This invention relates to a display arrangement which is capable of presenting bright, but readily alterable displays with modest power consumption and without being excessively bulky.
According to a first aspect of this invention a display arrangement includes a sealed envelope containing a mesh electrode positioned between an electron emissive cathode arranged to emit a divergent flood beam of electrons and a fluorescent screen which forms part of the envelope so that the flood beam falls upon a predetermined area of said mesh electrode; and field electrode means positioned closely adjacent to the cathode for controlling the strength and polarity (with respect to the cathode) of the electric field in which the cathode is situated, so as to determine whether or not electrons are able to reach said screen to cause it to fluoresce.
According to a second aspect of this invention, a display arrangement includes a sealed envelope containing a mesh electrode positioned between an electron emissive cathode arranged to emit a divergent flood beam of electrons and a fluorescent screen which forms part of the envelope so that the flood beam falls upon a predetermined area of said mesh electrode; and field electrode means positioned so that the cathode is between at least a part of said means and said mesh electrode, the field electrode means being positioned closely adjacent to the cathode for controlling the strength and polarity (with respect to the cathode) of the electric field in which the cathode is situated so as to determine whether or not electrons are able to reach said screen to cause it to fluoresce.
The strength and polarity of the electric field in which the electron emissive cathode is situated is dependent on the potential of the cathode itself as well as the potentials on the mesh electrode and the field electrode. Since the potential on the mesh electrode is primarily dictated by other considerations, it is preferable to use the potential upon the field electrode to control the passage of electrons from the cathode to the fluorescent screen. By arranging that the electron emissive cathode is situated in an electric field which is more negative than the potential of the cathode, free electron emission is inhibited, and electrons are tightly confined to the vicinity of the cathode surface and thus are unable to reach the fluorescent screen. Conversely, by altering the polarity of the electric field with respect to the cathode, electrons are freely emitted and accelerated towards the mesh electrode which is at a potential somewhat more positive than that of the cathode. Once they reach this mesh electrode the electrons are rapidly accelerated towards the fluorescent screen, which typically has a potential of several thousand volts upon it. Increasing the magnitude of the electric field provides a control over the quantity of electrons which reach the screen and hence the brightness of the display, although the brightness is primarily dependent on the magnitude of the accelerating potential on the screen. The presence of the mesh electrode effectively isolates the cathode and the field grid from the effects of the high potential on the screen, and thus the display can be switched on and off by means of very low voltages in a rapid and reliable fashion.
According to a third aspect of this invention a method of operating a display arrangement comprising a sealed envelope containing a mesh electrode positioned between an electron emissive cathode arranged to emit a divergent flood beam of electrons and a fluorescent screen which forms part of the envelope so that the flood beam falls upon a predetermined area of said mesh electrode; and field electrode means positioned so that the cathode is between at least a part of said means and said mesh electrode, the field electrode means being positioned closely adjacent to the cathode for controlling the strength and polarity (with respect to the cathode) of the electric field in which the cathode is situated so as to determine whether or not electrons are able to reach said screen to cause it to fluoresce, includes the steps of applying a first predetermined potential difference between the field electrode means and the cathode so as to cause the divergent flood beam of electrons of predetermined size to be emitted from said cathode so as to cause fluoresence of said screen; and applying a second selectable predetermined potential difference between the field electrode means and the cathode so as to prevent electrons reaching the mesh electrode.
The invention avoids the need to position control electrodes between a cathode structure and the screen to achieve selective illumination thereof, and it permits the thickness of a display arrangement to be very small indeed, since the control electrode, which comprises, in effect, the field electrode, can be positioned on that side of the cathode which is remote from the screen. Advantageously, one or both of the cathode and the field electrode are of a segmented nature, so that the selection of particular segments constrains electrons to call upon selected locations of the screen to permit complex display patterns to be generated and rapidly altered. In principle, the mesh electrode can be of a segmented nature, so that it can also be used to selectively address locations of the fluorescent screen, but this is less preferred.
The display arrangements can take a number of different physical forms. For example, it can be arranged to generate a stylised symbol or character, usually a seven stroke character based upon the numeral eight. Alternatively, it can be used to select from a matrix of possible points or small patches of light just those points which act together to represent the required display pattern. Other variations are described subsequently with reference to the drawings. The use of an appropriate fluorescent screen enables a colour display to be provided, if necessary.