The demand for lamps having a reflector envelope equipped with a halogen incandescent insert lamp of modified light is gradually increasing. They are very advantageous for illumination of objects arranged in a shop-window, but they are very often used for general lighting purposes owing to the fact that they provide concentrated illumination at low energy consumption.
Lamps having a reflector envelope first came into general use in open embodiments, i.e. they had no front element, as shown, for example, in U.S. Pat. Nos. 4,021,659, 4,169,237 and 4,169,238.
However, illuminating lamps having a closed reflector envelope are now in demand. In these lamps the conical reflecting body was united with a front element and so a closed envelope is formed. The front element is either simply a light transmitting plate matching the front surface of the conical element or an edge extending from the front surface the conical element, or it constitutes a special optical element modifying the light emitted by the insert lamp due its structure.
There is known a lamp wherein the conical body is united with the front element by mechanical means, for example, by the use of a metallic pressure ring having edges from two sides, as can be seen, for example, in the U.S. Pat. No. 4,213,170.
Further, there is known also a lamp, wherein the conical reflecting body is united with a front element by adhesive, as can be seen in German Laid-Open Appln. No. DE-2 228 684.
The known solutions prescribing uniting a conical reflecting body and a front element have not fulfilled the requirements as expected.
The rigid fixation made by adhesive is not durable enough, especially in the case of an insert light source consisting of a halogen incandescent lamp. For example, if the front element arranged in a recess made along the front surface of the conical reflecting body is fixed by an adhesive of high thermoresistivity, the recess made on the front surface of the conical reflecting body should be prepared so that the mantle surface of the front element matches the edge with a substantially small matching clearance. This is important because the conical reflecting body made of hard glass having a linear thermal expansion coefficient of about 36.10.sup.-6 cm/cm..degree.C. should be kept together with the front element of generally very different thermal expension coefficient by the adhesive applied. If the matching clearance between the outer mantle surface of the front element and the recess surrounding the edge of the conical reflecting body is greater than sometimes ten microns, the thermal expansion and contractions cause the adhesive bond to crack and gradually to decompose during the life of the lamp. The adhesive bond will slowly be lost and between the front element and the conical body a harmful resonance comes into being. If the lamp is applied upwardly or in a horizontal position it is a further problem that the cracked adhesive bond after a time cannot hold the front element, which falls out from the conical body.
If the fixation is realized only by mechanical means, it is not stable enough. An accidental vibration load can cause light movement of the front element and this has a harmful effect especially when the front element performs an optical function.