Television camera system having differentiated illumination between fields

A system includes a television camera (10), a first source of illumination (17) operable to provide pulsed illumination of a scene to be viewed by the camera, and a second source of illumination (18) operable to provide constant illumination of the scene. Illumination control means (16) is operable to activate the first source (17) to provide a pulse of illumination prior to the commencement of each alternate field period of the television raster scan cycle. Camera control means (13) controls a gate (12) in the camera system so that, during each of these alternate field periods the camera (10) provides a first video signal representing the scene illuminated by the first source (17) at a predetermined time after the pulse of illumination. The camera control means (13) also operates the gate (12) so that, during each other frame period, the camera provides a second video signal representing the scene illuminated by the second source throughout the entire field period. A video output terminal (14) is provided, to which the first and second video signals are applied. Means may be provided for measuring the distance to an object in the scene to be viewed.

Conventional television cameras function well with adequate lighting and 
good visibility of the scene to be viewed. It is common practice to use 
floodlights to enable such cameras to operate in conditions of low ambient 
light level. If, however, visibility becomes reduced, such as by the 
presence of smoke or mist, or other situations where optical scattering 
occurs, the presence of floodlighting may actually be a disadvantage. This 
is because light reflected back from the particles causes the lack of 
visibility due to the backscattered light producing a high level of glare 
in the camera. 
The problem of backscatter may be avoided by using a pulsed illumination 
system together with a gated television camera. The camera is synchronised 
to the pulse of light so that the camera responds only to light reflected 
from an object at a predetermined distance. Light from objects closer to, 
or further from, the camera reaches the camera at a different time, and is 
rejected. Whilst such a system may give a clear image of an object at a 
certain distance from the camera, the remainder of the scene is missing. 
Although this system may be useful for determining the range of an object, 
it will not show a scene of any depth. Such a system has therefore, 
operational limitations for certain applications. 
It is an object of the present invention to provide a television system in 
which the effects of backscatter may be substantially reduced. 
According to the present invention there is provided a television system 
which includes a raster scan television camera, a first source of 
illumination operable to provide pulsed illumination of a scene to be 
viewed by the camera, a second source of illumination operable to provide 
continuous illumination of the scene, illumination control means operable 
to activate the first source to provide a pulse of illumination prior to 
the commencement of each alternate field period of the television raster 
scan cycle, camera control means operable to enable the television camera 
to provide, during each of said alternate field periods, a first video 
signal representing the scene illuminated by the first source at a 
predetermined time after the pulse of illumination and, during each other 
field period, to provide a second video signal representing the scene 
illuminated by the second source throughout the entire field period, and a 
video output terminal to which the first and second video signals are 
applied.

Referring now to FIG. 1, a television system includes a television camera 
10 having a lens 11 which receives light from a scene to be viewed. A gate 
12 is positioned between the lens and the camera and is operable under the 
control of a camera control unit 13 to pass or block the light from the 
lens system as required. The camera 10 produces a video output signal 
which is applied to a video output terminal 14. An external input R to the 
camera control unit 13 enables the timing of the operation of gate 12 to 
be varied at will. 
The scene to be viewed is illuminated by two sources. One of this is a 
pulsed source, such as a laser 15 which is fired by trigger pulses from an 
illumination control unit 16. Since a laser generally produces a narrow 
beam of light it may be necessary to use a beam-expanding lens system 17. 
The other source of illumination is constant and may comprise floodlights 
shown schematically at 18, or the illumination may be natural ambient 
light. 
Gate 12 may conveniently be an electronically-gatable proximity-focussed 
microchannel plate detector. In such a device electrons emitted by a 
photo-cathode on to which the optical image is focussed are normally 
accelerated towards one side of a microchannel plate. The opposite side of 
the microchannel plate emits secondary electrons which are detected by an 
anode. This anode may be a phosphor screen. The detector may be gated off 
by changing the potential of the photocathode relative to the adjacent 
surface of the microchannel plate. Gating is very rapid, taking less than 
5 nanoseconds in some instances. 
The operation of the system of FIG. 1 will now be described with reference 
also to the waveforms of FIG. 2. In FIG. 2, waveform (a) represents the 
triggering of the pulsed illumination source, waveform (b) shows the 
operation of the gate and waveform (c) represents the camera video output. 
As wil be seen from FIG. 2, the pulsed light source is arranged to fire 
during alternate television field blanking periods FB, say before each 
even-numbered field period FP2, FP4 and so on. The gate is arranged to be 
open during odd-numbered field periods FP1, FP3 and so on, and is opened 
only for a very short time during each of the even-numbered field periods. 
The point in each even-numbered field period when the gate is open is 
variable and determines the range from the camera of any object "seen" by 
the camera during such period. 
As already stated the first source of illumination provides constant 
illumination of the scene. Hence during the odd-numbered field periods 
such as FP1 a video waveform will be produced such as that indicated in 
FIG. (2c). During the following even field period, since the gate is open 
only for a short time the video waveform should represent only light 
reflected from those parts of the scene at a certain distance from the 
camera. However, due to storage effects in the television camera a certain 
small amount of the video waveform from the previous field will still be 
present, then the video waveform produced during the field period FP2 will 
represent the combined effects. In the next field period there is no 
pulsed illumination and the gate is again open during the entire period. 
The above procedure is followed during each successive field period, so 
that the video output signal presented to the video output terminal is of 
the form shown in FIG. (2c). 
When the video signal output is applied to a display the time constant of 
the display screen phosphor has an integrating effect, giving a picture in 
which the content of the gated and ungated field periods are superimposed. 
Clearly the amounts of light received by the television camera during the 
gated and ungated field periods will differ considerably. It is necessary 
to arrange that the camera automatic gain control circuit is able to 
change the gain levels between alternate field periods, so as to provide a 
normalised video signal level throughout. FIG. 3 illustrates one way in 
which this might be done. The camera automatic gain control circuit 30 is 
shown separate from the camera 10 and receives control inputs from two 
sample-and-hold circuits 31 and 32. Each of these circuits has a timing 
input from the camera control unit 13 of FIG. 1, circuit 31 receiving an 
input OFP during odd field periods whilst circuit 32 receives an input EFP 
during even field periods. 
The camera video output signal is sampled by each sample-and-hold circuit 
during the appropriate field period and the held value is used during the 
next alternate field period to vary the camera gain. 
The automatic gain control circuit 30 may also be used to control an iris 
incorporated in the camera lens by way of an iris control circuit 33. 
FIG. 4 illustrates an alternative optical arrangement in which two lenses 
40 and 41, eahc with its own gate 42 and 43 respectively are provided. 
Suitable combining optics, such as a totally reflecting mirror 44 and 
semi-reflecting mirror 45, are used to direct light passing through the 
two optical systems to the single camera 10. Each gate has a control input 
from the camera control unit. 
Yet another possible arrangement involves the use of two separate cameras; 
each with its own lens and gate. FIG. 5 illustrates such an arrangement 
and shows cameras 50 and 51 with lenses 52 and 53 and gates 54 and 55. The 
video outputs of the cameras are combined, for example by a frame store 
56, to provide a common video signal at video output terminal 14. The 
camera control unit 13 controls the operation of the frame store 56. 
The above description has been concerned with the use of an image 
intensifier tube as a gate. Other forms of gate may be used if suitable. 
Whilst it is unlikely that electro-mechanical systems will operate with 
sufficient speed and accuracy, electro-optical gates may exist or be 
developed which will satisfy the requirements of fast and accurate 
operation. 
Brief mention has already been made to the measurement of range. This is 
possible because the camera control unit 13 of FIG. 1 already produces an 
accurately-timed gate control pulse to control gate 12. In order to 
provide an absolute measurement of range it is necessary to calibrate the 
gate delay interval with reference to the instant of firing of the pulsed 
illumination source. FIG. 6 is a block diagram of a suitable circuit. This 
has a clock pulse generator 60 which generates a train of clock pulses. 
These are applied to the clock pulse input of a range counter 61 and, 
through a preset delay counter, 62 into a bistable circuit 63. A second 
input to the bistable circuit is applied from the output of camera control 
unit 13 of FIG. 1 and the output of the bistable circuit is applied to the 
enabling input of trange counter 61. The counter output is applied to a 
range read-out 64. 
In operation the illumination source trigger pulse TP starts the preset 
delay counter 62, which counts a number of pulses equal to the delay 
period before the emission of a light pulse from the source. After this 
delay the next clock pulse sets the bistable circuit 63 and allows the 
range counter to count clock pulses. When a pulse is received to operate 
gate 12 of FIG. 1, from camera control unit 13, the bistable circuit 63 
again changes state and the range counter stops counting. The number of 
pulses counted represents the time elapsed between the illumination pulse 
and the operation of gate 12. If gate 12 is operated at such a time that 
an object whose range is required is just illuminated, then that time 
period represents the range of the object from the source of illumination. 
The speed of passage of light through the medium between source and object 
must, of course, be taken into account. 
The illumination means may operate at any wavelength to which the camera 
will respond. Whilst many applications will require the use of visible 
light, the system will operate at other wavelengths, such as in the 
infra-red region of the spectrum.