GB2329809A - Providing pairs of gating pulses to a low-light camera overcomes flicker or strobing - Google Patents

Abstract

A low light television camera is provided with an image intensifier or CCD imaging device, the operation of which is gated to control the sensitivity of the camera in response to the level of light which illuminates a viewed scene. The gating pulses occur in pairs spaced apart by a quarter wave period of the a.c. power used to illuminate the scene instead of being linked to the field or frame rate, to avoid undesirable strobe effects in the video signal.

Description

Camera Arrangements This invention relates to camera arrangements, and is particularly applicable to such arrangements which are intended to view scenes which are lit by a periodically fluctuating artificial illumination. For example, in the UK the power supply frequency is 50Hz whereas in the USA it is 60Hz, and thus a light source powered by such a supply frequency will flicker, although such flicker is generally not perceived by the human eye.

The flicker effect will be more pronounced if the lighting is, for example, a fluorescent light rather than an incandescent type.

Such a flicker generally does not cause difficulties if the camera arrangement is provided with a power supply at the same frequency, but the inventor has realised that if the supply frequencies are not the same, a very undesirable strobing fluctuation in signal output can result, and that this can be a particularly serious problem with low light television camera arrangements.

According to this invention, a camera arrangement includes gated image forming means, means for controlling the sensitivity of the camera arrangement by varying the gating duration of the image forming means, and means for producing gating pulses in pairs, in which the two pulses of a pair are spaced apart from each other by a period corresponding to substantially a quarter cycle period of the a.c. power used to illuminate a scene which the camera arrangement is to view.

The invention is advantageously applicable to a low light surveillance television camera having a CCD imaging device and an image intensifier. In such cameras the sensitivity to light (i.e. the brightness of a viewed scene) is varied by altering the proportion of time during. which the image intensifier or the CCD imaging device is operative, usually the former.

Conventionally, the image formed in a television camera arrangement is scanned in a raster like fashion to produce a serial output signal. A complete TV frame (picture) may be scanned in one pass (progressive scan) or in two interlaced fields, one covering the odd numbered raster lines, and the second the even lines. Preferably, the pair of pulses has a repetition rate of one field period. Additional pairs of pulses may be provided interleaved with the first pair, the two pulses of each pair are spaced apart by the one quarter cycle period of the illumination power.

The use of such a system minimises the strobing effect which would otherwise occur if the individual gating pulses had a repetition rate unrelated to the fluctuating brightness of the illuminated scene.

It is usual for the TV picture rate (frame frequency) to be related to the power supply frequency of the country of usage. Thus, in the UK, a 50Hz power supply frequency is used, along with a 25Hz TV frame frequency, whereas in the USA, the power supply frequency is 60Hz and the TV frame frequency is 30Hz. Usually, of course, a scene which is illuminated by artificial light has a fluctuating brightness which is in tune with the TV operating rate. However, if for example, a TV camera system operating at a 30Hz frame rate is viewing a scene illuminated by light powered at 50Hz, a 1OHz brightness flicker may be seen in the picture. The effect can become very pronounced with a gated low light TV camera system to the extent that such a system is very difficult to use. It is extremely difficult to remove such a flicker, or strobe effect, by means of filters, or frequency tracking circuits. By means of the invention such a difficulty is largely overcome. It is not necessary that the pulse pair repetition period be exactly the same as a quarter cycle period of the illuminant power supply, although desirably this should be the case. In many instances, it may not be possible to make the periods exactly the same, since the two frequencies will generally be generated quite independently of each other.

The invention is further described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates a camera arrangement in accordance with the invention, and Figures 2, 3 and 4 are explanatory diagrams.

Referring to Figure 1, a video camera 1 consists of a lens 2, an image intensifier 3, a charge coupled device (CCD) imaging device 4 and a video output stage 5. The camera is designed to operate with a 30Hz frame rate with interfaced fields.

In this instance, the camera 1 views a scene which is illuminated by lamps powered by a 50Hz electrical power supply, such that light 6 entering the lens 2 is modulated. The nature of the modulation is further explained with reference to Figure 3 subsequently.

The video signal 7 generated by the camera 1 is utilised as required, and is also fed to synchronisation circuitry 8 and to a pulse length circuit 9. A gating waveform generator 10, generates the gating pulses illustrated in Figure 2 and which are used to control the gate-on periods of the image intensifier 3. From Figure 2, it is seen that the gating pulses occur in pairs, in which the repetition period of a pair corresponds to the television field period, in this case 16.667mS since the field rate is 60Hz, and this repetition period is determined by the synchronisation circuitry 8.

The two pulses 21, 22 of each pair have rising edges spaced apart by SmS as shown, and the duration of the pulses 21 and 22 is variable and is determined by the pulse length circuit 9, in dependence on the sensitivity requirement of the camera arrangement. The camera is intended to view low light scenes, and depending on the optical gain of the image intensifier 3, it is able to produce a usable video signal under conditions of almost complete darkness. However, if the scene is well lit, this can result in overload of the CCD imaging device, and it is therefore necessary to reduce the gain of the image intensifier under such conditions by reducing the sensitivity of the camera to light, and this is achieved by adjusting the duration of the pulses 21 and 22. A control signal is applied via input 11 to the pulse length circuit 9 for this purpose, and is generated in dependence on the level of the video signal. The pulse length circuit 9 is also operative to control the spacing of the pulses, so that control signals applied to it via input 11 alter the pulse spacing in dependence on the modulation frequency of the light of the scene which is viewed by the camera arrangement.

These gating pulses are used to turn on the image intensifier for short periods during which an amplified complete optical image of the viewed scene is projected onto the CCDimaging device. Obviously if the gating pulses 21, 22 are very short, little light is allowed through to the CCD imaging device.

Instead of applying the gating pulses to the image intensifier, they may instead be used to gate the operation of the CCDimaging device directly.

Typically, the duration of each pulse is variable from 1,xS to a maximum of 5mS. If greater optical gain is required than can conveniently be provided by a single pulse pair, additional pulse pairs can be used within a single field period, but the spacing between the two pulses of each pair is still a quarter period of the illuminant frequency, ie 5mS in this example.

Figure 3 illustrates the strobing effect which can arise in the absence of the invention.

The waveform represents the relative brightness of a lamp powered by a 50Hz power supply. The period of the power cycle is therefore 20mS, and as the lamp responds equally to positive and negative polarities, the lamp flicker has a period of 10mS. The waveform approximates to a sine wave, as the brightness of a lamp is generally related to the square of the a.c. waveform which energises a lamp, and thus the solid line in Figure 3 represents the modulation of the light received by a lens 2. With a conventional gated camera system operating at a 60Hz field rate, the undesirable strobe effect is particularly noticeable, having a flicker frequency of 10Hz represented by the solid dots on Figure 3. Thus, there is a substantial cyclic variation in video signal level under such circumstances.

Figure 4 directly compares the effect of the invention, in which two very short pulses spaced apart by 5mS (solid dot) is compared with a continuous gate-on time of 5mS (open squares), the latter giving a substantial variation in brightness as represented by the video signal.

The effectiveness of the technique can be demonstrated mathematically.

The brightness of the 50Hz illumination is assumed to be proportional to the square of the applied voltage, the brightness at time t is: ill= 1.{1 + sin(2#100t)} .....(1) 2 This is simplified as: ill=1.( 1 +sin(0)) (2) 2 where o is the angular offset, in radians, of the first sample relative to the illumination waveform.

The delay for the second of the two samples is one quarter cycle of 50Hz. For the 100Hz illumination rate it becomes one half cycle or Tt radians.

The total light from the two samples can be evaluated as: hght=l.(2+sin(O)+sin(O+)} (3) 2 Expanding the second term we get: light=-. (2 +sin(0) +sin(8).cos(z)+cos(#).sin(#)}....(4) 2 or: light=1.{2+sin(0) -sin(0)} = 1 (5) 2 Thus, the light collected is constant, regardless of the position of the first sample.

Claims (7)

1. Claims 1. A camera arrangement including gated image forming means, means for controlling the sensitivity of the camera arrangement by varying the gating duration of the image forming means, and means for producing gating pulses in pairs, in which the two pulses of a pair are spaced apart from each other by a period corresponding to substantially a quarter cycle period of the ac. power used to illuminate a scene which the camera arrangement is to view.
2. 2. An arrangement in which the gated image forming means includes an optical image intensifier.
3. 3. An arrangement in which the gate image forming means includes a CCD imaging device.
4. 4. An arrangement as claimed in any of the preceding claims and wherein the pulse pair has a repetition rate corresponding to the field period of the camera arrangement.
5. 5. An arrangement as claimed in any of the preceding claims and wherein means are provided for varying the spacing apart of the two pulses in said pair.
6. 6. An arrangement as claimed in any of the preceding claims and wherein means are provided for varying the duration of each of said pulses, the duration of each pulse of said pair being substantially equal.
7. 7. A camera arrangement substantially as illustrated in and described with reference to Figure 1 of the accompanying drawings.