HEATED WINDOWS

Abstract: A heatable window, especially for optical equipment, comprising an elongate heating element located in one or more grooves formed in the window and maintained in place by adhesive material.

Description: This invention relates to electrically heated windows, particularly, but not exclusively, windows for use in optical equipment which is subjected to extreme environmental conditions. In certain atmospheric conditions mist or ice may form on the surface of windows and impair the performance of optical equipment behind the window. A method of overcoming icing or fogging is to heat the window electrically. However, in some applications, such as when a high power laser is to pass through the window or if sensitive optical equipment is used to receive low light levels, it is necessary to either avoid obscuring the window with heating elements or to minimise the effect. In this application the surface of the window must be heated evenly so that the optical properties of the window remain uniform and this is difficult to achieve for large areas of window. An array of heating wires could be bonded onto the outside surface of the window but this may cause smearing during wiping of the window and the wires could be damaged by the wiping. Another method is to bond wires to the inside surface of the windows. This enables the smooth outer surface to be wiped cleanly but a longer thermal path (ie the thickness of the window) is introduced between the elements and the outside surface of the window; consequently the window takes longer to de-ice and more energy is required. In another method wires are sandwiched between two layers of glass. As it is important optically that the outer surfaces of the final assembly are parallel to each other, these faces must be re-ground and polished after assembly, and is therefore expensive. Another approach is to lay a conductive film over the entire surface of the window but this reduces the optical transmissive properties of the window by up to 30% at some wavelengths. DE-C-928,310 discloses a glass pane having grooves in the surface which are coated with or contain heating elements of silver, copper or gold, thereby providing heat conducting members. According to one aspect of this invention there is provided an electrically heatable window, for use in optical equipment for example, having an elongate electrical heating element fixed into at least one groove in the window surface, characterised in that the heating element lies in the groove below said surface of the window and is fixed therein by adhesive material, which adhesive material substantially just fills the groove up to said surface to render this surface substantially uniform. Advantageously, the window has a further elongate electrical heating element wound around its periphery and bonded in place by means of adhesive. In one embodiment, the conducting elements are arranged to include parallel portions extending at spaced intervals across the window. Conveniently the conducting elements are arranged in parallel groups. Preferably the conducting elements in each group are arranged so that the total length of wire in a group is substantially the same as the total length of wire in any other group. In one embodiment, the conducting elements are arranged to include parallel portions extending at spaced intervals across the window. The spacing of the intervals may be regular or irregular. The conducting elements may be electrically connected in parallel, series - parallel groups or in series, preferably the conducting elements in each group are arranged so that the total resistance of wire in a given group is substantially the same as the total resistance of wire in any other group. The circuit can be arranged to give any desired heat distribution. The present arrangement provides a smooth surface which can be easily wiped and in which the heating elements are in close contact with the deposits they are to disperse. The arrangement also enables a window to be de-iced rapidly. The invention will now be described by way of example only referring to the figures in which: Figure 1 shows the top portion of a window and illustrates the arrangement of one group of conducting wires; Figure 2 is an enlarged section view through the window of Figure 1 along the line II; Figure 3 Is a schematic diagram of the heating elements in Figure 1 showing the arrangement of the groups; and Figure 4 is a perspective view of a window wound circumferentially with a heating element, said heating element being held in position with adhesive. Referring to figures 1 to 3 there is illustrated an embodiment of heated window in which the heating element is placed in parallel grooves in the external surface of the window. In the embodiment illustrated, the window 15 has a series of parallel slots 16 scored into one surface at a regular pitch to achieve a uniform temperature gradient across the surface of the window; however they may be irregularly spaced to create a different temperature gradient across the window. A further advantage of having irregularly spaced heating elements is that they disrupt the optical interference effects which might otherwise occur. The width of each slot must be minimal to reduce obscuration of the optical system and to improve thermal conduction from the wires. A wire 17 is placed in slot 16 and a low viscosity epoxy resin 18 or other suitable bonding agent is placed in the slot to bond the wire in place. The resin 18 substantially just fills the slot 16 up to the level of the surface of the window 15 so maintaining that surface substantially uniform. The wires 17 are arranged in several groups over the area of the window. Figure 2 shows a group of four wires 17 in parallel on the upper part 15 of a window. In the group shown a wire A is inlaid in slots 12, 2, and 1; a second wire B is inlaid in slots 11, 4, 3; a third wire C inlaid in slots 10, 6 and 5 and a fourth wire D inlaid in the slots 9, 8 and 7. The four wires in the group are the same length; however, because of the circular shape of the window the length of slots progressively increase down the window from slot 12 to slot 1. To overcome the difference in slot length each wire is inlaid into a combination of slot lengths so that it traverses approximately the same length of window as the other wires and so that the total length of each combination of slots is approximately the same. Figure 3 is a schematic circuit diagram of the wiring arrangement in Figure 1 and shows how the group is connected in a series parallel arrangement. The wiring arrangement disclosed in Figures 1 and 3 is an optimum arrangement which enables a uniform heat distribution to be achieved across a window at the same time if a wire in a group should fail the failure will be localised to the single wire in that group and the rest of the group will continue to function properly. As shown in Figure 4, to provide additional heating of the window, one or more coils of electrical heating wire may be wound around the periphery of the window and adhered to that periphery by epoxy resin. The slots scored into the window surface need not be straight parallel lines but could instead have a different shape appropriate to achieve particular heat distribution effects. For instance the slots might be scored as a circle around the periphery of the window or as a circle or a series of concentric circles and the wires arranged in coiled formation within the circle or circles. In these cases the power density for a given window can be varied by using wire of a different resistivity and/or by varying the number of turns in each coil. Any cross-section of wire heating element can be used.

Claims: An electrically heatable window, for use in optical equipment for example, having an elongate electrical heating element fixed into at least one groove in the window surface, characterised in that the heating element (17) lies in the groove (16) below said surface of the window and is fixed therein by adhesive material (18), which adhesive material substantially just fills the groove up to said surface to render this surface substantially uniform. A window according to claim 1, including a further elongate electrical heating element wound around the periphery of the window member and bonded in place by means of adhesive.

Inventors: CHANTLER, IAN DAVID BRITISH AEROSPACE PLC, PRICE, NORMAN WILLIAM BRITISH AEROSPACE PLC
Applicants: BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1987
Main classification: B64C1
IPC codes: B64C1, H05B3