High Performance Flash! by Excelitas

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1 High Performance Flash! by Excelias Fundamenals Elecronic flashubes are gas discharge devices for pulse operaion. Xenon is used as a fill gas for mos applicaions. For laser exciaion Krypon is also used. Modes of Operaion Single flash: Random reques for single flashes. In comparison o he flash duraion (normally beween 10 µs and 10 ms), he off imes are very long, ypically a few seconds. Sroboscopic and beacon operaion: The discharges are produced periodically and ofen over a longer period of ime. Typical sroboscopic frequencies are beween 1 Hz and several khz. Higher frequencies require special flashubes and circuis. Coninuous wave discharge: Elecronic flashubes are unsuied for his mode of operaion. Auhors Ingo Dünisch Peer Srzelczyk Dr. Rainer Heise Excelias Wenzel-Jaksch-Sr Wiesbaden Germany

2 Configuraion All flashubes consis of a ube made of hard glass or quarz glass wih sealed-in elecrodes (anode / cahode) a each end. In addiion o linear glass ubes, various oher shapes, such as U, ring, or helix shape are also available. The cahode conains emier subsances o reduce he elecron work funcion. When connecing flashubes, polariy mus be observed. Non-polarized ubes wih wo cahodes are also available. Many flashubes are equipped wih a capaciive rigger elecrode -- for example, a wire wrapped around he ube, a silver sripe or a ransparen conducive coaing on he ouside of he glass. Leads on elecrodes can be supplied wih solid or flexible lead wires, according o Excelias sandards or cusomer specificaions. Lamps can also be supplied wih sockes, end caps and proecive covers. Flashubes are ligh sources wih high power densiy. Therefore, hey are only made of high emperaure resisive maerials. Discharge ube characerisics Apar from he glass maerial and elecrodes, flashubes feaure hree key characerisics: 1. The arc lengh (e) 2. The inner ube diameer (r) 3. The Xenon (or Krypon) fill pressure (p) inner diameer r cahode rigger elecrode anode - arc lengh e Schemaic drawing of a flash ube By deermining e, r, and p, all condiions and requiremens such as flash energy, expeced life, rigger qualiy, size, opical projecion and specral disribuion of he ligh mus be fulfilled. Tubes characerized by low e/r raios (ypically e/r < 5) are called elecrode sabilized and used for shor pulses and high luminance. The plasma is mainly guided by he anode and he cahode ips - no ouching he glass. For ubes characerized by higher e/r raios, wo ypical flash discharges have o be disinguished: The plasma fills he cross secion of he ube compleely, limiing he peak curren. The discharge is referred o as wall sabilized. Mos flashubes are designed for 5 < e/r < 20, as his range obains he bes ligh efficiency. Typically, for sroboscopic applicaions he plasma does no fill he ube cross-secion compleely, bu is guided along he glass envelope. The flash capacior or an elecronic circui limis he peak curren. The luminous efficiency rises wih an increase in Xenon fill pressure, whereas he abiliy o rigger he lamp decreases inversely. Discharge sequence Flashubes are conneced wih wo differen elecronic circuis: 1. The rigger circui: operaes in he rigger phase. 2. The main discharge circui: operaes in he main discharge phase. V rigger phase VZ arrows: progress of ime VS main discharge VR imax i The impedance characerisic of he flash discharge 2

3 The impedance characerisic - anode volage (V) versus discharge curren (i) - feaures he same form for all flash discharges. Is slope represens he characerisic of he discharge. Firs, he rigger volage V z (ypically 2 o 20 kv) causes an ionizaion in he ube. This requires energy (ypically 1 o 100 mj) and ime (ypical rigger delay is 1 o 10 µs). Main discharge circuis Free capacior discharge The energy E [J] sored in he flash capacior C B [F] a an operaing volage V 0 [V] is defined as: flash energy E = 1/ 2 C B V 0 2 A low percenage of residual energy in C B is negleced when he discharge exinguishes. Discharge conrol by semiconducor An IGBT offers high peak curren and high frequency swiching wih very low loss and a simple driving circui. This concep is ideal for discharge conrol of flashubes. The IGBT can also operae he lamp s rigger circui. All pulse paerns, preflashes and any manipulaions of he mainflash are possible. The main discharge can be subdivided ino he curren rise and he curren decay. During he decay phase, under normal condiions, mos of he ligh oupu is generaed. The following inernal impedance of he ube R i can be defined as: CB Ri CB IGBT rigger circui R i = V S / i max (ypical 0.1 o 5 Ohms) Finally, he discharge exinguishes a a residual volage of V R (ypically 10 o 100 V). i imax i IGBT charge conrol τ Free capacior discharge Afer he peak curren i max has been reached, an almos exponenial discharge of C B akes place, since he inernal resisance R i of he ube remains consan. The ime consan τ = R i C B is a measure for he flash duraion (~ 1/3 value). In sroboscopic applicaions, he ube s medium power load P [W] resuls from he energy E [J] of he individual pulse and he repeiion rae f [Hz] P = E * f 3

4 Simmer operaion The consan curren source (S) mainains a simmer curren in he lamp. When he semiconducor (T) is operaed, any pulse discharge paern can be superimposed o he simmer curren wihou an addiional high volage rigger impulse. The simmer curren should be raher small, bu mus be adaped o he lamp s requiremens. T CB VS i Simmer operaion is S Trigger circuis The flashube discharge is iniiaed by means of a high volage pulse Vz (rigger volage) which mus be higher han he saic breakdown volage of he ube. Typical V z values range beween 2 kv and 20 kv. The difference beween V z and V 0 mus be sufficien o avoid sponaneous riggering. is generaed by a pulse ransfor- Vz mer (rigger coil). Typical ransformaion raios are 1:20 o 1:100. A semiconducor or mechanical swich discharges a rigger capacior C z via he primary side of he rigger coil. On he secondary side a damped high volage oscillaion is produced. The oscillaion frequency, ampliude and damping depend srongly on he rigger coil and he exernal circuiry. 1. Capaciive exernal riggering This is he simples form of riggering. The rigger elecrode of he ube is insulaed from anode o cahode and exends over he enire arc lengh. VZ - T R 5 kv 1µs Capaciive exernal riggering CZ VZ CB Since he capaciy of he rigger elecrode is in he range of some pf agains he cahode and anode, he secondary side of he rigger coil can be highly resisive, resuling in a compac consrucion of he coil. The polariy of he firs half wave of he high volage rigger oscillaion can influence he abiliy o rigger. Advanages of exernal capaciive riggering are: Low primary and secondary currens. Small size, low cos componens. One disadvanage of he exernal riggering is he producion of elecromagneic inerference, especially in he case of long wires wihin he rigger circui. 4

5 2. Direc series riggering The secondary winding of he rigger coil is eiher on he anode or cahode side, in series wih he lamp, and conducs he enire discharge curren. VZ 5 kv R CZ H VZ CB - T 1µs Direc series riggering There is no need of an isolaed rigger elecrode exending over he arc lengh. In special cases, an opional rigger elecrode can be conneced wih he anode or cahode. In comparison o he capaciive exernal riggering, he advanage of direc series riggering is shorer rigger delays and lower emission of elecromagneic inerferences. Some disadvanages of direc series riggering include he larger size and higher cos of componens. Excelias rigger ransformers for exernal and series riggering are included in his applicaion noe. 3. Trigger wih doubling of anode volage This circui considerably improves he riggering abiliy of flashubes wih high gas fill pressure. When firing he hyrisor T, he discharge of C Z produces he usual high volage rigger oscillaion in he secondary rigger coil. Simulaneously, C inv is discharged, bringing down he cahode poenial of he flashube o -V 0 for a few microseconds. Ideally, his doubles he effecive anode volage for riggering. CZ - T Trigger wih doubling of anode volage 4. Booser circui Generaion of ligh by Xenon discharge Wihin he group of discharge lamps, flashubes fall ino he caegory of high-pressure arc lamps. Xenon fill pressures of 50 o 3000 Torr and curren densiies beween 100 and A/cm 2 provide all he characerisics of an arc discharge : Plasma emperaures beween 7000 o K. A coninuum radiaion, very similar o sunligh. Among he noble gases Xenon has he highes phoomeric radiaion efficiency - approxi- R - maely 40 lm/w. Cinv D Xenon flashubes have he highes luminance of all ligh sources, apar from lasers. The booser is an auxiliary anode volage V H > V 0 which is applied o a capacior C H << C B and is blocked agains V 0 by a diode D. The effecive anode volage for riggering is V H. This circui is ideal in cases of high variaions in he operaing anode volage or when he fill pressure is high. - T R In flashubes, he Xenon specrum consiss of a coninuum - he disribuion o he radiaion of a black body and a characerisic line specrum - ha mainly conribues o he infrared region beween 880 o 1000 nm. The densiy of he discharge curren specifically influences he relaion beween lines and coninuum. By deermining his curren densiy, he porions of UV and IR radiaion added o he visible ligh can be D seleced. I is no possible o emi CZ CB colored ligh only. This applies CH VH Capaciive riggering wih addiional booser for oher iner gases as well. Curren densiies in he range of several hundred A/cm 2 can only be achieved in ubes wih high inernal impedance. In his case, he porion of IR radiaion is srongly predominan. 5

6 relaive inensiy 100 A / cm wavelengh (nm) Specrum of a high impedance flashube Curren densiies of A/cm 2, used in mos sudio, beacon and sroboscopic flashubes, feaure IR as well as a srong coninuum. UV radiaion can also be considerable. relaive inensiy 2000 A / cm 2 black body radiaion wavelengh (nm) Specrum of a sudio flashube Curren densiies beween A/cm 2 feaure minimal IR lines. The coninuum is now very similar o sunligh (7000 K). These ubes provide he highes visual radiaion efficiency (up o 40 lm/w), paricularly in phoo flashubes. relaive inensiy 6000 A / cm 2 black body radiaion wavelengh (nm) Typical specrum of a phoo flashube The large porion of UV and blue in he specrum of hese ubes can inerfere wih phoographic applicaions, bu can be correced hrough filering. relaive inensiy Envelope maerials Flashubes are divided ino wo groups, classified by power: Hard glass ubes. Quarz glass ubes (wihsand hree o en imes more power han hard glass ubes). 1. Hard glass There are four borosilicae glasses, seleced and characerized by heir excepional resisance o he arc and by opical qualiy. Borosilicae glass B1 (sandard glass): Auomaic processing abiliy. Many ubes diameers available. Borosilicae glass B2: Wihsands approx. 30% more power han B1. Requires manual processing. Borosilicae glass B3: Exra ransparency for UV radiaion. Auomaic processing abiliy, similar o B1. Borosilicae glass B4: Auomaic processing abiliy. Allows up o double flash energy in phooflash applicaions compared o B wavelengh (nm) Specrum of a shor arc flashube Shor arc lamps wih very ho as well as very cold plasma zones due o he missing wall sabilizaion can have IR lines, UV lines and a disincive coninuum. ransparency (%) 100 B B1 B B Transparency of borosilicae glasses 6

7 2. Quarz glasses Quarz glass gains is unique resisance o arc and hermal shock from he high bonding energy of pure SiO 2 and he negligible small expansion of 4 x 10-7 /K. Quarz glass usually requires manual processing. Quarz glass Q1: UV ransparen. Quarz glass Q2: Reduced generaion of ozone. Quarz glass Q3: No generaion of ozone. Quarz glass Q1, Q2 and Q3 differs mainly in heir UV ransparency. Synheic quarz glass is available on reques. ransparency (%) Q Q2 Q wavelengh (nm) hickness 1mm Color correcive coaings and colored lamps Hard glass and quarz glass can be coaed wih a yellow layer ha absorbs he excessive blue radiaion for film exposure. The color emperaure is lowered by 1000 o 2000 K. For special applicaions, flashubes can be colored wih uniform and crack-free colored layers. Typical colors are red, blue, amber, green, and purple. Life expecancy Flashubes age in erms of ligh oupu reducion and decreased abiliy o rigger he lamp. Saemens on he life of a specific flashube require exac knowledge of he following operaing condiions: flash energy, anode volage, flash frequency, flash capacior and is effecive series resisance, resisors and inducances in he discharge circui, reflecor, cooling condiions, crieria defining he end of life. All deails of he life expecancy menioned in he caalog refer o nominal operaing condiions ha are described in he individual specificaions. The flashube drawings shown on he following pages are schemaic skeches only. Furher deails, as well as olerances of he mechanical dimensions, are provided in our daa shees, available on reques. Transparency of quarz glasses 7

8 Your Parner of Choice Wih a broad cusomer base in all major markes, buil on niney years of solid rus and cooperaion wih our cusomers, Excelias is recognized as a reliable parner ha delivers high quaniy, cusomized, and superior onesop soluions. Our producs - from lamps o rigger ransformers, reflecors, power supplies, and more - mee he highes qualiaive and environmenal sandards. Our worldwide Ceners of Excellence along wih our Cusomer and Technical Suppor eams always work wih you o find he bes soluions for your specific needs. Excelias Excelias is a global echnology leader providing marke-driven, inegraed soluions for a wide range of applicaions, which leverage our lighing, sensors, and imaging experise. Our echnologies, services and suppor are fueling he medical, genomic and digial revoluions by enhancing our cusomers produciviy, opimizing performance, and acceleraing ime-o-marke. So conac us and pu Excelias s experise o work in your demanding lighing applicaions. Le us show you how our innovaions will help you deliver he perfec produc. European Headquarers Asia Headquarers Excelias Technologies Excelias Technologies Wenzel-Jaksch-Sr Ayer Rajah Crescen # Wiesbaden, Germany Singapore Telephone: (49) Telephone: (65) Fax: (49) Fax: (65) generalinquiries@excelias.com Worldwide Headquarers Excelias Technologies Chrisy Sree Fremon, CA Telephone: Toll free: (Norh America) OPTO (6786) Fax: For a complee lising of our global offices, visi Excelias Technologies Corp. All righs reserved. The Excelias logo and design are regisered rademarks of Excelias Technologies Corp. All oher rademarks no owned by Excelias Technologies or is subsidiaries ha are depiced herein are he propery of heir respecive owners. Excelias reserves he righ o change his documen a any ime wihou noice and disclaims liabiliy for ediorial, picorial or ypographical errors _01 APP0904P