GB2035749A

GB2035749A - Display screen photography arrangement for a monitor employing a circular-image form of frame blanking

Info

Publication number: GB2035749A
Application number: GB7936792A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1978-10-26
Filing date: 1979-10-23
Publication date: 1980-06-18
Also published as: FR2440017B1; SE7908745L; DE2846540C2; FR2440017A1; JPS6358432B2; BE879613A; JPS5560386A; GB2035749B; DE2846540A1; IT7926728A0; IT1125575B

Abstract

A radiography system in which a video image 1 derived via a television camera (not shown) from an X-ray image intensifier output screen (not shown), is applied to a display monitor tube 7 and photographically recorded from the display screen 12 by a camera 11. Exposure contrast is normally controlled by measuring image brightness using a sensor 12 and controlling the X-ray tube current. Exposure results tend to vary due to display tube aging and voltage fluctuation. In the improved arrangement the optical path between display screen 12 and the camera 11, is enclosed by an optical screen 10 to exclude ambient light and the displayed image has a circular boundary 13 because the intensifier output is a circular image. The sensor 12 measures light in an image region 14 and provides contrast control by controlling the gain of a video amplifier 4 feeding the tube 7. A further sensor 16 measures the light from the blanked outer region, eg 17, to provide a black-level control via the brightness control circuit 9. The arrangement thus enables a radiographic image stored on videotape to be replayed and photographed. <IMAGE>

Description

SPECIFICATION Display screen photography arrangement for a monitor employing a circular image form of frame blanking The invention relates to a display screen photography arrangement for a monitor in which a circularimage form of frame blanking is employed, including a photographic camera for photographing an image displayed by the monitor, a constrast control circuit including a photoelectric sensor arranged to sense light emitted from a predetermined region of said image displayed by said monitor, and a switching circuit for stabilizing the black-level, and such an arrangement will be referred to herein as an arrangement of the kind referred to.

An arrangement of this kind is essentially known from Netherlands Patent Application 281 676 laid open to public inspection; therein, however, no circular image form of frame blanking is considered.

Such a circular-image form of frame blanking is frequently used in present X-ray installations, since an image is commonly picked up by a television camera from the exit image screen of an X-ray intensifier which is circular, and therefore the video signals usually supplied to the monitor provide an image display region also shaped as a circle.

The known arrangement attempts to provide correctly exposed photographs of the display screen of the monitor, but the black-level is liable to change when the characteristic of the picture tube of the monitor changes due to aging, or when the high voltage or the mains voltage fluctuates. This may result in incorrectly exposed photographs.

The present invention has for an object to construct an improved arrangement of the kind referred to, by means of which photographs of the display screen can be more uniformaly exposed.

According to the invention there is provided a display screen photography arrangement for a monitor in which a circular-image form of frame blanking is employed, including a photographic camera for photographing an image displayed by the monitor, a contrast control circuit including a photoelectric sensor arranged to sense light emitted from a predetermined region of said image displayed by the monitor, and a switching circuit for stabilizing the black-level, characterised in that there is provided an optical screening device which, when connected to the monitor, screens the optical path from the display screen of the monitor to the camera, in a light-tight manner from ambient illumi nation, the switching circuit for stabilizing the blacklevel being formed by a control circuit which includes a second photoelectric sensor arranged to sense light emitted by the display screen of the monitorfrom a region lying outside the circular boundary of the displayed image.

Thus, the black-level is adjusted by measuring the brightness of the display screen outside the boundary of the circular image region by means of the second photoelectric sensor and thereafter comparing the measured brightness with a reference value which is chosen so that for the selected setting of the exposure time and the aperture of the camera, the exposure density of the film material lies at a predetermined point near the lower end of the characteristic gradation curve of the material. The screening device prevents falsification of the measurement result obtained by the second photoelectric sensor (and also that obtained by the first photoelectric sensor) by incident ambient light.

Moreoever, the screening device is simply constructed in such a way that between the display screen of the monitor and the camera a well-defined distance is provided, so that a predetermined common respective exposure time, distance and aperture setting of the camera can be used for all exposures.

In a modification in accordance with the invention, the video signal is applied to the picture tube of the monitor via a contrast control circuit, the gain of which is controlled by the signal supplied by the first photoelectric sensor.

In the known circuit, the contrast is controlled by varying the current through the X-ray tube in dependence on the signal of the first photoelectric sensor. As a result, not only is the dose applied to the patient changed, but so also is the mean value of the image brightness. Moreover, such a kind of contrast control is possible only when the monitor receives the video signal from the teleivision camera which directly picks up the X-ray image at the time of convertion into a visible image. Contrast control of this kind is not possible in the case of display of a television image recorded on a magnetic tape.

However, the modification of the arrangement in accordance with the invention enables the contrast also to be controlled when an (X-ray) television signal recorded on tape is displayed.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawing; the single figure of which illustrates an embodiment.

The embodiment illustrated in the figure forms part of an X-ray system in which an X-ray source (not shown) irradiates a body under examination and a corresponding radiographic image of the body is formed on the input screen of an X-ray intensifier (not shown). The optical output image from the X-ray image intensifier is applied to a television pick-up tube (not shown) to provide a corresponding video signal 1 which is applied to the input terminal 2 of a circuit associated with a picture tube 7 of the monitor, together with corresponding synchronization signals. The synchronization signals are separated in a separation circuit 3 and are applied to a contrast control circuit 4, the gain of which can be varied in dependence on a control voltage present on a control line 5.In a mixing stage 6, the video signal whose amplification has been controlled, and the synchronization signals are recombined and applied to the cathode of the picture tube 7 of the monitor.

In the case in which the video signal and the associated synchronization signals are supplied separately, the separation circuit 3 can be dispensed with and the synchronization signals applied directly to the circuit 6.

Moreover, the synchronization signals are treated in a pulse generating circuit 8 which generates line-frequency and frame-frequency pulses which can be delayed pulses, for controlling the contrast control circuit 4 and a brightness control circuit 9, the output of which is connected to the Wehnelt cylinder of the picture tube 7 and determines the black-level of the image displayed by the picture tube 7.

The picture tube 7, or the monitor (not shown) of which the picture tube 7 forms part, is connected, via an optical screening device (tube) 10, to a photographic camera 11 by means of which the display screen 12 of the monitor, or at least the circular image 13 formed thereon, can be photographed.

When a tube 10 of this kind is used for screening the display screen 12 and the camera 11 against incident ambient light, the camera 11 occupies a defined position with respect to the display screen 12, so that photographs can be made with a fixed setting of distance, aperture and exposure time. The exposure time should amount to several frames (approximately 160 ms) so that the photograph does not need to be exposed in synchronism with the television images.

Afirst photoelectric sensor 15 is mechanically supported by the screening device 10, the lightsensitive surface of said sensor being arranged in a manner not shown, only to receive light from an image region 14 which is concentric with the image boundary 13, the diameter of said region 14 amounting to approximately two thirds of the diameter of the image 13. The output signal of the first photoelectric sensor 15 is applied, via the line 5, to the contrast control circuit 4 in which the mean value with respect to time is formed and compared with a reference value.

Furthermore, there is provided a second photoelectric sensor 16, for example, a phototransistor, onto the light-sensitive surface of which light from a region 17 on the display screen and lying outside the circular X-ray image 13, is projected. The region of the display screen projected onto the second photoelectric sensor, may be situated, instead of at the bottom left as shown in the figure, elsewhere outside the boundary of the image 13 on the display screen 12. Light from said region 17 represents the black-level, the instantaneous value of which appears on the output line 18 of the second photoelectric sensor 16 which is connected to the brightness control circuit 9. In the circuit 9, the peak value of the light representing the black-level region outside the image 13, is rectified throughout one frame period, and the peak value thus formed is stored in a sample-and-hold circuit for at least one frame period and is compared with a reference value. This reference value is chosen so that in the photo-picture outside the region occupied by the television image 13, a photographic density of approximately 0.1 results for the selected setting of aperture and exposure time for the camera 11.

The control circuits for the contrast and the black level can thus be arranged to create reproducible photographic exposure conditions, so that optimally exposed photographs can be obtained with defined settings of the camera 11.

Claims

1. A display screen photography arrangement for a monitor in which a circular-image form of frame blanking is employed, including a photographic camera for photographing an image displayed by the monitor, a contrast control circuit including a photoelectric sensor arranged to sense light emitted from a predetermined region of said image displayed by the monitor and a switching circuit for stabilizing the black-level, characterised in that there is provided an optical screening device which, when connected to the monitor, screens the optical path from the display screen of the monitor to the camera, in a light-tight manner from ambient illumination, the switching circuit for stabilizing the blacklevel being formed by a control circuit which includes a second photoelectric sensor arranged to sense light emitted by the display screen of the monitor from a region lying outside the circular boundary of the displayed image.

2. An arrangement as claimed in Claim 1, characterised in that an input video signal is applied to the picture tube of the monitor via said contrast control circuit, the gain of which is controlled by a signal supplied by said first photoelectric sensor.

3. A display screen photography arrangement for a monitor in which a circular image form of frame blanking is employed, said arrangement being substantially as herein described with reference to the accompanying drawing.