Reflector lamp

Abstract

 The reflector lamp has a second wall portion (3), which is internally provided with a metal layer (13), and a third wall portion (4), which adjoins the second wall portion (3) and is light-scattering. According to the invention, the third wall portion (4) has on the inner side an electrostatic powder coating (14), which extends also in an edge zone (7) of the second wall portion. The metal layer (13) also extends in the edge zone (7) on the inner side of the powder coating (14).

Description

[0001] The invention relates to a reflector lamp provided with
a blown lamp vessel sealed in a vacuum-tight manner comprising
a first neck-shaped wall portion
a second wall portion internally coated with a metal layer and adjoining the first wall portion
opposite to the first wall portion a third light-scattering wall portion adjoining the second wall portion,
a light source arranged in the lamp vessel, and
current supply conductors extending from the light source through the wall of the lamp vessel to the exterior.

[0002] Such reflector lamps are well known and are described in, for example, EP 0 237 104 A1.

[0003] The light generated by the light source of the lamp emanates for the major part through the third wall portion, either directly or after reflection on the second wall portion. In order that, when projecting the light beam formed onto a screen, a spot is obtained without abrupt differences in brightness, the third wall portion is made light-­scattering. For this purpose, the inner surface of the lamp vessel is etched.

[0004] Etching lamp vessels has a number of disadvantages. An important disadvantage is that the environment is loaded with residues of etching acid. Other disadvantages are that the mechanical strength of the lamp vessel is diminished by etching and that the metal layer of the second wall portion extends entirely over an etched surface. As a result, said metal layer lacks on the outer side a brilliant, reflecting appearance.

[0005] The invention has for its object to provide a lamp of the kind described in the opening paragraph, in whose manufacture the use of etching acid is avoided.

[0006] According to the invention, this object is achieved in a lamp of the kind described in the opening paragraph in that the third wall portion is provided on its inner side with an electrostatic powder coating, which extends in an adjoining edge zone of the second wall portion, and in that the metal layer on the second wall portion extends in the edge zone over the inner side of the powder coating.

[0007] An electrostatic powder coating can be obtained in a simple manner in that a powder, to which an electric charge is supplied, is dusted near a heated wall having an electric charge of a sign opposite to that of the powder. Under the influence of a potential difference of several kV, for example 10 or 15 kV, the powder then adheres to the wall.

[0008] An electrostatic powder coating is distinguised by a very low density and a high degree of unevenness of the powder surface from a coating comprising the same powder obtained from a suspension. Another difference is that upon observing an electrostatically coated transparent lamp vessel along a tangent line to said lamp vessel, the thickness of the wall of the lamp vessel is clearly observed. This is not the case with a lamp vessel coated with a suspension.

[0009] It is possible to apply electrostatic powder coatings only locally in that a lamp vessel is heated and provided with a charge only locally and the powder is prevented by means of a shield from getting into contact with the remaining surface of the lamp vessel. Lamp vessels only locally provided with an electrostatic powder coating are known from US 3 279 937 A.

[0010] However, it has been found that it is not possible to provide a lamp vessel having a wall portion mirror-coated with a metal layer with an electrostatic coating on the wall portion adjoining said mirror-coated wall portion. Each time an uncoated strip is obtained between the metal layer and the electrostatic coating. This strip leads to unacceptable optical effects.

[0011] Such an uncoated strip is avoided in that the electrostatic coating is provided on the third wall portion and on the adjoining edge zone of the second wall portion before the metal layer is provided on the second wall portion. The two-fold coating of the edge zone, first with the powder layer and then with the metal layer on the powder layer, is necessary to make certain that no uncoated regions occur in the second and/or in the third wall portion. The metal layer can be provided in the manner usual for reflector lamps in that a metal, for example aluminium, gold or silver, is evaporated in vacuo in the not yet sealed envelope by current passage through a resistance element, with which this metal is in contact within the envelope. The third wall portion is then shielded from the metal vapour by a hood in the proximity of the resistance element, for example by the shield used in the aforementioned US 3 279 937 A.

[0012] The electrostatic coating will generally be only very thin, for example amount to 20 µm or less, and will consist of a colourless substance or a colourless mixture, for example of SiO₂ or mixtures of SiO₂ powder of different origins.

[0013] The nature of the light source, a filament or a pair of electrodes in an ionizable medium is, like the shape of the mirror-coated wall portion, not essential to the invention. This portion may be conical, parabolic, elliptical, spherical or may be curved in a different manner.

[0014] An embodiment of the lamp according to the invention is shown in the drawing.
In the drawing:

Figure 1 is a side elevation partly broken away of the lamp,

Figure 2 shows a part of the lamp vessel of the lamp shown in Figure 1 with the wall coatings in sectional view.

[0015] The reflector lamp shown in Figure 1 is provided with a blown lamp vessel 1 of glass, which is sealed in a vacuum-tight manner and comprises a first neck-shaped wall portion 2 and a second wall portion 3, which is internally coated with a metal layer 13 (Figure 2) and adjoins the first wall portion 2.
Opposite to the first wall portion 2, the lamp vessel 1 has a third light-scattering wall portion 4, which adjoins the second wall portion 3.
A light source 5, i.e. a filament, is arranged in the lamp vessel 1 and current supply conductors 6 extend from this source through the wall to the exterior. In the lamp shown, the second wall portion 3 is curved parabolically and a lamp cap 8 is present, to which the current supply conductors 6 are connected.
The third wall portion 4 is provided on the inner side with an electrostatic powder coating 14 (Figure 2), which extends in an adjoining edge zone 7 of the second wall portion 3. The metal layer 13 (Figure 2) on the second wall portion 3 extends in the edge zone 7 over the inner side of the powder coating 14 (Figure 2).
In the embodiment shown, the powder coating 14 consisted of equal parts by weight of a hydrophilic and a hydrophobic SiO₂ powder. The coating had a thickness of about 10 µm. This is not more than one tenth of the thickness of a coating on lamp vessels coated entirely or for the major part with an electrostatic coating to cause a lamp to irradiate diffuse light. The metal layer consisted of aluminium. During operation at mains voltage, the lamp consumed a power of 60 W and formed a light beam of the same quality as a lamp commercially available having an etched third wall portion.

Claims

A reflector lamp provided with
a blown lamp vessel sealed in a vacuum-tight manner
comprising
a first neck-shaped wall portion,
a second wall portion, which is internally coated with a metal layer and adjoins the first wall portion,
opposite to the first wall portion a third light-scattering wall portion, which adjoins the second wall portion,
a light source arranged in the lamp vessel, and
current supply conductors extending from the light source through the wall of the lamp vessel to the exterior,
characterized in that
the third wall portion is provided on the inner side with an electrostatic powder coating, which extends in an adjoining edge zone of the second wall portion, and in that the metal layer on the second wall portion extends in the edge zone over the inner side of the powder coating.

Drawing