Method And Means For Providing A Lighted Display Having A Constant Contrast Ratio

Abstract

A display system has a closed loop circuit for controlling the brightness of a cathode-ray tube. The circuit includes sensing means providing an output corresponding to the brightness of the display and to ambient brightness. The output is compared to a reference signal. The signal applied to the grid of the cathode-ray tube is amplified in accordance with the comparison to control the brightness of the display so as to maintain a constant contrast ratio C=(B .sub.C +B.sub. A)1 B.sub. A where B.sub. C is the brightness of the cathode-ray tube display and B.sub. A is the reflected ambient brightness.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to display systems and, more particularly, to a display system with automatic brightness control.

2. Description of the Prior Art

Cathode-ray tube display systems of a type disclosed in U.S. Pat. No. 3,404,226 uses automatic brightness control which are responsive only to ambient light conditions. In providing brightness control dependent on the ambient illumination, the control system is an open loop system. Open loop brightness control systems have a transfer function of the ambient light condition to the cathode-ray tube display brightness which depends on the transfer function of each element of the system. When the voltage or current of any element of the system drifts, the drift causes inaccuracies in the brightness control.

The present invention distinguishes over U.S. Pat. No. 3,404,226 by providing for a closed loop brightness control to maintain a constant contrast ratio and includes swing of the output E.sub.6 sensing of the brightness of the cathode-ray tube display as well as ambient brightness reflected by the cathode-ray tube. A drift of any element in the closed loop system is automatically compensated for by the feedback of the closed loop system.

SUMMARY OF THE INVENTION

A circuit for controlling the brightness of a display in which the display provided by a display system illuminates a sensor. The sensor provides a signal corresponding to the brightness of the display. A difference network provides a signal corresponding to the difference between the signal from the sensor and a signal corresponding to a reference brightness. A control network controls the brightness of the display in accordance with the difference signal from the difference network.

One object of the present invention is to provide a lighted display having a constant contrast ratio.

Another object of the present invention is to provide a closed loop brightness control system for a lighted display system to maintain a constant contrast ratio.

Another object of the present invention is to use a brightness control system in a lighted display system which considers the brightness of the display as well as the ambient brightness.

Another object of the present invention is to use a photoelectric cell which is illuminated by a lighted display and ambient light and provides an output corresponding to the brightness of the lighted display and the ambient brightness.

The foregoing and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawing wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for illustration purposes only and is not to be construed as defining the limits of the invention.

DESCRIPTION OF THE DRAWING

The figure is a simplified block diagram of a cathode-ray tube display system using an automatic brightness control system constructed in accordance with the invention to maintain a constant contrast display.

DESCRIPTION OF THE INVENTION

It is desirable to change the brightness of a cathode-ray tube display when the ambient illumination changes or when the display brightness changes to maintain a constant contrast ratio. The contrast ratio is defined as the ratio of what an observer sees as white to what is seen as black. What the observer sees as white is the display brightness and the reflected ambient brightness, while what the observer sees as black is the reflected ambient brightness. The contrast ratio C as set forth in the abstract of the disclosure may be rewritten in logrithmic form as

log C=log(B.sub.C +B.sub.A)-logB.sub. A

Referring to the Figure, there is shown a display system having an automatic brightness control system and including a voltage means 1 providing voltages E.sub.1 and E.sub.2 and a brightness voltage E.sub.3. Voltage means 1 may be a conventional type TV receiver without the cathode-ray tube, or it may be a symbol display system as disclosed in U.S. application Ser. No. 879,069 by John F. Yurasek and Abner Owens, Jr. and assigned to The Bendix Corporation, assignee of the present invention. When voltage means 1 is a TV receiver, voltages E.sub.1, E.sub.2 are horizontal and vertical sweep voltages, respectively, and voltage E.sub.3 is the video voltage. When voltage means 1 is a symbol display system, voltages E.sub.1, E.sub.2 are horizontal and vertical deflection voltages, respectively, and voltage E.sub.3 is the bright-up plus voltage. Although voltage means 1 is necessary to the overall combination, it is not necessary to one skilled in the art to know the detail of voltage means 1 in order to understand the invention.

Voltage means 1 provide voltages E.sub.1, E.sub.2 to horizontal and vertical deflection coils 4 and 5, respectively, having a common connection to ground 8, of a cathode-ray tube 10. Brightness voltage E.sub.3 is applied to a conventional-type variable-gain amplifier 14 whose gain is varied to control the brightness of cathode-ray tube 10. The amplified brightness voltage is provided by amplifier 14 to a conventional-type video amplifier 17 which in turn provides it to a grid 16 of cathode-ray tube 10. All elements, except cathode-ray tube 10 and grid 16, are connected to ground 8 as shown in the Figure. Cathode-ray tube 10 provides the display in accordance with the voltages applied to its deflection coils 4, 5 and grid 16.

A photoelectric cell 24, which may be a Fairchild FPM 200 type photoelectric cell, provides an output E.sub.6 corresponding to a logarithm of the light illuminating it. A photoelectric cell providing a linear output and a logarithmic amplifier may be used instead of the FPM 200 type photo cell.

The largest amplitude of output E.sub.6 from photoelectric cell 24 occurs when cathode-ray tube 10 is providing a display and corresponds to the observed brightness B.sub.C of the display plus the reflected ambient brightness B.sub.A from cathode-ray tube 10. The minimum amplitude of output E.sub.6 from photoelectric cell 24 occurs when cathode-ray tube 10 provides no display and corresponds to the reflected ambient brightness B.sub.A from cathode-ray tube 10. Since output E.sub.6 corresponds to the logarithm of the light sensed by photoelectric cell 24, we may write the voltage difference in equation form as follows:

E.sub. 6 max-E.sub. 6 min=.increment.E.sub. 6 =K(log (B.sub.C .sub.+B.sub.A).sub.-log

B.sub.A) 2

where K is a constant of proportionality.

Substituting from equation 1, equation 2 may be rewritten as

.increment.E.sub. 6 =KlogC 3.

From equations 2 and 3, it can be seen that the swing of output E.sub. 6 from photoelectric cell 24 corresponds to the contrast ratio.

A conventional-type direct current restorer and peak rectifier 26 converts output E.sub. 6 to a direct current voltage E.sub. 8 which corresponds to the voltage swing of output .sub.6 and hence to the contrast ratio of the display provided by a cathode-ray tube 10. A conventional-type difference amplifier 27 provides a direct current voltage E.sub.10 corresponding to the difference between direct current voltage E.sub.8 and a direct current reference voltage E.sub.12. Reference voltage E.sub.12 corresponds to a desired contrast ratio and is provided by a source 28 of fixed direct current voltage. The difference voltage E.sub.10 is applied to a variable-gain amplifier 14 for controlling the gain of amplifier 14 accordingly.

When the brightness of the display on cathode-ray tube 10 decreases, or the ambient brightness increases, the swing of output E.sub. 6 from photoelectric cell 24 decreases causing direct current voltage E.sub.8 from the direct current restorer and peak rectifier 26 to decrease. Difference amplifier 27 changes its output E.sub.10 accordingly thereby increasing the gain of amplifier 14 to increase the brightness of the display of cathode-ray tube 10. When the brightness of the display of cathode-ray tube 10 increases or the ambient lightness decreases, the swing of output E.sub. 6 increases causing direct current voltage E.sub.8 to increase accordingly. Difference amplifier 27 reduces output E.sub.10 accordingly to reduce the gain of variable-gain amplifier 14 thereby decreasing the brightness of the display on cathode-ray tube 10.

The device of the present invention provides a lighted display having a constant contrast ratio. The contrast ratio is maintained by a closed loop brightness control system using a photoelectric cell which considers the brightness of the lighted display as well as ambient brightness.

Claims

What is claimed is:

1. Means for controlling the brightness of a display, comprising means for providing the display, means illuminated by the display for providing a signal corresponding to the brightness of the display, a source providing a reference signal, means connected to the reference signal source and to the brightness signal means and providing an output corresponding to the difference between the reference signal and the brightness signal, and means connected to the display means and to the difference means and responsive to the difference signal for controlling the brightness of the display; the brightness signal means including a photoelectric cell illuminated by the lighted display provided by the display means and ambient light, the photoelectric cell providing a logarithmic output varying in accordance with the brightness of the display and the ambient brightness, and a direct current restorer and peak rectifier circuit connected to the photoelectric cell for providing a direct current voltage corresponding to the voltage swing of the output from the photoelectric cell; the difference means including a difference amplifier connected to the direct current restorer and peak rectifier circuit and to the reference signal source and providing an output corresponding to the difference between the direct current voltage from the direct current restorer and peak rectifier circuit and a direct current signal corresponding to a reference contrast ratio; the display means including a cathode-ray tube having deflection coils, each coil being connected to ground, and a grid connected to control means, and voltage means connected to the deflection coils and to the control means and providing deflection voltages to the deflection coils of the cathode-ray tube and a brightness voltage to the control means; and the control means including variable-gain amplifying means connected to the voltage means, to the grid of the cathode-ray tube and to the difference amplifier for amplifying the brightness voltage from the voltage means in accordance with the difference output from the difference amplifier and for providing the amplified brightness voltage to the grid of the cathode-ray tube to control the brightness of the display provided by the cathode-ray tube.

2. Means for controlling the brightness of a display, comprising means for providing the display, means illuminated by the display for providing a signal corresponding to the brightness of the display, a source providing a reference signal, means connected to the reference signal source and to the brightness signal means and providing an output corresponding to the difference between the reference signal and the brightness signal, and means connected to the display means and to the difference means and responsive to the difference signal for controlling the brightness of the display; the brightness signal means providing a signal corresponding to the brightness B.sub.C of the display and to the ambient brightness B.sub.A so that the display brightness may be controlled to maintain a constant contrast ratio C for the display where

C=(B.sub. C +B.sub. A)/B.sub. A.

3. Brightness control means of the kind described in claim 2 in which the brightness signal means includes a photoelectric cell illuminated by the brightness of the display and ambient brightness and providing an output varying in accordance with the logarithm of the brightness of the display and the ambient brightness, and means connected to the photoelectric cell and to the difference means for converting the output from the photoelectric cell to a direct current voltage corresponding to the contrast ratio of the lighted display.

4. Brightness control means of the kind described in claim 3 in which the converting means is a direct current restorer and peak rectifier circuit.