselenide-coprecipitate and cadmium sulphide/ultramarine blue, respectively, as pigment.

For comparison, conventional lamps were manufactured with an electrostatically applied coating comprising a single layer consisting of: 40% by weight of pigment, 20% by weight of hydrophylic Si0₂, 40% by weight of hydrophobic SiO₂. On an average the layer thickness was 145 µm and the weight was 140 mg.
The lamps according to the invention in operation and out of operation had optical properties which could not or substantially not be distinguished from those of the conventional lamps. In lamps according to the invention, however, the pigment weight was only 28.6 % of that of the conventional lamps.
During the application of the coating layers and layer, respectively, the lamp envelope was given a positive potential of at least 12 kV with respect to the powder mixture to be applied.
An embodiment of the lamp according to the invention is shown in the drawing in side elevation, partly broken away.
In the Figure, the lamp envelope 1 has at its inner surface an electrostatically applied colour coating 2a,2b. The coating comprises two layers. The outer layer 2a consists of a mixture of light-scattering powder and at least one cadmium pigment: the inner layer 2b mainly consists of light-scattering powder. A filament 3 is arranged in the lamp envelope as a light source. Current-supply conductors 4 carry the light source 3 and extend through the wall of the lamp envelope 1, which is sealed in a vacuum-tight manner, to the exterior,where they are secured to contacts of a lamp cap 5 fixed to the lamp envelope.

Claims

An electric lamp having a glass lamp envelope which is sealed in a vacuum-tight manner and in which is arranged a light source connected to current-supply conductors extending through the wall of the lamp envelope to the exterior, said lamp envelope having on its inner surface an electrostatically applied coloured coating which comprises a light-scattering powder and at least one cadmium pigment, characterized in that the coating comprises a layer on said surface of a mixture of light-scattering powder and pigment powder, and an overlying layer of mainly light-scattering powder.