Device for the [servo-] control of the cut-off voltage of a cathode-ray tube by measurement of luminance

The disclosure relates to cathode-ray tubes that have to be used at a low level of luminance. A zone of the screen of the tube is covered with a luminophore of high sensitivity and is associated with a luminance sensor by an optic fiber that measures the luminance of the tube at chosen instants corresponding to an excitation of said luminophore by an electron beam deflected towards the part of the zone facing the fiber. The luminance signal is compared with a desired value and the result of the comparison is used to modify the cut-off voltage of the tube through action on the voltage of the cathode or of the Wehnelt gate of the tube. This device can be applied to cathode-ray tubes used in aircraft.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to cathode-ray tubes and, more particularly, to a 
device for the automatic modification of the cut-off voltage of a 
cathode-ray tube as a function of the luminance measured on the screen of 
the tube. 
2. Description of the Prior Art 
A cathode-ray tube 10 (FIG. 1) comprises in a chamber 11 under vacuum: a 
cathode 12 comprising a heated filament 16 that emits electrons and an 
anode 13 that is brought by means of a terminal 19 to a positive potential 
(HT) higher than the potential VK of the cathode so as to attract the 
electrons towards a surface 14 which constitutes the screen of the 
cathode-ray tube. The internal wall of the screen is coated with 
luminophores which get illuminated when they receive the electrons emitted 
by the cathode. This enables luminous images to be made to appear on the 
external wall of the screen by deflecting the path of the electrons, 
notably by means of variable magnetic fields created by deflection coils 
15. 
In order to achieve greater control over the path of the electrons and 
modulate the intensity of the electron beam, the electrons emitted by the 
cathode 12 go through a structure constituted by three electrodes or gates 
G1, G2 and G3 which are carried to potentials appropriate to their role. 
It is thus that the gate G1, better known as the Wehnelt gate, is 
positioned in the vicinity of the cathode and is at a negative potential 
VG1 with respect to this cathode so that it can stop or let through 
electrons going towards the screen. The gate G2, known as the acceleration 
electrode, is placed in the vicinity of the gate G1 towards the screen and 
is at a positive potential VG2 with respect to the cathode. Finally, the 
gate G3, known as the focusing gate, is placed before the deflection coils 
15 and is at a positive potential VG3 with respect to the cathode. 
In FIG. 1, the potentials of the different cathodes are obtained 
schematically by potentiometers 17, 18 and 101. The potentiometer 17 is 
connected between a terminal at +100 volts for example and a terminal 
connected to the ground. The potentiometer 18 is connected between the 
ground and a high voltage (HT) of 16 kilovolts for example. The 
potentiometer 101 is connected between the ground and a potential of -200 
volts. 
The cathode 12 is connected to the output terminal of the potentiometer 17 
and its potential VK can therefore vary from 0 to +100 volts. The Wehnelt 
gate G1 is connected to the output terminal of the potentiometer 101 and 
its potential VG1 may therefore vary from 0 to -200 volts. The accelerator 
gate G2 is connected to a first output terminal of the potentiometer 18 
and its potential VG2 may therefore vary from 0 to some thousands of 
volts. The focusing gate G2 is connected to a second output terminal of 
the potentiometer 18, and its potential VG3 may therefore reach several 
thousand volts. 
It will be understood that the intensity of the electron beam and, hence, 
that of the luminous dot on the screen can be modulated by the 
modification of the voltage VGK1. To this effect, the gate G1 is biased at 
a voltage Vco, called a cut-off voltage, and a variable modulation voltage 
is applied to it to obtain a variable beam electron current and hence a 
variable luminance of the light dot on the screen. 
The cut-off voltage Vco corresponds to the difference in potentials VKG1 
which is just enough to prevent the passage of electrons towards the 
screen. 
FIG. 2 is a graph showing the variation of the cathode current Ik which 
corresponds substantially to the luminance of the dot on the screen, as a 
function of the voltage VKG1 between the cathode and the gate G1. The 
curve 20, which is quasi-logarithmic, shows that the current Ik is zero 
for VKGl=Vco and that it reaches the value Iko for VKG1=0. 
To obtain a linear characteristic between the signal applied to the gate G1 
and the luminance on the screen, it is necessary, firstly, to linearize 
the curve 20 and, secondly, to hold the cathode-ray tube at its cut-off 
voltage in the absence of a modulation signal. This holding is all the 
more critical as the tube operates at low values of luminance, which it 
does when the cathode-ray tube is used in a dim environment. 
To guarantee the stability of the low-level luminance, it is necessary: 
always to bias the tube at its cut-off voltage; 
to keep the voltage VKG2 stable between the cathode and the accelerator 
gate; 
to keep the cathode heating power stable, i.e. ensure a certain precision 
and stability of the voltage Vf which is applied to the heating filament 
16; 
to keep the difference in potentials VKA between the cathode and the anode 
stable. 
To solve these problems, it has been proposed to bias the tube with 
voltages VKG2, Vf and VKA that are as constant as possible, but it is 
difficult to maintain these voltages with a precision higher than 1%. 
Furthermore, the characteristics of the tube, notably the cut-off voltage, 
change: 
during the thermo-mechanical stabilization of the electron gun, when 
starting the system and 
in the course of ageing during the life of the tube. 
The result thereof is that the bias voltages would have to be readjusted in 
the course of time. 
To compensate for these drifts, devices have been proposed for the 
servo-control of the cut-off voltage of the tube by the measurement of the 
cathode current. This servo-control is done at regular intervals, for 
example during the frame flyback or retrace of the image, and its value is 
memorized during the next frame. 
The acquisition of the servo-control value is done in two steps: 
a first step of applying, to the gate G1, a voltage greater than the 
cut-off voltage and of measuring the cathode leakage currents. The result 
of this measurement is subtracted from the measurement made in the second 
step and makes it possible to do away with the effects of the leakage 
currents; 
a second step of applying, to the tube, a low modulation voltage of a known 
value and of servo-controlling the potential VKG1 so as to measure a 
cathode current Ik which is the sum of the leakage currents measured 
during the first step and of a constant current Iks corresponding to the 
value that would be generated by the desired value or set value of the 
modulation which is applied. 
Such a method is satisfactory when the dynamic range of cathode current is 
between 10 micro-amperes and 2 milliamperes, which corresponds to 
servo-control currents Iks that are appropriate when the minimum light 
conditions are what are known as drawing room conditions as is the case 
with family television sets. 
When the tube is placed in a very dim environment and/or when it is very 
sensitive (because of the high output of the luminophores), the 
servo-control should be done at cathode current values far lower than one 
microampere. This is difficult to achieve because of the values of the 
insulation resistance and of the inter-electrode parasitic capacitances. 
Furthermore, this prior art method does not take account of the variation 
of the sensitivity of the luminophores, namely their light output, in the 
course of time. 
Other devices have been proposed, using a photodetector placed before or 
optically coupled to a part of the screen that is not normally used. 
Because of the performance characteristics required of the screen, the 
single phosphor or luminophore that is used generally has a fairly lengthy 
response time of at least several milliseconds. 
Consequently, if it is sought to obtain an electrical signal with an 
amplitude representing the luminance of the phosphore, it is necessary to 
maintain the test pulse for the same length of time. 
For applications where it is acceptable to interrupt the display of the 
image temporarily to carry out the operation for the automatic correction 
of the cut-off voltage of the tube, the build-up time of the phosphor is 
not an inconvenient factor. 
The particular feature of the invention is that it overcomes the problem of 
the build-up or raising time of the phosphor because any elimination, even 
a very occasional one, of a frame of the displayed image is considered to 
be inacceptable in many applications. 
SUMMARY OF THE INVENTION 
The object of the present invention, therefore, is to make a device for the 
servo-control of the cut-off voltage of a cathode-ray tube that enables 
operation at very low luminance while at the same time eliminating the 
problems related to the ageing of the cathode-ray tubes and to the 
temporal drifts of their characteristics. 
The invention relates to a device for the servo-control of the cut-off 
voltage of a cathode-ray tube, the screen of which has at least one first 
operational zone of the tube used to make the images appear on the screen 
and one second zone positioned outside the first zone, said device 
comprising a luminance sensor that is positioned facing the second zone to 
measure the luminance of the associated surface of the second zone, 
scanning means to direct the electron beam, at certain chosen instants, 
towards said surface associated with the sensor and means to modify the 
cut-off voltage of the tube as a function of the value of the luminance 
measurement in relation to a desired value, wherein the luminophore 
covering the second zone has a build-up or raising time to 99% that is 
smaller, by at least one order of magnitude, than that of the luminophore 
covering the first zone. 
Preferably, this build-up time to 99% is of the order of a few microseconds 
for the luminophore of the second zone whereas it is of the order of a few 
milliseconds for the luminophore of the first zone.

MORE DETAILED DESCRIPTION 
The invention consists in modifying the cathode current Ik as a function of 
the measurement of the luminance of a point of the screen located outside 
the part that is normally used; preferably, this point of the screen is 
made with a luminophore having particular characteristics that shall be 
defined hereinafter. 
To this effect, the screen 14 comprises (FIG. 5), in addition to a useful 
surface S made with the usual luminophores, a zone S.sub.o made with a 
luminophore Lo having features suited to its role in the device according 
to the invention. This zone S.sub.o is located at the periphery of the 
screen and is not normally scanned by the electron beam emitted by the 
cathode to make an operational image. 
The luminophore Lo of the zone S.sub.o is chosen according to the following 
criteria: 
its wavelength should be such that any visible parasitic radiation towards 
the operational surface of the tube is avoided; preferably, it will be in 
the infrared range; 
its build-up time to 99% should be as short as possible so that the 
duration of the measurement phase of the luminance should be as short as 
possible; a build-up time of the order of a few microseconds is acceptable 
for a measurement during the frame flyback; 
its energy yield should be as high as possible so as to servo-control the 
system as closely as possible to the cut-off voltage. 
It will be noted that the build-up time to 99% of the luminophore of the 
operational zone S is generally of the order of a few milliseconds which 
must be compared with the build-up time of a few microseconds of the 
luminophore Lo of the zone S.sub.o. It is recommended that the build-up 
time of the luminophore Lo be preferably far smaller than that of the 
luminophore of the zone S, smaller by at least by one order of magnitude 
(in a ratio of 10) to several orders of magnitude in the example described 
hereinabove (a ratio of 1000). 
With this zone S.sub.o, there is associated a photoelectric sensor 30, of 
the photoconductive diode or similar type, which gives an electrical 
signal representing the luminance of a dot of the zone S.sub.o. Of course, 
the sensor 30 may be in the immediate vicinity of the zone S.sub.o or it 
may be at a distance therefrom and, in this case, may be connected to it 
by an optical fiber 31 as shown in FIG. 3. 
The luminance sensor 30 should have the following characteristics: 
its sensitivity should be the maximum for the wavelength of the luminophore 
Lo; 
its surface area should be great enough to increase the energy received and 
facilitate its positioning; 
its parasitic capacitance should be low enough for the passband of the 
detection system to allow the measurement to be done swiftly. 
The electrical signal given by the sensor 30 is applied to a preamplifier 
32, the output signal of which is applied to a circuit 73 which eliminates 
the component due to the dark current of the sensor 30 from the signal. 
To this effect, the circuit 73 samples the value of the dark current during 
the phase P0 (FIG. 4a) and this sampled value is memorized for the rest of 
the time to be deduced from the signal coming from the preamplifier 32. 
The circuit 73 may be designed in different ways and notably according to 
the diagram indicated within the rectangle 73. It includes an amplifier 
56, the negative feedback loop of which is constituted by a 
sample-and-hold circuit and a subtractor circuit 57. The sample-and-hold 
circuit comprises a first amplifier 75, one input terminal of which is 
connected to the output terminal of the amplifier 56 and the other input 
terminal of which is connected to a reference voltage source REF. The 
output terminal of this first amplifier is connected to a terminal of a 
memory capacitor C' by means of a switch 74, the other terminal being 
connected to the ground. An amplifier 76 connects the capacitor C' to the 
subtractor circuit 57. The opening and closing of the switch 74 are 
activated by the phase signal P0 (FIG. 4a). 
The output signal of the circuit 73 is applied to a sample-and-hold circuit 
33 to be compared, in an error amplifier comparator 45, with a set value 
of luminance Vc that is adjusted by means of a resistive voltage divider 
connected between a voltage source REF and the ground and comprising a 
resistor RO and a potentiometer PTO. This sample-and-hold circuit 
comprises, mainly, the error amplifier comparator 45 which carries out 
this comparison and gives, at its output terminal, a correction voltage 
Vcor which, as shall be described further below, will enable the 
increasing or reducing of the cut-off voltage of the tube during the 
sampling phase P2 (FIG. 4c). 
Through the closing of a switch 44 during this phase P2, this correction 
voltage Vcor is memorized in a capacitor Cm throughout the rest of the 
time when the switch 44 remains open. 
The output stage of the sample-and-hold circuit 33 is constituted by an 
amplifier 46, the output terminal 34 of which is at a voltage V.sub.o 
which is equal to the charging voltage of the capacitor Cm at a point A 
(terminal A). 
The voltage V.sub.o is subtracted from a voltage Vv corresponding to a 
video signal VDO in a subtractor circuit 35, the output terminal 36 of 
which is connected to the cathode of the cathode-ray tube 10 by means of 
an amplifier 37. 
The video signal VDO is applied to the subtractor circuit 35 by means of a 
corrector circuit 38, better known as a gamma corrector circuit, which has 
the effect of linearizing the luminance of the screen as a function of the 
luminance control signal constituted by the video signal VDO. 
Furthermore, according to the invention, the video signal is not applied 
permanently to the gamma corrector circuit owing to the interposition of a 
three-position selector 39 that enables the connection of the gamma 
corrector 38 and, consequently, of the subtractor circuit 35: 
either to the video signal VDO in normal operation (position 1), 
or to the ground (position 3), 
or to a reference circuit 48 (position 2) during the servo-control phases. 
This reference circuit 48 comprises a resistive voltage divider comprising 
resistors R'0 and R"0, the resistor R'0 being connected to the reference 
voltage REF and the resistor R"0 being connected to the ground. 
The switching over of the selector 39 to the ground is obtained by the 
phase signal P0 (FIG. 4a) or by a signal ST when the cathode-ray tube is 
turned on. 
The switching over of the selector 39 towards the output terminal of the 
reference circuit 48 is obtained by a phase signal P1 (FIG. 4b). The phase 
signals P0 and P1 are cyclical and may be synchronized, for example, with 
the scanning signals. The signal P0 precedes the signal P1 without 
overlapping. 
With the tube 10 and, more specially, with the deflection coils 15, 
deflection amplifiers are associated in a known way. One of these 
deflection amplifiers, referenced 40, is for the scanning along the x-axis 
X'X (FIG. 5) and the other one, referenced 41, is for the scanning along 
the y-axis Y'Y. These amplifiers 40 and 41 receive television or 
directed-beam type scanning signals from a scanning circuit 9 but, 
according to the invention, they also receive signals for the positioning 
of the electron beam for the duration of the phase signal P1 to direct 
said beam towards the zone S.sub.o of the screen and, more specifically, 
in front of the end of the optical fiber 31. 
To this effect, the input terminal of each deflection amplifier 40 and 41 
is respectively connected to a selector 42 and 43 of a selector circuit 8, 
activated by the signal P1, in such a way as to receive the scanning 
signals during the frame flyback or the directed-beam trace and to receive 
reference signals during the frame flyback or the time planned for the 
test during the directed-beam trace, i.e. in the duration of the signal 
P1. In FIG. 3, these reference signals are created by a resistive divider 
circuit supplied by a reference voltage REF and comprising the resistors 
Rx and R'x for the deflection along the axis X'X and the resistors Ry and 
R'y for the deflection along the axis Y'Y. 
The luminance of the zone S.sub.o of the screen is measured by the 
sample-and-hold circuit 33 for a part of the duration of the signal P1. To 
this effect, the switch 44 of the sample-and-hold circuit 33 is controlled 
by a signal P2 (FIG. 4c) which appears during P1. As FIG. 3 shows, this 
signal P2 controls the charging of the capacitor Cm by means of the switch 
44 positioned between the comparator 45 and the terminal A of the 
capacitor Cm, the other terminal of which is connected to the ground. The 
charging voltage of the capacitor Cm is applied to the subtractor 35 as 
described hereinabove. 
Furthermore, the terminal A of the capacitor Cm is connected to a source 
102 by a switch 49 activated by the signal ST so as to charge the 
capacitor Cm at a determined voltage when the tube 10 is turned on and so 
as to obtain maximum cut-off for the tube. 
The signal ST as well as the signals P0, P1 and P2 are provided by the 
scanning circuit 9. 
To obtain variation in the luminance threshold, the signal given by the 
sensor 30 is compared, as described hereinabove, with a set value Vc in 
the error amplifier 45; this set value is obtained by the resistive 
divider circuit that is connected between the reference voltage REF and 
the ground and comprises the resistor RO and the potentiometer PTO. This 
value Vc can be adjusted by the user to adapt the background luminance of 
the screen. 
The tube then works as follows, assuming that the tube is in normal 
operation. Cyclically, for example at each frame flyback, the signal P0 
switches over the selector 39 to the ground and enables the circuit 73 to 
sample the dark current of the sensor 30 while the beam current is almost 
zero and the beam is not positioned on the zone S.sub.o, i.e. on the 
sensor 30. The circuit 73 will deduct this value from the servo-control 
measurement which shall be done subsequently. 
Then, the signal P1 activates the selectors 42 and 43 so that the electron 
beam excites the point of the zone S.sub.o facing the end of the fiber 31. 
It also activates the selector 39 so that the subtractor 35 is connected, 
through the gamma corrector 38, to the reference voltage source 48 
(position 2). 
If the luminance, which is measured by the sensor 30, is greater than the 
set value Vc, the capacitor Cm gets discharged, for example during the 
signal P1, and the voltage of the point A diminishes; this has the effect, 
through the amplifier 46 and the subtractor 35, of diminishing the cathode 
current Ik and hence of increasing the difference in potentials VKG1, the 
effect of which is to diminish the electron current of the tube and hence 
the luminance of the tube. 
It will be understood that the reverse effect is obtained when the 
luminance measured by the sensor 30 is smaller than the set value Vc. 
When the voltage is turned on in the tube, it is important for the tube to 
be cut off to the maximum and the result thereof is that, according to the 
operation described hereinabove, the voltage at the point A should be the 
minimum: this is the role of the source 102 which may be zero or negative 
and which, when the voltage is turned on, is connected to the point A by 
the closing of the switch 49 through the signal ST. 
For the same reason, the signal ST also activates the switch 39 to connect 
the gamma corrector 38 to the ground (position 3). 
The invention has been described in providing for a measurement of 
luminance of the zone S.sub.o during the frame flyback but it is clear 
that other instants may be chosen depending on the applications of the 
servo-control device according to the invention.