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The flash time characteristics of indirectly ignited photographic flash lamps
Physica IV, no 5
Mei 1937
THE FLASH TIME CHARACTERISTICS OF INDIRECTLY IGNITED PHOTOGRAPHIC FLASH LAMPS by J. A. M. VAN LIEMPT Physisch
Chemisch
and J. A. DE VRIEND
Laboratorium der N.V. Eindhoven-Holland
Philips’
Gloeilampenfabrieken,
Summary
The flash time characteristics of indirectly ignited photographic flash lamps are given and conclusions are drawn in relation to practical use specially with synchronizers.
It is a well-known fact that when a photographic flash lamp with thin aluminium foil is closely surrounded by one or more identical lamps, the electrical ignition of a single lamp suffices to ignite the additional lamps. This is caused by the radiation of the first lamp, a large amount of the radiated energy being absorbed by the surrounding lamps, which energy is sufficient to heat the aluminium foil to a temperature where it readily bums in pure oxygen. It seems interesting to know the flash characteristics of lamps ignited in this way and to compare them with those of a single lamp especially to see at what point of the light-time curve of the original lamp the light emission of the indirectly ignited lamps starts. The flash time curves were determined with a cathode ray tube installation as will be described briefly below (Fig. 1). The apparatus is composed of two parts, one for the horizontal movement of the electron beam spot of the cathode ray tube and one for the vertical movement. Horizontally the spot is moved across the
screen of the cathode ray tube at a constant speed. For this purpose the vertical deflecting-plates of the cathode ray tube, C.R.T., are given a regularly decreasing voltage obtained from a condenser; -
353 -
354
J. A. M. VAN LIEMPT
AND J. A. DE VRIEND
Cr, which is charged through the battery B,, and discharged by the anode current of a diode with tungsten filament VI.
I’1 I II
r-7 c. I?. r.
Fig.
1. Principle
.C\r
of measuring
.+
apparatus
The vertical movement of the cathode beam is caused by the influence of the photocurrent of a vacuum-cesium photo-electric cell, of high sensitivity, PIz, which is illuminated by the flash lamp F. This current is directly proportional to the illumination of the photo-electric cell, so to the momentary luminous intensity of the flash lamp. This current gives rise to a’potential difference at the
CHARACTERISTICS
OF FLASH
355
LAMPS
extremities of a resistance, R, connected with the horizontal deflecting-plates of the cathode ray tube. So as to measure the time lag of x lob
LUMEN.
Fig.
x105
2. One lamp.
LUMEN.
SEC.
Fig.
3. Two lamps;
one ignited
electrically.
the ignition the same resistance is taken up in the anode-current circuit of a triode V,. If the ignition wire of the flash lamp is switched
356
J. A. M.
VAN
LIEMPT
AND
J. A.
DE
VRIEND
on the Pvolt accumulator, Bi, by the contact S,, the grid tension of the triode V,,which was already negative by battery B,, is diminished by an amount of 4 volts. This cuts off the triode, i.e. the anode current is reduced to zero. It was this anode current that in the beginning deflected the cathode ray a little bit upwards. In this way the moment of the disappearance of the anode current and thus of
SEC.
,
0
x lo-2 Fig.
4. Three
lamps;
the middle-one
ignited
electrically,
the deviation of the cathode ray beam coincides exactly with the moment of switching on the flash lamp. S4 switches on the two relays Si and S,, so that the horizontal and the vertical movement take place simultaneously. The moving electron beam spot is photographed by a camera with low focus number. The speed of the horizontal movement can be controlled by a separate measurement whereby by means of S, a low ,altemating voltage is applied to the horiiontal deflecting plates instead of the potential difference of the coupling
CHARACTERISTICS
OF FLASH
357
LAMPS
resistance. The light output of the same lamp was measured by collecting the emission of the photo-electric cell in a condenser C,. The difference in voltage of the condenser before and after switching off the flash lamp is proportional to the light output and is determined with an’electrostatic voltmeter M. The capacity of condenser C2 is large in comparison with that of the voltmeter M. x 105
LUMEN.
SEC
0
5
W
15 x 10-Z
Fig.
5. Three
lamps:
the first-one
ignited
electrically.
The switch S5 serves to discharge the condenser before measuring the flash lamp. An incandescent lamp burning under known conditions and during a time which is exactly known, also gives a voltage difference corresponding with a certain amount of lumenseconds. So we can calculate the proportionality-factor by which the measured voltage difference must be multiplied to get the output in lumen-seconds. The area of the light-time curve is also proportional to the light output and because the time-scale is known, the light-scale can now easily be calculated. To imitate a pocket battery the primary lamp was ignited with an accumulator of 4 volts in series with a resistance of 0.5 L2. The lamps measured were ordinary Al-foil lamps as available on the European market.
358
J. A.
M.
VAN
LIEMPT
AND
J. A. DE
VRIEND
The flash characteristics are given below, the zero of the time axis is the ignition-point of the lamp. Fig. 2. A single foillamp, electrically ignited. Fig. 3. Two lamps the first of which is ignited electrically and the second indirectly. Fig. 4. Three lamps, the centres lying on a straight line, the middle lamp of which was ignited electrically, whereas the two outside lamps were indirectly ignited. LUMEN.
Xd 4
SEC.
Fig.
6. Three
lamps
ignited
electrically.
Fig. 5. Three lamps, the centres lying on a straight line, the ftist lamp of which was ignited electrically and the two others indirectly.
CHARACTERISTICS
OF
FLASH
LAMPS
359
Fig. 6. Three lamps ignited electrically in parallel. The dotted line is the curve of a single lamp electrically ignited. (Fig. 2). In each case the lamps were put close together and bound with an elastic. From Fig. 2 we see that the single lamp has a time lag of light emission of 0.015 sec. and a peak value of 1.85 lumens. In Fig. 3 we see that due to the light absorption of the second lamp this peak value has dropped to 1.4 lumens and the time lag to 0.0 18 sec. It is interesting to see that the second lamp emits practically no Iight before the first has almost completely burned out. The peak value of the second lamp reaches about its normal value because the absorbing influence of the aluminia vapour in the first lamp may be neglected. Moreover, it should be mentioned that the peak values of individua lamps are not alkays identical and that an indirectly ignited lamp starts its light emission from the outside of the foil whereas a directly ignited lamp starts from the centre. From Fig. 4 we note that due to the absorption of light by the two surrounding bulbs the time lag of the first lamp has still more increased, whereas the peak value has still more decreased. The two outside lamps ignite now about at apparently not exactly the same time; due to this and to absorption they do not reach a peak value much higher than that of one original lamp, while the flash time is longer than for a single lamp. Fig. 5 speaks for itself. Finally from Fig. 6 we see that by igniting three lamps in parallel the peak value is about twice that of one lamp. The lamps ignite at about the same time, but not exactly, this causes a lower peak value than was to be expected, but on the other hand the flash curve is broadened. The question raised in this paper is of special importance when combining the flash lamps with synchronizers, especially if these are used with a short shutter-time. From Fig. 3, 4 and 5 it is obvious that when using a synchronizer which is adjusted for a single lamp, the final result in using the indirect ignition method gives only worse results, if adjusted for the secondary ignition, (which, however, is practically hardly possible due to small deviations in distance of the lamps which influence the time lag of the secondary lamps), the result is but scarcely better. In any case if we want a higher lumen output the method of switching more lamps in parallel on a battery is to be advised (Fig. 6)
360
CHARACTERISTICS
OF
FLASH
LAMPS
In this case we need not readjust our synchronizer whereas the light emission reaches its maximum in about the same time as in the case of a single lamp. If more lamps are to be ignited in parallel the use of more batteries ‘in parallel (not in series !) is to be recommended. Without the use of a synchronizer the flash time in the case of Fig. 3,4 and 5 is largely increased in respect of the single lamp or more lamps in parallel; the indirect igniting method is therefore useless for focal plane shutters and bad working for Compurshutters even with a long shuttertime. Received
March
13. 1937.
Mei 1937
THE FLASH TIME CHARACTERISTICS OF INDIRECTLY IGNITED PHOTOGRAPHIC FLASH LAMPS by J. A. M. VAN LIEMPT Physisch
Chemisch
and J. A. DE VRIEND
Laboratorium der N.V. Eindhoven-Holland
Philips’
Gloeilampenfabrieken,
Summary
The flash time characteristics of indirectly ignited photographic flash lamps are given and conclusions are drawn in relation to practical use specially with synchronizers.
It is a well-known fact that when a photographic flash lamp with thin aluminium foil is closely surrounded by one or more identical lamps, the electrical ignition of a single lamp suffices to ignite the additional lamps. This is caused by the radiation of the first lamp, a large amount of the radiated energy being absorbed by the surrounding lamps, which energy is sufficient to heat the aluminium foil to a temperature where it readily bums in pure oxygen. It seems interesting to know the flash characteristics of lamps ignited in this way and to compare them with those of a single lamp especially to see at what point of the light-time curve of the original lamp the light emission of the indirectly ignited lamps starts. The flash time curves were determined with a cathode ray tube installation as will be described briefly below (Fig. 1). The apparatus is composed of two parts, one for the horizontal movement of the electron beam spot of the cathode ray tube and one for the vertical movement. Horizontally the spot is moved across the
screen of the cathode ray tube at a constant speed. For this purpose the vertical deflecting-plates of the cathode ray tube, C.R.T., are given a regularly decreasing voltage obtained from a condenser; -
353 -
354
J. A. M. VAN LIEMPT
AND J. A. DE VRIEND
Cr, which is charged through the battery B,, and discharged by the anode current of a diode with tungsten filament VI.
I’1 I II
r-7 c. I?. r.
Fig.
1. Principle
.C\r
of measuring
.+
apparatus
The vertical movement of the cathode beam is caused by the influence of the photocurrent of a vacuum-cesium photo-electric cell, of high sensitivity, PIz, which is illuminated by the flash lamp F. This current is directly proportional to the illumination of the photo-electric cell, so to the momentary luminous intensity of the flash lamp. This current gives rise to a’potential difference at the
CHARACTERISTICS
OF FLASH
355
LAMPS
extremities of a resistance, R, connected with the horizontal deflecting-plates of the cathode ray tube. So as to measure the time lag of x lob
LUMEN.
Fig.
x105
2. One lamp.
LUMEN.
SEC.
Fig.
3. Two lamps;
one ignited
electrically.
the ignition the same resistance is taken up in the anode-current circuit of a triode V,. If the ignition wire of the flash lamp is switched
356
J. A. M.
VAN
LIEMPT
AND
J. A.
DE
VRIEND
on the Pvolt accumulator, Bi, by the contact S,, the grid tension of the triode V,,which was already negative by battery B,, is diminished by an amount of 4 volts. This cuts off the triode, i.e. the anode current is reduced to zero. It was this anode current that in the beginning deflected the cathode ray a little bit upwards. In this way the moment of the disappearance of the anode current and thus of
SEC.
,
0
x lo-2 Fig.
4. Three
lamps;
the middle-one
ignited
electrically,
the deviation of the cathode ray beam coincides exactly with the moment of switching on the flash lamp. S4 switches on the two relays Si and S,, so that the horizontal and the vertical movement take place simultaneously. The moving electron beam spot is photographed by a camera with low focus number. The speed of the horizontal movement can be controlled by a separate measurement whereby by means of S, a low ,altemating voltage is applied to the horiiontal deflecting plates instead of the potential difference of the coupling
CHARACTERISTICS
OF FLASH
357
LAMPS
resistance. The light output of the same lamp was measured by collecting the emission of the photo-electric cell in a condenser C,. The difference in voltage of the condenser before and after switching off the flash lamp is proportional to the light output and is determined with an’electrostatic voltmeter M. The capacity of condenser C2 is large in comparison with that of the voltmeter M. x 105
LUMEN.
SEC
0
5
W
15 x 10-Z
Fig.
5. Three
lamps:
the first-one
ignited
electrically.
The switch S5 serves to discharge the condenser before measuring the flash lamp. An incandescent lamp burning under known conditions and during a time which is exactly known, also gives a voltage difference corresponding with a certain amount of lumenseconds. So we can calculate the proportionality-factor by which the measured voltage difference must be multiplied to get the output in lumen-seconds. The area of the light-time curve is also proportional to the light output and because the time-scale is known, the light-scale can now easily be calculated. To imitate a pocket battery the primary lamp was ignited with an accumulator of 4 volts in series with a resistance of 0.5 L2. The lamps measured were ordinary Al-foil lamps as available on the European market.
358
J. A.
M.
VAN
LIEMPT
AND
J. A. DE
VRIEND
The flash characteristics are given below, the zero of the time axis is the ignition-point of the lamp. Fig. 2. A single foillamp, electrically ignited. Fig. 3. Two lamps the first of which is ignited electrically and the second indirectly. Fig. 4. Three lamps, the centres lying on a straight line, the middle lamp of which was ignited electrically, whereas the two outside lamps were indirectly ignited. LUMEN.
Xd 4
SEC.
Fig.
6. Three
lamps
ignited
electrically.
Fig. 5. Three lamps, the centres lying on a straight line, the ftist lamp of which was ignited electrically and the two others indirectly.
CHARACTERISTICS
OF
FLASH
LAMPS
359
Fig. 6. Three lamps ignited electrically in parallel. The dotted line is the curve of a single lamp electrically ignited. (Fig. 2). In each case the lamps were put close together and bound with an elastic. From Fig. 2 we see that the single lamp has a time lag of light emission of 0.015 sec. and a peak value of 1.85 lumens. In Fig. 3 we see that due to the light absorption of the second lamp this peak value has dropped to 1.4 lumens and the time lag to 0.0 18 sec. It is interesting to see that the second lamp emits practically no Iight before the first has almost completely burned out. The peak value of the second lamp reaches about its normal value because the absorbing influence of the aluminia vapour in the first lamp may be neglected. Moreover, it should be mentioned that the peak values of individua lamps are not alkays identical and that an indirectly ignited lamp starts its light emission from the outside of the foil whereas a directly ignited lamp starts from the centre. From Fig. 4 we note that due to the absorption of light by the two surrounding bulbs the time lag of the first lamp has still more increased, whereas the peak value has still more decreased. The two outside lamps ignite now about at apparently not exactly the same time; due to this and to absorption they do not reach a peak value much higher than that of one original lamp, while the flash time is longer than for a single lamp. Fig. 5 speaks for itself. Finally from Fig. 6 we see that by igniting three lamps in parallel the peak value is about twice that of one lamp. The lamps ignite at about the same time, but not exactly, this causes a lower peak value than was to be expected, but on the other hand the flash curve is broadened. The question raised in this paper is of special importance when combining the flash lamps with synchronizers, especially if these are used with a short shutter-time. From Fig. 3, 4 and 5 it is obvious that when using a synchronizer which is adjusted for a single lamp, the final result in using the indirect ignition method gives only worse results, if adjusted for the secondary ignition, (which, however, is practically hardly possible due to small deviations in distance of the lamps which influence the time lag of the secondary lamps), the result is but scarcely better. In any case if we want a higher lumen output the method of switching more lamps in parallel on a battery is to be advised (Fig. 6)
360
CHARACTERISTICS
OF
FLASH
LAMPS
In this case we need not readjust our synchronizer whereas the light emission reaches its maximum in about the same time as in the case of a single lamp. If more lamps are to be ignited in parallel the use of more batteries ‘in parallel (not in series !) is to be recommended. Without the use of a synchronizer the flash time in the case of Fig. 3,4 and 5 is largely increased in respect of the single lamp or more lamps in parallel; the indirect igniting method is therefore useless for focal plane shutters and bad working for Compurshutters even with a long shuttertime. Received
March
13. 1937.