Flash Technical Primer
Introduction:
At the heart of any electronic flash system is a Xenon discharge
tube, which belongs to the class of triggerable gas discharge
devices known as 'Thyratrons'. Other types of thyratron once
found service in situations where the light output was not important,
i.e., they were used as switches, but were replaced by the semiconductor
equivalent: the 'Thyristor' (AKA 'Silicon Controlled Rectifier'
or 'SCR'). Development of the Xenon tube as a synchronised photographic
light source is mainly due to the work of Prof. Harold Edgerton
of MIT in the 1930s. Prof. Edgerton formed part of the scientific
team which sailed with J Y Cousteau on the Calypso expeditions,
in which environment he was known as 'Papa Flash'. |
Flash tubes:
A basic gas discharge tube is simply a glass bulb or tube, with
two electrodes inserted into it, which has been evacuated and
filled with an appropriate gas at somewhat less than atmospheric
pressure. All such tubes have the property that, as the voltage
applied across the electrodes is increased, there comes a point
called the 'strike voltage', at which the gas in the tube ionises,
and due to an avalanche effect, a low resistance current path
is formed. Once an arc has struck, the tube resistance remains
low until the applied voltage falls below a point called the
'extinction voltage'. The extinction voltage is usually considerably
lower than the strike voltage, i.e., the arc is difficult to
get going, and then difficult to stop once started.
If the gas pressure is very low, the tube emits light at one
or more characteristic wavelengths associated with transitions
between electronic states of the gas atoms or molecules. Devices
with such spiky spectra (e.g. yellow sodium-vapour street lights)
are no use as photographic light sources because they give poor
or non-existent colour rendering, but as the gas pressure is
increased, the spikes become broader. This 'pressure broadening'
is a direct consequence of the Heisenberg Uncertainty Principle,
and stems from the fact that as the pressure increases, the gas
molecules undergo more and more collisions, thus reducing the
average time for which a molecule can remain in a given electronic
state. The Heisenberg principle states that as the lifetime of
given state decreases, its energy becomes less and less well
defined, the net result being that the characteristic emission
wavelengths are smeared-out into bands. Xenon gas discharge tubes
can be operated at a pressure at which the characteristic wavelengths
are sufficiently smeared out, that the output approximates pure
daylight; or more specifically, black-body radiation with a colour
temperature of about 6000K.
A discharge tube becomes a Thyratron upon the addition of a third
electrode. Such devices are operated by applying, across the
main electrodes, a voltage which is above the extinction voltage
but below the strike voltage. The device is triggered by applying
a voltage pulse between the third electrode and one of the main
electrodes, the resulting small discharge starting an avalanche
which then spreads to the whole tube. In Xenon tubes, it is usually
sufficient to apply a high voltage trigger pulse (3-6KV) to the
outside of the glass envelope. As the tube is operated closer
and closer to it strike voltage, it can also be triggered by
radioactive particles*, and by the light from other flash tubes
being fired in its vicinity. Flash tubes are usually operated
sufficiently well away from the strike voltage that accidental
triggering is rare.
(* A Geiger-Müller tube is a discharge tube to which an
electron-capture agent (usually bromine) has been added to prevent
the discharge from spreading) |
 |
Early streamer in a Xenon tube. As the avalanche progresses,
the arc grows to fill the whole tube. |
Photographic Flash.
When used as a photographic light source, the Xenon tube is connected
across a large high-voltage capacitor, which is charged from
a mains or battery power supply. The device which produces a
high voltage output from a low-voltage battery is known as an
'inverter', but may also be referred to as a 'DC to DC converter'.
Common preferred operating voltages are 330, 360, 400 and 600V,
depending on the application, whereas the pressure dependant
strike voltage of the tube is considerably in excess of 1000V.
The tube extinction voltage is usually about 50V. When the tube
is triggered, its resistance drops to something in the order
of 1W. Consequently, the discharge
current may peak at several hundred Amps, before it ceases abruptly
as the capacitor voltage falls to 50V. Note that ordinary electrolytic
capacitors are not designed to withstand being discharged by
short-circuit, and special 'Photoflash' grade electrolytics are
used.
The burst of energy from a typical photographic flash tube lasts
for about 1 milli-second (1ms). This time can be reduced by minimising
the resistance in the tube - capacitor circuit, or by interrupting
the current in the tube. The flash duration can be increased
by inserting an inductance (known to this author as a 'delay
coil') in series with the tube. The delay coil is used particularly
in TTL and auto flash systems, where, by spreading the discharge
over a longer interval, it permits more accurate determination
of the point at which the light-burst should be terminated for
proper exposure.
X-Sync.
Because the burst of energy from a flash unit is very short,
the level of film exposure cannot be controlled by a mechanical
camera shutter. Exposure is controlled instead by the camera
aperture and the (fixed or variable) guide-number of the flash.
For this system to work however, the camera shutter must be fully
open when the flash is fired, and this triggtering regime is
known, for historical reasons, as "X-Synchronisation".
In focal-plane shutter systems (film cameras), X-synchronisation,
is achieved by placing a switch at the end of travel of the first
shutter curtain. Note however, that for high shutter speeds in
focal-plane systems, the second curtain is released before the
first curtain has finished traveling, and the film is exposed
by the slit between the moving curtains. Consequently, X-sync
is not possible above a certain shutter speed. For a given camera,
the highest shutter speed at which the shutter opens fully is
called the "X-sync speed". Synchronisation at speeds
below the X-sync speed is of course possible.
The 'Ready' Signal:
Photographers using completely manual flash systems must suffer
the irritation of spoiling photographs by forgetting to reduce
the shutter speed to something below the X-sync limit. The single
most important ergonomic improvement for a film camera system
is therefore to provide a signal from the flash unit to the camera,
which forces the camera to the X-sync speed if a higher speed
has been selected. This signal is known as the "Flash Ready"
signal, and is a feature of all modern automatic flash systems.
Auto and TTL exposure control:
Photographic exposure is the product: Intensity x
time. Automatic control of the instantaneous intensity of a flash
is difficult, but the duration of the flash can be curtailed
in one of two ways:
1) Dump the remaining energy in the capacitor by switching
a low resistance device across it. The device in question is
usually a small Xenon trigger-tube with high electrode area and
small inter-electrode distance, called a 'Quench Tube'. A quench
tube does emit light, but it is placed inside the flash-unit
electronics compartment and cannot contribute to the exposure.
The disadvantage of the charge-dump system is that it wastes
the unused energy and results in a system which must recharge
the capacitor from scratch after each firing.
2) Interrupt the current in the tube using a series control
element, originally a 'Gate Turn-Off Thyristor' (GTO-SCR), but
nowadays an Insulated-Gate Bipolar Transistor (IGBT). The idea
was origianlly patented by Vivitar Inc. This is the modern system,
which results in reduced recycling time after a partial discharge.
In order to determine the correct point at which to quench the
flash, It is necessary to integrate the amount of light falling
on the subject over time. In film TTL and auto systems, this
is done by using the current passing through a phototransistor
to charge a capacitor. The capacitor is connected to a comparator,
which changes state when the capacitor voltage crosses over a
threshold set by the ISO film-speed control.
In TTL digital camera systems, a controlled pre-flash is fired
and the level of exposure obtained is found by reading data from
the sensor and averaging it. This information is used to calculate
a suitable burst time for the main flash, using stored data relating
to the time vs intensity profile of the flash output.
OK:
The signal sent from the camera to the flash unit to terminate
the light burst is called the 'Quench', 'Q' or 'TTL Stop' signal.
If there is insufficient light to complete the exposure, a quench
signal will not be sent. Many flash units have an 'OK' indicator,
often a green lamp, which tells the user that a quench signal
has been received. |
Afterglow.
A potentially serious problem in high-power flash system design
is afterglow, which can occur intermittently, and may cause the
circuit to self-destruct if not controlled. Afterglow occurs
when the charging circuit has sufficient output to keep the capacitor
charged to a point above the tube extinction voltage while the
gas is conducting. In this case, the gas continues to glow indefinitely
after triggering, and system meltdown will occur unless preventative
measures are taken. In TTL and automatic exposure flash systems,
arterglow can be prevented by sending a late quench pulse to
the series-control element (i.e., by waiting to see if the camera
sends a quench signal, and switching-off the current anyway if
nothing is received after several milliseconds). In full-output
systems, one possible solution is to sense the tube current and
inhibit power-supply output (stop the inverter) while the tube
is conducting. |
|
Flash Energy:
The energy stored in a capacitor is given by the relationship:
 .
(E in Joules, C in Farads, V in Volts)
Thus the energy dissipated by the tube (neglecting resistance
losses) is:
 ,
where Vm ("V-max") is the starting voltage, and Vx
("V-ex") is the extinction voltage.
The relationship between the flash energy and the photographic
guide number G is given by:
 ,
i.e., the guide number is proportional to the square root of
the flash energy.
K is a system constant which depends on the design of the reflector
and any losses which may occur due to light absorption.
Guide Numbers:
When the flash tube is operated in air, the surrounding medium
is so thin that it can be treated as a vacuum. In this case,
insofar as the flash tube and reflector assembly can be regarded
as a point source, the level of illumination produced is inversely
proportional to the square of the distance. This results in a
very simple rule for calculating exposure:
In metric countries, the flash guide number is the aperture
setting required for ISO 100 film sensitivity when the flash
to subject distance is 1m (In North America, the guide number
is the 100ASA aperture setting for a distance of 1 foot. Divide
the American guide number by 3.28 to get it in metres). Once
the Guide number G has been determined (usually by trial and
error), the required aperture for any ISO film speed and distance
(in air) is obtained thus:
Where 'film speed' is the ISO index for the film or the ISO light
sensitivity setting of a digital camera.
Thus, if a flash has a guide number of (say) 32, its guide table
looks like this: |
|
|
Flash to subject distance / metres |
|
ISO |
1 |
1.4 |
2 |
2.8 |
4 |
5.6 |
8 |
11 |
16 |
|
25 |
16 |
11 |
8 |
5.6 |
4 |
2.8 |
2 |
1.4 |
1 |
|
50 |
22 |
16 |
11 |
8 |
5.6 |
4 |
2.8 |
2 |
1.4 |
|
100 |
32 |
22 |
16 |
11 |
8 |
5.6 |
4 |
2.8 |
2 |
|
200 |
|
32 |
22 |
16 |
11 |
8 |
5.6 |
4 |
2.8 |
|
400 |
|
|
32 |
22 |
16 |
11 |
8 |
5.6 |
4 |
|
800 |
|
|
|
32 |
22 |
16 |
11 |
8 |
5.6 |
|
1600 |
|
|
|
|
32 |
22 |
16 |
11 |
8 |
|
Underwater "Guide Numbers".
Unfortunately, when flash lighting is used underwater, the guide
number rules applicable in air break down; for two reasons: Firstly,
the flash has to be fired at short range and so no longer acts
as a point source, and secondly, the intervening medium absorbs
light. An exposure guide table can still be constructed, but
it has to be calculated in a different way or determined empirically,
and it will usually be based on the assumption that the flash
unit and the camera are both situated at about the same distance
from the subject.
When a ray of light passes through an absorbing medium, the
attenuation (reduction in intensity) over a given distance can
be calculated using the Beer-Lambert law:
I = I0
exp{-eL)
Where I is the final intensity, I0 is
the initial intensity, e (epsilon)
is the extinction coefficient, and L is the path length in units
compatible with e (the quantity within
the exponent brackets must be dimensionless, so if L is in metres,
e must be in metres^-1, i.e., "per
metre"). The Beer-Lambert law represents the fact that light
intensity decays exponentially with distance in an absorbing
medium. This decay is in addition to the angular dilution effect
(inverse square law) which applies in a non-absorbing medium.
Note also that the extinction coefficient e
varies with wavelength, and in water, the attenuation is much
greater at the red end of the spectrum than at the blue end.
The upshot is that flash illumination only works for a very short
range underwater; and the range which permits red to be recorded
effectively is much shorter than the range which permits blue
or monochrome photography.
As a rough starting approximation, light absorption by water
reduces the effective guide number of a flash by a factor of
3. (e.g., a strobe with a guide number of 24 in air becomes a
strobe with a guide number of about 8). The water also acts as
a filter with a density of about 0.12 red per metre of light
path; which means that you lose a whole stop of red for every
2.5m, and since the light must travel from flash to subject,
and then from subject to camera, you lose 1 stop of red when
you are only 1.25m away from the subject. |
D. W. Knight. © Cameras Underwater,
2002, 2006 |
