WO2004008742A2

WO2004008742A2 - A light source triggering device

Info

Publication number: WO2004008742A2
Application number: PCT/GB2003/003111
Authority: WO
Inventors: John Dyson; Ian Bailey
Original assignee: John Dyson; Ian Bailey
Priority date: 2002-07-17
Filing date: 2003-07-17
Publication date: 2004-01-22
Also published as: WO2004008742A3; AU2003281022A8; GB2406738A; AU2003281022A1; GB2406738B; GB0216598D0; GB0501193D0

Abstract

A light source triggering device for use with a light source (3) to illuminate a scene being recorded by a moving picture camera (1). The light source triggering device comprises means to cause the light source (3) to emit at least one flash of light substantially in synchronisation with exposures of the moving picture camera (1) and to cause the light source to emit at least one additional flash of light between each pair of exposures of the moving picture camera (1).

Description

A LIGHT SOURCE TRIGGERING DEVICE

The present invention relates to a light source triggering device and a method of triggering a light source.

There are certain conditions where the lighting arrangement for television cameras can become very difficult, for instance when lighting a subject in front of a window where the outside view also needs to be recorded by the camera. If the outside scene is lit by full sunlight, then high intensity lighting will be neede_.d to illuminate the subject so that both the subject and the outside scene will be viewed in balance by the camera. A typical example would be for instance a reporter, standing in the shade in front of a building, which is lit by full sunlight. The camera must be able to expose correctly both the background (the building) and the foreground (the reporter) in order to obtain a correctly exposed and balanced scene. Traditionally, this has meant lighting the reporter in such a way that the reporter is very brightly lit to match the background lighting level. A TV crew would normally use an HMI light, these are approximately four times more efficient than an equivalent tungsten halogen lighting system but they still consume a lot of power and are expensive. A common portable HMI lighting system is around 300 watts, which produces roughly the equivalent light output of a 1.2 k tungsten halogen light. Battery life is in the order of around 30 to 40 minutes using a large heavy-duty battery (30 volt / 7 ampere hour) or the light can be mains powered (if available) . However, the light output is still normally not sufficient to correctly illuminate the person, even with the light placed very close to that person. In fact, this kind of bright light is extremely dazzling and uncomfortable to the subject, and can cause them to squint. It also requires high electrical power. In some locations, this can be a problem if mains electrical power is not available.

US-A-5, 382, 974 discloses a movie camera and a method of photographing a still with the movie camera in which a strobe device automatically produces flashing light in synchronism with the shutter of the camera.

US-A-5, 546, 121 discloses a camera which applies a signal to a strobe light to cause the strobe light to flash.

Most television cameras are fitted with an electronic shutter. When activated, this feature controls the active time period of the cameras imaging sensor (s) during the video field period. The electronic shutter of a television camera is so designed that only light falling on the camera-imaging sensor (s) during the shutter period is integrated to form the output of the camera. Light falling on the imaging sensor (s) outside this shutter period has no effect on the camera output. By varying the duration of the active shutter period of a camera, the exposure of a scene viewed by the camera can be similarly varied.

If the subject being recorded by a camera with an active shutter is only illuminated during the cameras active shutter period by the use of a synchronised flash system, the flash illumination upon the subject will appear comparatively bright to the camera. However, the relative background illumination (from ambient lighting) will be reduced as the effective exposure time is now also reduced due to the shutter effect of the camera.

We have appreciated that this technique can dramatically decrease power requirements for television lighting. However, light flashing at the same TV field repetition rate as the shutter can produce unpleasant flickering effects to a subject.

Preferred embodiments of the present invention provide additional flashes of light between exposures such that the illumination appears to be substantially constant, i.e. the flicker present at lower rates cannot be seen.

Further embodiments provide the ability to control the period of the electronic shutter of a camera, when used with the synchronised lighting system to be described, can also be used to generate special lighting effects .

The invention in its various aspects is described in the independent claims below to which reference should now be made. Advantageous features are set forth in the appendant claims .

The invention will now be described in more detail, by way of example, with reference to the drawings in which:

Figure 1 shows a schematic diagram of a device embodying the present invention;

Figure 2 shows a timing diagram for the shutter timing of a frame transfer CCD video camera;

Figure 3 shows a block diagram of a system embodying the present invention;

Figure 4 shows an implementation of the timing section of the system shown in Figure 3; Figure 5 shows a timing diagram of signals produced by the timing section of Figure 4;

Figure 6 shows a flash unit controlled by the system of Figure 3;

Figure 7 shows a circuit diagram of the system of

Figure 4;

Figure 8 shows a circuit diagram of the flash unit of Figure 6;

Figure 9 shows an alternative implementation of the system embodying the present invention; and

Figure 10 shows another system embodying the present invention.

The embodiments described maintain synchronism between the active shutter period of a video camera and a pulsed light source or a plurality of pulsed light sources. There are several possible ways by which this may be achieved.

By way of example only, one possible solution is^' shown in Figure 1.

Camera 1 produces a video or synchronising output which is fed to a pulse control unit 2. The pulse control unit 2 produces a pulse during or immediately before the active shutter period of the camera 1 for every video field. The exact timing will depend on the time taken for a flash unit 3 to trigger and reach maximum light output. The pulse output of the control unit 2 is used to trigger the flash unit 3. The light output of the flash unit 3 will in turn illuminate the camera subject 4 during the active shutter period of video camera 1 which is viewing the subject. The power supply units 5 and 6 are used to provide power for the pulse control unit 2 and the flash unit 3 respectively.

The pulse control unit 2 is required to produce a pulse during or just prior to the active shutter period of the camera 1. This could be achieved by triggering a timer from the field interval pulses of the video signal from the camera 1. The timing of this pulse may be different for various camera models/manufacturers, so the control unit 2 would need to be adjustable to suit a particular camera 1.

The flash unit 3, on receipt of a pulse, is required to produce a high energy short duration flash and can be constructed for example by using a typical xenon flash tube stroboscope arrangement.

The power supply units 5 and 6 are used to provide the necessary powering for the pulse control unit 2 and the flash unit 3. For convenience it may be desirable to combine these two power supplies 5 and 6 into a single unit.

The improvement in effective lighting power can be seen by the following example. Consider a camera operating at 50 fields per second and with a shutter period of l/lOOOth second. The field period is 20 milliseconds and the active shutter period is 1 millisecond. If the pulsed light is only present during the period the shutter is active, then the average illumination will be l/20th of that which would be needed if the same illumination level was maintained during the whole field period. Assuming similar efficiencies for flash tubes and conventional halogen lights, this would mean a one kilowatt halogen light could be replaced by a flash system running with a peak power equivalent to one kilowatt but only having a mean power input of 50 watts. Higher shutter speeds will give a greater improvement providing that the flash duration is kept within the active shutter period.

In another example, consider a scene where a subject requires a conventional 1 kwatt tungsten halogen light source to correctly expose the subject in balance with a brightly-lit background. If the shutter of the camera is now enabled, and set to 1/lOOth second (the "non shutter" duration is effectively l/50th second) then there will be a drop in the output level of the camera of one stop. Increasing the exposure of the lens can easily compensate for this drop in level. However, the camera is now effectively "blind" for half of its field period, so the light source may be switched off during this period, thus saving 500 watts of power without any change in the output of the camera. Faster shutter speeds will give a greater improvement. Further reductions in intensity of the video image due to the shutter effect can be compensated for by adjusting the iris of the camera lens, removal of any neutral density filters in the cameras optical path or by applying electronic gain within the camera. Thus, provided the average lighting power level from the light source during the active shutter period is equivalent to a 1 kwatt halogen light, the average power level required over the field period is substantially reduced with shorter shutter speeds, and a corresponding shorter "flash" duration.

Table 1

Table 1 shows the effect of reducing the shutter speed, and the resulting improved energy efficiency saving.

The energy per flash has been calculated by using Power (in watts) = Energy (in joules) / Time (in seconds)

Thus the energy per flash in joules is Power (in watts) / 50 as there are 50 flashes per second.

Table 1 shows that, in theory, a camera operating with a shutter speed of 1/10, 000th second requires only a 5 watt stroboscopic light source to effectively replace a 1 kwatt constant light source. This is a considerable power saving and the subject is now exposed to only a 5 watt light source so dazzle has been eliminated.

The results shown in Table 1 assume a similar efficiency between the 1 kwatt tungsten halogen light and the strobing light source. The TV camera is operating at the UK specification of 50 fields/second.

The flash duration can be substantially shorter than the shutter period provided that the energy level per flash is maintained (i.e. a more energetic flash but over a shorter period) . The flash must still fall within the active shutter period.

In practice, it is easier to build a stroboscopic light source having a constant energy output per flash.

Table 2 shows the increase in lighting efficiency by varying the shutter speed of a 0.5 joule flash (25 watts at 50 Hz) .

Table 2.

Table 2 shows that a stroboscopic light source consuming a mean power of just 25 watts can have the same equivalent lighting power to a 5 kwatt constant light source at a 1/10, 000th second camera shutter speed. The relationship between shutter speed and increased effective lighting efficiency is an inverse function i.e. halving the shutter speed doubles the apparent efficiency.

By maintaining a constant energy level per flash, the effect as seen by the camera, is that the background level can be made to vary in intensity as the shutter speed is adjusted. The foreground however, being lit by the light, stays at a relatively constant level (dependent on how much ambient light is falling on the subject) . Thus the camera operator can effectively change the background lighting level to correctly balance the foreground (subject) lighting level by simply adjusting the shutter speed of the camera. This effect can be very useful when competing against dynamically varying background levels due to changing cloud cover, for example.

The CCD shutter effect helps in making this light operate in its highly efficient manner. Most TV cameras use a frame transfer CCD array as the light sensor, this has the advantage that the integrating time period for the whole image sensor array can be varied before the charge is transferred to a secondary layer to be read out. This gives the frame transfer CCD its shuttering effect. That is to say, the CCD of a camera only detects light for the integrating time period. This is known as the active shutter period. Older generation CCD arrays used a line transfer arrangement, which did not have this facility.

Figure 2 shows the typical effect of different shutter timings of a frame transfer CCD camera across a TV field period. For clarity, only crude field sync pulses 8 of the output video are shown. In this example, the period 9 of one field is 20 milliseconds.

Figure 2 shows that the active shutter period 10 starts at the same time 12 for different shutter periods. Figure 2 (a) shows a shutter period 10 of 1/lOOth second, (b) shows a shutter period 10 of l/200th second and (c) shows a shutter period 10 of l/500th second. Therefore, if a short duration flash is to be triggered during this period 10, it makes sense to do it just after the start of the shutter period 12 as this time does not change position with respect to the field pulse 8.

However, the precise position of the start of the shutter period 12 can vary between cameras and it can be before the field pulse 8 and so adjustments should be made accordingly.

An example of the type of system 19 described above is illustrated in Figure 3. A xenon flash tube is used as the light source. However, any light source that can produce a high energy, short duration flash would suffice. The advantages of xenon flash tubes are that:

1) They have a colour temperature output that can be made to be similar to daylight.

2) They have a high efficiency.

3) They are robust. 4) They are comparatively cheap.

5) They can produce a short duration flash.

6) They can be easily dimmed without substantial colour temperature change. White, or a combination of coloured LEDs may also be used if they can provide a suitable short time and high energy level flash.

The system 19 comprises a battery 20, to provide power for the system 19, a low voltage power supply (5V) 22 for the timing circuits, a high voltage power supply (320V) 24 for the xenon flash head and the flash head itself. The flash head is shown in Figure 6. The flash head is separate from the rest of the system 19. It is connected to the system 19 by a multicore cable 26. Line 25 is connected to ground.

The video camera 28 produces a composite video signal 30, which is input to the timing section 32 of the flash unit 26. This timing section 32 produces two outputs, one to trigger the flash unit 34 and the other 36 to start the charging process for the xenon tube storage capacitor (not shown) . In this example, the outputs are variable with respect to the field sync pulse 8 to correctly position the flash within the shutter period for different cameras.

Xenon tubes, after firing, need a short time to recover due to residual ions present within the tube. Any voltage applied to the tube during this time could cause a continuous arc within the tube, which is not desirable because it could damage the tube. Therefore a recovery period is required, typically around 2 milliseconds. The charging of the capacitor is achieved by gating a high voltage MOSFET 38 which in turn sinks current via a current limit resistor 40. Figure 4 shows an example arrangement of the timing section 32 using monostables 50,54,58,62,64,68.

The composite video signal 30 is inputted into a sync pulse separator 50. The output of sync pulse separator 50 forms a field drive 52 and is input to monostable 54 (the pre-delay) . The output of monostable 54 forms a pre-delay pulse 56 which is input to monostable 58. This sets the flash position. The output 60 of monostable 58 is input into two monostables (62 and 64) . The output 60 into monostable 62 forms a primary flash trigger. The output 66 from monostable 64 forms a secondary flash trigger and is input into monostable 62. The output of monostable 62 is connected to the flash unit (see Figure 6 and described below) and to another monostable 68. The output of monostable 68 controls a storage capacitor for the flash unit.

Figure 5 shows the timing relationships of the monostables 50,54,58,62,64,68. Time t is shown extending from left to right.

Monostable 54 is triggered by the negative going edge

100 of the signal 102 from the sync pulse separator 50 and produces a fixed duration pulse 104 of around 18ms. This pulse 104 expires before the shutter start period 12. Monostable 58 triggers off the negative going edge 106 of the signal 104 from monostable 54 and provides a variable pulse length 108, which can span either side of the field interval between shutter start periods 12. The variable pulse length 108 is adjusted for correct flash timing with the shutter. Monostable 64 is triggered off the negative going edge 110 of monostable 58 and produces a fixed pulse 112 with a length of 10ms. Such a pulse is used to trigger the flash unit when the active shutter is closed. Monostable 62 produces a pulse 114 for triggering the flash unit from monostable 58 when the shutter is open and also a pulse 116 from monostable 64 to trigger the flash unit when the shutter is closed. The duration of the pulses 114 and 116 also defines the charging delay time 118 of the tube capacitor after the flash unit has fired. Monostable 68 produces a charging pulse 118 (inverted) by triggering off the negative edge of pulses 114 and 118 and defines for how long the capacitor charges

The flash unit is illustrated in Figure 6. To initiate the flash 150, a positive going pulse 114,116 is applied along line 153 to the HT trigger generator 152, this in turn provides an HT pulse along line 154 (typically 5kV) to the trigger electrode 156 of the flash tube 150. The flash tube 150 then ignites and energy stored in the capacitor 158 is discharged. When there is no pulse 114,116 the capacitor 158 is charged. A short time after the flash tube 150 has been initiated, the capacitor 158 is allowed to charge up again ready for the next flash. The energy dissipated by the flash tube 150 is in practice a little less than the theoretical total energy stored in the capacitor. This is because of the series resistance of the capacitor 158, and the fact that the capacitor 158 is not completely discharged when the flash tube 150 is ignited.

A flash light operating at the required 50Hz field rate in the UK, that is one flash every time the shutter is opened, can be a little distracting to any subjects being lit by this system, even when taking into account persistence of vision. A camera could be made to operate the shutter more than once within the field period. So by using a suitable synchronising arrangement with such a camera the flash rate could now be correspondingly increased giving a far more pleasing light output. For cameras without this feature, one solution, as described above, is to operate the flash system at a higher frequency, typically at a multiple of 50Hz. For example, at 100Hz, the flash rate is far less distracting but efficiency is now reduced by a half as the camera would not view flashes falling outside the active shutter period. However, this solution is still highly efficient when compared to a conventional lighting system.

The secondary flash, described above as the flash during the period when the shutter is closed, may not need to be as energetic as the flash when the shutter is open, thus some of the lost efficiency could be recovered. A 50% reduction in the power of the secondary flash would only result in a slight flicker. Furthermore, the precise timing of the secondary flash is not critical.

In one embodiment, a camera could perform two shutter operations within the field period. Hence, if the secondary flash fell within this new shutter period, efficiency would be restored.

For a US type 60Hz TV system, a strobe light operating at 60Hz is also rather unpleasant, a system as that described above could be adjusted to 60Hz and used to make the light flicker acceptable.

The circuit diagram for the system described above is shown in Figure 7. In this example, HT generation was obtained from a modified mains inverter and, for safety reasons, opto-isolators were used between the low voltage and high voltage sections . The circuit diagram for the flash unit example described above is shown in Figure 8. In an alternative embodiment a microprocessor- controlled system could be used. An example of such a system is illustrated in Figure 9.

In this example, the flash unit is synchronised by a composite video signal output from a video camera. The composite video signal is applied along lines 200 to a video distribution amplifier 202, this allows for extra video feeds for monitors and other destinations. One output of the video distribution amplifier 202 is input to a video sync, separator 204, this in turn produces line sync. 206, field sync. 208 and field ident. 210 outputs to port inputs 212 of a microprocessor 214.

The microprocessor 214 uses internal counters, which are reset by the field sync, pulse from output 208 and clocked by the line sync, pulses from output 206 to produce the flash trigger output 216 at the required time 12. The flash trigger output is output to the trigger control 218, which in turn triggers the flash unit 220. Alternate fields may require slight differences in this timing, in which case software controlling the microprocessor 214 adjusts the timing of the trigger output 216 depending on the field ident. signal from output 210.

The high voltages required for the flash tube 220 are produced, in this example, by a simple voltage multiplier arrangement 222, which is driven by the microprocessor 214. Pulses from an output 224 of the microprocessor, control a driver stage 226, which in turn drives a step-up transformer 228. Charge is then directly pumped via a diode (not shown) into a storage capacitor 225. This arrangement removes the need for a current limit resistor. Feedback of the voltage across the capacitor 225 is provided via the voltage divider (isolated) so the microprocessor 214 can stop the charging operation when the required voltage is reached.

An average picture level detector 229 is connected between the video distribution amplifier 202 and the microprocessor 214.

The precise timing of the flash of the flash unit 220 can be achieved, as before, by the user manually adjusting the timing by operating user control buttons 230 on the microprocessors ports 232. In an alternative embodiment, an automatic arrangement could be provided whereby the microprocessor 214 changes the flash trigger position until synchronisation is achieved. In this situation, an average picture level detector 229 is used (see Figure 9) to determine when the camera and the flash are synchronised. In operation, software controlling the microprocessor 214 gradually changes the timing of the flash until the shutter period of the camera and the flash coincide. When the shutter period of the camera and the flash coincide, the video signal from the camera would be larger than when the shutter and the flash are not synchronised. The video signal is larger because light from the flash is detected by the camera and hence the shutter and flash are synchronised.

In one embodiment, the correct synchronised timing could be stored in memory so that if a different camera, with a different shutter period was used with the system, the timing parameters could be easily changed back if the original camera was later used again. In one arrangement, a user selects the shortest shutter speed of a particular camera and allows the camera to view the light output from the flash before starting the synchronisation procedure described above.

The microprocessor 214 can also control the power level of the flash tube output by varying the voltage across the storage capacitor, this control could be dynamic, if required, to produce differing light level outputs per flash to create special lighting effects.

The use of a microprocessor 214 as the main control element allows for very flexible operation of the light and many extra features can be added such as battery voltage monitoring and/or automatic switching between 50Hz and 60 Hz TV operation and/or monitoring of the light output and/or warning of a failing flash tube.

The control system described above may be separate to the camera and the flash unit. Alternatively, it may be built into a camera or a flash unit.

If the light is built into a camera, the control microprocessor could be integrated with the camera control system.

Sometimes it may be desirable that a plurality of synchronised flash light sources be used. One possible solution is to arrange for all the flash heads to use the same output pulse from the pulse control unit (timing section) 32, or alternatively to trigger the extra light sources by means of photo sensors which trigger the secondary light source directed towards the light from the primary flash source. These photo sensors are connected to a control device which triggers the secondary flash source or sources. When the photo sensors detect the flash from the primary light source, the control device detects this and the secondary flash units are triggered to flash by the control device .

Such a system provides the advantage that cables are not required to connect the primary and secondary light sources together.

By using more than one camera, with the cameras operating at different relative and non-overlapping active shutter timings, a variety of original effects can be produced. A flash head synchronised to one camera shutter would not be seen by another camera. Hence, using different flash head positions/colour filters synchronised to different cameras, the same scene could produce totally different outputs from the cameras. This would enable many original creative effects to be obtained that would be impossible by any other means. For instance, one camera could see the scene as being in full daylight, whilst a second camera could simultaneously see the scene as being shot at night. Such effects are not possible with existing camera and lighting systems .

As described above, the shutter speed control of a camera can adjust the background level of the picture if the foreground subject is lit by a stroboscopic light source. This effect can usefully be used to remove the problems caused when filming, using a video camera, through a window when the background light level changes. A suitable system is shown in Figure 10.

Three video cameras 300,302 and 304 are shown. They are all driven off the same video synchronising source for example, "black and burst", from a sync pulse generator. Hence, all three cameras 300,302,304 have the same shutter timings. A light level detector 306 picks up the background lighting level and the signal it produces is fed into a processing unit 308. In response to the background lighting level, the processing unit 308 produces a signal suitable to be fed into a camera control unit 310,312,314 each of which is associated with one of the cameras 300,302,304. The processing unit 308 can also be setup with user parameters such as response time.

The output of each of the camera control units 310,312,314 is shown being fed into a vision mixer 316.

Black and burst synchronising pulses are fed into the vision mixer 316 and all three camera control units 310,312,314 to correctly time the system.

As the background lighting level changes in intensity, the change is detected by the light level detector 306 and the shutters of the cameras 300,302,304 are adjusted to compensate for this change. The foreground subjects stay at the same lighting level as they are lit by a stroboscopic light or synchronised stroboscopic lights, of the type described above.

Embodiments of the present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention.

Claims

1. A light source triggering device for use with a light source to illuminate a scene being recorded by a moving picture camera the light source triggering device comprising means to cause the light source to emit at least one flash of light substantially in synchronisation with exposures of the moving picture camera and to cause the light source to emit at least one additional flash of light between each pair of exposures of the moving picture camera.

2. A light source triggering device according to claim 1, further comprising an input for receiving a synchronisation signal from the ••camera, and an output for outputting at least two signals to the light source wherein the light source responds to each signal by emitting a single flash, one of the flashes being within the active shutter period of the camera .

3. A light source triggering device according to claim 1, wherein the at least one flash substantially in synchronisation with exposures of the camera is brighter than the at least one additional flash between each pair of exposures.

4. A control system for controlling the timing of the operation of a light source such that the light source operates during the active shutter period of a camera, the control system comprising adjusting means to adjust the time when the light source is caused to flash and average picture level detecting means to measure the average picture level received by a camera operating within the range of illumination of the light source, whereby the adjusting means adjusts the time at which the light source is caused to flash until the average picture level detecting means indicates that the average picture level received by the camera is maximised when the light source is operated during the active shutter period.

5. A secondary strobe light source for triggering by a primary strobe light source, the secondary strobe light source comprising a signal detector means to detect a first signal from the primary strobe light source, control means to trigger the secondary strobe light source, and a strobe light source, in use, when the light detector means detects a first signal from the primary strobe light source a second signal is provided from the control means which causes the strobe light source to light up.

6. A secondary strobe light source according to claim 5, wherein the first signal is carried on a light carrier.

7. A camera comprising an active shutter, the active . shutter being operable at least twice per video field period.

8. A video camera system for viewing a subject, the system comprising at least two cameras and at least one flash unit, wherein each camera has an active shutter period that does not overlap in time with the other camera or cameras' active shutter period, and the flash unit flashes light during the active shutter periods of one of the cameras, such that the subject is viewed at a different level of illumination with each camera.

9. A method of triggering a light source for illuminating a scene being recorded by a moving picture camera, the method comprising the steps of: causing the light source to emit flashes of light substantially in synchronisation with exposures of the moving picture camera, and causing the light source to emit at least one additional flash of light between each pair of exposures of the moving picture camera.

10. A method of triggering a light source according to claim 9, the method comprising the steps of: receiving a synchronisation signal from the camera, outputting at least two signals to the light source, and the light source responding to each signal by emitting a single flash of light, one of the flashes being within the active shutter period of the camera.

11. A method of controlling the timing operation of a light source such that the light source operates during the active shutter period of a camera, the method comprising the steps of: measuring the average picture level received by a camera operating within the range of illumination of the light source, and adjusting the time when the light source is caused to flash until the measured average picture level received by the camera is maximised when the light source is operated during the active shutter period.

12. A method of triggering a secondary strobe light source from a primary strobe light source, the method comprising the step of: lighting the secondary strobe light source, in response to detecting a signal from the primary strobe light source at the secondary strobe light source .

13. A method of triggering a secondary strobe light source from a primary strobe light source according to claim 12, wherein the signal is carried on a light carrier.

14. A method of operating a camera, the method comprising the steps of: operating the active shutter of the camera at least twice per video field period.

15. A method of viewing a subject with video cameras, the method comprising the steps of: viewing the subject with at least two cameras, wherein each camera has an active shutter period that does not overlap in time with the other camera or cameras' active shutter period, and producing a flash of light during the active shutter period of one of the cameras, such that the subject is viewed at a different level of illumination with each camera.

16. A method of illuminating a foreground of a scene in which the background is more brightly lit than the foreground, the foreground and the background being filmed with a camera, the method comprising the steps of: detecting the active shutter period of the camera and producing at least one flash of light from a light source within the active shutter period, so that the output from the camera indicates that the foreground and the background are evenly lit.

7. A method according to claim 16, wherein the method further comprises the step of: adjusting the active shutter period of the camera to compensate for variations in the lighting level of the background.