High Performance Flash! by PerkinElmer

A P P L I C A T I O N
LIGHTING SOLUTIONS
High Performance Flash !
by PerkinElmer
N O T E
Fundamentals
Modes of Operation
Electronic flashtubes are gas
discharge devices for pulse
operation. Xenon is used as a fill
gas for most applications. For
laser excitation Krypton is also
used.
• Single flash: Random request
for single flashes. In comparison to the flash duration
(normally between 10 µs and
10 ms), the off times are very
long, typically a few seconds.
• Stroboscopic and beacon
operation: The discharges are
produced periodically and
often over a longer period of
time. Typical stroboscopic
frequencies are between 1 Hz
and several kHz. Higher
frequencies require special
flashtubes and circuits.
• Continuous wave discharge:
Electronic flashtubes are
unsuited for this mode of
operation.
w w w. o p t o e l e c t ro n i c s . p e r k i n e l m e r. c o m
Authors
Ingo Dünisch
Peter Strzelczyk
Dr. Rainer Heise
PerkinElmer Optoelectronics
Wenzel-Jaksch-Str. 31
65199 Wiesbaden
Germany
Configuration
All flashtubes consist of a tube
made of hard glass or quartz glass
with sealed-in electrodes (anode /
cathode) at each end. In addition to
linear glass tubes, various other
shapes, such as U, ring, or helix
shape are also available. The
cathode contains emitter substances to reduce the electron work
function. When connecting flashtubes, polarity must be observed.
Non-polarized tubes with two
cathodes are also available.
Many flashtubes are equipped with
a capacitive trigger electrode -- for
example, a wire wrapped around
the tube, a silver stripe or a transparent conductive coating on the
outside of the glass.
Leads on electrodes can be supplied
with solid or flexible lead wires,
according to PerkinElmer standards
or customer specifications. Lamps
can also be supplied with sockets,
end caps and protective covers.
Flashtubes are light sources with
high power density. Therefore,
they are only made of high temperature resistive materials.
Discharge tube
characteristics
Apart from the glass material and
electrodes, flashtubes feature three
key characteristics:
1. The arc length (e)
2. The inner tube diameter (r)
3. The Xenon (or Krypton) fill
pressure (p)
inner diameter r
trigger
electrode
anode
cathode
+
–
arc length e
Schematic drawing of a flash tube
By determining e, r, and p, all conditions and requirements such as
flash energy, expected life, trigger
quality, size, optical projection and
spectral distribution of the light
must be fulfilled.
Tubes characterized by low e/r
ratios (typically e/r < 5) are called
electrode stabilized and used for
short pulses and high luminance.
The plasma is mainly guided by
the anode and the cathode tips –
not touching the glass.
For tubes characterized by higher
e/r ratios, two typical flash discharges have to be distinguished:
• The plasma fills the cross
section of the tube completely,
limiting the peak current. The
discharge is referred to as “wall
stabilized.” Most flashtubes are
designed for 5 < e/r < 20, as
this range obtains the best light
efficiency.
• Typically, for stroboscopic applications the plasma does not fill
the tube cross-section completely, but is guided along the glass
envelope. The flash capacitor or
an electronic circuit limits the
peak current.
The luminous efficiency rises with
an increase in Xenon fill pressure,
whereas the ability to trigger the
lamp decreases inversely.
Discharge sequence
Flashtubes are connected with two
different electronic circuits:
1. The trigger circuit: operates in
the trigger phase.
2. The main discharge circuit:
operates in the main discharge
phase.
V
VZ
arrows:
progress of time
trigger
phase
V0
VS
main
discharge
VR
imax
i
The impedance characteristic of the flash
discharge
2
The impedance characteristic –
anode voltage (V) versus discharge
current (i) – features the same form
for all flash discharges. Its slope
represents the characteristic of the
discharge.
First, the trigger voltage Vz (typically 2 to 20 kV) causes an ionization
in the tube. This requires energy
(typically 1 to 100 mJ) and time
(typical trigger delay is 1 to 10 µs).
The main discharge can be subdivided into the current rise and the
current decay. During the decay
phase, under normal conditions,
most of the light output is generated. The following internal impedance of the tube Ri can be defined
as:
Main discharge circuits
Discharge control
by semiconductor
Free capacitor discharge
An IGBT offers high peak current
and high frequency switching with
very low loss and a simple driving
circuit. This concept is ideal for
discharge control of flashtubes. The
IGBT can also operate the lamp’s
trigger circuit. All pulse patterns,
preflashes and any manipulations
of the mainflash are possible.
The energy E [J] stored in the flash
capacitor CB [F] at an operating
voltage V0 [V] is defined as:
flash energy E = 1/2 CB V0 2
A low percentage of residual
energy in CB is neglected when the
discharge extinguishes.
+
V0
V0
CB
Ri
CB
IGBT
trigger circuit
Ri = VS / i max
(typical 0.1 to 5 Ohms)
i
i
Finally, the discharge extinguishes
at a residual voltage of VR (typically
10 to 100 V).
imax
t
t
IGBT charge control
τ
Free capacitor discharge
After the peak current imax has been
reached, an almost exponential
discharge of CB takes place, since
the internal resistance Ri of the tube
remains constant. The time constant
τ = Ri CB is a measure for the flash
duration (~ 1/3 value).
In stroboscopic applications, the
tube’s medium power load P [W]
results from the energy E [J] of the
individual pulse and the repetition
rate f [Hz]
P=E*f
w w w. o p t o e l e c t ro n i c s . p e r k i n e l m e r. c o m
3
Simmer operation
Trigger circuits
1. Capacitive external triggering
The constant current source (S)
maintains a simmer current in the
lamp. When the semiconductor (T)
is operated, any pulse discharge
pattern can be superimposed to the
simmer current without an additional high voltage trigger impulse.
The simmer current should be
rather small, but must be adapted to
the lamp’s requirements.
The flashtube discharge is initiated
by means of a high voltage pulse
Vz (trigger voltage) which must be
higher than the static breakdown
voltage of the tube.
This is the simplest form of triggering. The trigger electrode of the tube
is insulated from anode to cathode
and extends over the entire arc
length.
T
V0
iS
VS
CB
S
i
t
Simmer operation
Typical Vz values range between
2 kV and 20 kV. The difference between Vz and V0 must be sufficient
to avoid spontaneous triggering.
Vz is generated by a pulse transformer (trigger coil). Typical transformation ratios are 1:20 to 1:100.
A semiconductor or mechanical
switch discharges a trigger capacitor
Cz via the primary side of the trigger
coil. On the secondary side a
damped high voltage oscillation is
produced. The oscillation frequency, amplitude and damping depend
strongly on the trigger coil and the
external circuitry.
+
R
CZ
VZ
V0
–
CB
T
VZ
5 kV
1µs
t
Capacitive external triggering
Since the capacity of the trigger
electrode is in the range of some pF
against the cathode and anode, the
secondary side of the trigger coil
can be highly resistive, resulting in
a compact construction of the coil.
The polarity of the first half wave of
the high voltage trigger oscillation
can influence the ability to trigger.
Advantages of external capacitive
triggering are:
• Low primary and secondary
currents.
• Small size, low cost components.
One disadvantage of the external
triggering is the production of electromagnetic interference, especially
in the case of long wires within the
trigger circuit.
4
2. Direct series triggering
The secondary winding of the
trigger coil is either on the anode
or cathode side, in series with the
lamp, and conducts the entire
discharge current.
+
R
CZ
V0
H
VZ
CB
T
–
3. Trigger with doubling of anode
voltage
Generation of light by
Xenon discharge
This circuit considerably improves
the triggering ability of flashtubes
with high gas fill pressure. When
firing the thyristor T, the discharge
of CZ produces the usual high
voltage trigger oscillation in the
secondary trigger coil.
Simultaneously, Cinv is discharged,
bringing down the cathode potential of the flashtube to –V0 for a few
microseconds. Ideally, this doubles
the effective anode voltage for
triggering.
Within the group of discharge
lamps, flashtubes fall into the
category of high-pressure arc
lamps. Xenon fill pressures of 50 to
3000 Torr and current densities
between 100 and 10000 A/cm2
provide all the characteristics of an
arc discharge :
1µs
+V0
CZ
V0
–V0
R
Cinv
t
–
• Xenon flashtubes have the
highest luminance of all light
sources, apart from lasers.
D
T
5 kV
Trigger with doubling of anode voltage
4. Booster circuit
Direct series triggering
There is no need of an isolated trigger electrode extending over the arc
length. In special cases, an optional
trigger electrode can be connected
with the anode or cathode.
In comparison to the capacitive
external triggering, the advantage of
direct series triggering is shorter
trigger delays and lower emission of
electromagnetic interferences. Some
disadvantages of direct series triggering include the larger size and
higher cost of components.
PerkinElmer trigger transformers
for external and series triggering are
included in this application note.
The booster is an auxiliary anode
voltage VH > V0 which is applied to
a capacitor CH << CB and is blocked
against V0 by a diode D. The effective anode voltage for triggering is
VH. This circuit is ideal in cases of
high variations in the operating
anode voltage or when the fill pressure is high.
+
R
V0
–
D
CZ
CB
CH
T
• A continuum radiation, very
similar to sunlight.
• Among the noble gases Xenon
has the highest photometric
radiation efficiency – approximately 40 lm/W.
VZ
+
• Plasma temperatures between
7000 to 10000 K.
VH
Capacitive triggering with additional booster
In flashtubes, the Xenon spectrum
consists of a continuum – the
distribution to the radiation of a
black body and a characteristic line
spectrum – that mainly contributes
to the infrared region between 880
to 1000 nm.
The density of the discharge current
specifically influences the relation
between lines and continuum. By
determining this current density,
the portions of UV and IR radiation
added to the visible light can be
selected. It is not possible to emit
“colored light“ only. This applies
for other inert gases as well.
Current densities in the range of
several hundred A/cm2 can only be
achieved in tubes with high internal
impedance. In this case, the portion
of IR radiation is strongly predominant.
w w w. o p t o e l e c t ro n i c s . p e r k i n e l m e r. c o m
5
100 A / cm
2
relative
intensity
Current densities between 4000–
10000 A/cm2 feature minimal IR
lines. The continuum is now very
similar to sunlight (7000 K). These
tubes provide the highest visual
radiation efficiency (up to 40 lm/W),
particularly in photo flashtubes.
Envelope materials
Flashtubes are divided into two
groups, classified by power:
• Hard glass tubes.
• Quartz glass tubes (withstand
three to ten times more power
than hard glass tubes).
6000 A / cm2
100 300 500 700 900 1000
wavelength (nm)
Spectrum of a high impedance flashtube
Current densities of 1000–3000
A/cm2, used in most studio, beacon
and stroboscopic flashtubes, feature
IR as well as a strong continuum.
UV radiation can also be considerable.
1. Hard glass
relative
intensity
black body
radiation
Borosilicate glass B1 (standard glass):
100 300 500 700 900 1100
wavelength (nm)
Typical spectrum of a photo flashtube
2000 A / cm
There are four borosilicate glasses,
selected and characterized by their
exceptional resistance to the arc
and by optical quality.
• Automatic processing ability.
• Many tubes diameters available.
Borosilicate glass B2:
2
black body radiation
relative
intensity
The large portion of UV and blue
in the spectrum of these tubes can
interfere with photographic applications, but can be corrected
through filtering.
• Withstands approx. 30% more
power than B1.
• Requires manual processing.
Borosilicate glass B3:
• Extra transparency for UV radiation.
• Automatic processing ability,
similar to B1.
100 300 500 700 900 1000
wavelength (nm)
Spectrum of a studio flashtube
Borosilicate glass B4:
relative
intensity
• Automatic processing ability.
100 300 500 700 900 1000
wavelength (nm)
Spectrum of a short arc flashtube
Short arc lamps with very hot as
well as very cold plasma zones due
to the missing wall stabilization can
have IR lines, UV lines and a
distinctive continuum.
6
transparency (%)
• Allows up to double flash energy
in photoflash applications
compared to B1.
100
80
60
40
20
B3
B1 + B4
B2
0
0.2 0.3
1
2
Transparency of borosilicate glasses
3
5
2. Quartz glasses
Quartz glass gains its unique
resistance to arc and thermal shock
from the high bonding energy of
pure SiO2 and the negligible small
expansion of 4 x 10-7/K. Quartz
glass usually requires manual
processing.
Quartz glass Q1:
• UV transparent.
Quartz glass Q2:
• Reduced generation of ozone.
Quartz glass Q3:
• No generation of ozone.
Quartz glass Q1, Q2 and Q3 differs
mainly in their UV transparency.
transparency (%)
Synthetic quartz glass is available
on request.
Color corrective coatings
and colored lamps
Hard glass and quartz glass can be
coated with a yellow layer that
absorbs the excessive blue radiation
for film exposure. The color temperature is lowered by 1000 to 2000 K.
For special applications, flashtubes
can be colored with uniform and
crack-free colored layers. Typical
colors are red, blue, amber, green,
and purple.
Life expectancy
Flashtubes age in terms of light
output reduction and decreased
ability to trigger the lamp.
Statements on the life of a specific
flashtube require exact knowledge
of the following operating conditions:
• flash energy,
• anode voltage,
• flash frequency,
• flash capacitor and its effective
series resistance,
• resistors and inductances in the
discharge circuit,
• reflector,
• cooling conditions,
• criteria defining the end of life.
All details of the life expectancy
mentioned in the catalog refer to
nominal operating conditions that
are described in the individual
specifications.
100
Q1
80
60
Q2
Q3
40
20
0
150 200 250 300 350 400
wavelength (nm)
thickness 1mm
The flashtube drawings shown on
the following pages are schematic
sketches only. Further details, as
well as tolerances of the mechanical
dimensions, are provided in our
data sheets, available on request.
Transparency of quartz glasses
w w w. o p t o e l e c t ro n i c s . p e r k i n e l m e r. c o m
7
Your Partner of Choice
PerkinElmer Optoelectronics
With a broad customer base in all major markets, built on ninety years of solid trust and
cooperation with our customers, PerkinElmer is
recognized as a reliable partner that delivers
high quantity, customized, and superior “onestop” solutions. Our products – from lamps to
trigger transformers, reflectors, power supplies,
and more – meet the highest qualitative and
environmental standards. Our worldwide Centers
of Excellence along with our Customer and
Technical Support teams always work with you
to find the best solutions for your specific needs.
PerkinElmer Optoelectronics is a global technology leader providing market-driven, integrated
solutions for a wide range of applications,
which leverage our lighting, sensors, and imaging
expertise. Our technologies, services and support
are fueling the medical, genomic and digital
revolutions by enhancing our customers’ productivity, optimizing performance, and accelerating
time-to-market.
European Headquarters
PerkinElmer Optoelectronics
Wenzel-Jaksch-Str. 31
65199 Wiesbaden, Germany
Telephone: (+49) 611-492-269
Fax: (+49) 611-492-132
Email: [email protected]
Asia Headquarters
PerkinElmer Optoelectronics
47 Ayer Rajah Crescent #06-12
Singapore 139947
Telephone: (+65) 67704-306
Fax: (+65) 67751-008
Email: [email protected]
So contact us and put PerkinElmer’s expertise to
work in your demanding lighting applications.
Let us show you how our innovations will help
you deliver the perfect product.
Worldwide Headquarters
PerkinElmer Optoelectronics
44370 Christy Street
Fremont, CA 94538-3180
Telephone: +1 510-979-6500
Toll free: (North America) +1 800-775-OPTO (6786)
Fax: +1 510-687-1140
Email: [email protected]
www.optoelectronics.perkinelmer.com
For a complete listing of our global offices, visit www.optoelectronics.perkinelmer.com
©2004 PerkinElmer, Inc. All rights reserved. The PerkinElmer logo and design are registered trademarks of PerkinElmer, Inc. All other trademarks not owned by PerkinElmer, Inc. or its
subsidiaries that are depicted herein are the property of their respective owners. PerkinElmer reserves the right to change this document at any time without notice and disclaims liability
for editorial, pictorial or typographical errors.
600002_01 APP0904P
Printed in Germany