The present invention relates to a display device comprising an evacuated envelope having a display window with a display screen on its inner face and, in front of its outer face, which is substantially parallel to the display window, a second window having a good transparancy. A cooling fluid circulates between the display window and the second window, said windows being in contact in zones of the windows.
A display device, for example a display tube, comprising a window having a network of channels destined to ensure the circulation of cooling fluid is described in Netherlands Patent Application filed in the name of N. V. Philips' Gloeilampenfabrieken on Mar. 19, 1982 under No. 82 011 36. The window of said display tube is constituted by two glass plates connected together and having cooling means constituted by grooves in one of the plates. Said grooves may be obtained at the end of moulding or etching operations of one of the plates.
Said grooves provided parallel to one of the rectangular sides of the window of the display tube may have a cross section of a trapezoidal or sinusoidal shape. Said shape given to the grooves permits of eliminating the light losses by the lateral edges of the grooves. The cooling fluid has a refractive index which is equal to that of the glass constituting the plate comprising the grooves. The other plate may have a different refractive index, the assembly of said characteristics causing no distortion of the image whatsoever. The screen of such a display tube is generally constituted by a luminescent layer on which there is written by means of an electron beam. Under the influence of the electron bombardment the temperature of the screen increases such that a loss of luminous efficiency of the luminescent material occurs. This phenomenon makes itself felt in particular when the current densities of the electron beams are considerable, as in projection tubes for television. This phenomenon necessitates the cooling of the luminescent material.
In order that inequalities of the refractive index connected with thermal inhomogeneities and with turbulances are not going to distort the image, it is necessary to make arrangements to the inlet pipes and outlet pipes for the cooling liquid in the cooling chamber and to give the said cooling chamber such dimensions that the circulation of the cooling fluid in the said chamber is laminar. This is described in Netherlands Patent Application filed in the name of N. V. Philips' Gloeilampenfabrieken on the 13th of January 1983 under No. 83 001 14.
In all these cases inconveniences appear in the field of the optical realisation and cooling. In effect the geometrical dimensions of the grooves must be determined so that they are sufficiently small, with respect to the picture element, not to distort the picture and nevertheless sufficient to ensure the circulation of the cooling liquid.
The shape of the grooves, having a cross-section which is rectangular, triangular, sinusoidal or otherwise, must also be examined, as well as the state of the surface of the said grooves.
In practice, the index of the fluid also is not strictly equal to that of the glass constituting the place in which the grooves are provided and picture distortions occur if the flow of the fluid is not laminar or if there is a temperature gradient involving a variation of the refractive index.