A fiber array polar nephelometer of advanced design, the FAN I is capable of in-situ phase function measurements of scattered light from man-made or natural atmospheric particles. The scattered light is measured at 100 different angles throughout 360 degrees, thus providing a potential measurement of the asymmetry of irregularly shaped particles. Phase functions can be measured at 10 to 100 Hz rates and the range of measurable single particle sizes is from 5 micron m to as large as 8mm. For particles smaller than 5 micro m the ensemble average can be measured. The FAN I is microprocessor controlled and the data may be stored on floppy disk or printed out in tabular and/or graphical form. The optical head may be separated from the computer system for operation in field or adverse conditions. Examples of laboratory measured scattering phase functions obtained with the FAN I for spherical particles is given to illustrate its measurement capabilities

Burlbaw, E. J.

Deepak, A.

Farmer, W. M.

English

NASA Technical Reports Server

PARTICLE PHASE FUNCTION MEASUREMENTS BY A 
NEW FIBER ARRAY NEPHELOMETER: FAN I 
* 
W .  M ,  Farmer, Edward J, Eurlbaw, and A. Deepak 
Science and Technology Corporation 
Lae Crucee, New Mexico 
* Science and Technology Corporation, Hampton, Virginia 
i 'LLJEWNG PAGE BLANK NOT FUMED 
https://ntrs.nasa.gov/search.jsp?R=19860001989 2020-03-20T16:13:16+00:00Z
ABSTRACT 
A f i b e r  a r r a y  p o l a r  nephelometer  o f  advanced d e s i g n ,  t h e  FAN I is 
c a p a b l e  o f  i n - s i t u  phase  f u n c t i o n  measurements o f  s c a t t e r e d  l i g h t  from 
man-made o r  n a t u r a l  a tmosphe r i c  p a r t i c l e s .  The s c a t t e r e d  l i g h t  i s  
measured a t  100 d i f f e r e n t  a n g l e s  t h roughou t  360 d e g r e e s ,  t h u s  p r o v i d i n g  
a p o t e n t i a l  measurement o f  t h e  asymmetry o f  i r r e g u l a r l y  shaped 
p a r t i c l e s .  Phase f u n c t i o n s  c a n  be  measured a t  10 t o  100 Hz rates and 
t h e  r ange  o f  measurable  s i n g l e  p a r t i c l e  s i z e s  is  from Spm t o  as l a r g e  
a s  8mm. For  p a r t i c l e s  smaller t h a n  5pm t h e  ensemble a v e r a g e  c a n  be 
measured. The FAN I is  m i c r o p r o c e s ~ o r  c o n t r o l l e d  and t h e  d a t a  may be 
s t o r e d  on f l o p p y  d i s k  o r  p r i n t e d  o u t  i n  t a b u l a r  a n d / o r  g r a p h i c a l  form. 
The o p t i c a l  head may b e  s e p a r a t e d  from t h e  computer sys tem f o r  
o p e r a t i o n  i n  f i e l d  o r  a d v e r s e  c o n d i t i o n s .  
Fxamples o f  l a b o r a t o r y  measured s c a t t e r i n g  phase  f u n c t i o n s  o b t a i n e d  
w i t h  t h e  FAN I f o r  s p h e r i c a l  p a r t i c l e s  is g i v e n  t o  i l l u s t r a t e  i ts  
measurement c a p a b i l i t i e s ,  
I. INTRCDUCTION 
The FAN I p o l a r  nephelometer  is  a n  p a r t i c l e  
c h a r a c t e r i z a t i o n  system o f  advanced d e s i g n .  It is des igned  t o  measure,  
i n  s i t u ,  l i g h t  s c a t t e r e d  by a tmosphe r i c  p a r t i c l e s  such  as snow c r y s t a l s  
and was i n i t i a l l y  developed  f o r  t h a t  purpose .  The s c a t t e r e d  l i g h t  is 
measured a t  100 d i f f e r e q t  l o c a t i o n s  around a f u l l  360 d e g r e e  s c a t t e r i n g  
d i r e c t i o n  a t  sample r a t 2 s  which can  be s e t  between 10 and 100 Hz. I n  
i ts most s e n s i t i v e  mode o f  o p e r a t i o n ,  t h e  i n s t r u m e n t  c a n  respond t c  
s'rlgle p a r t i c l e s  a s  small a s  5pm' d i a m e t e r  s p h e r e s  o r  i n  i t s  l e a s t  
s e n s i t i v e  mode i t  c a n  respond t o  p a r t i c l e s  having  maximum dimens ions  o f  
approximate ly  8mn. The FAN I is  c o n t r o l l e d  by a mic rop roces so r ,  and 
t h e  a c q u i r e d  d a t a  is  permanent ly  s t o r e d  on f l o p p y  d i s k s .  Hardcopy o f  
t h e  d a t a  is  provided  by a computer p r i n t e r  w i t h  g r a p h l c s  c a p a b i l i t y .  
So f tware  is  s u p p l i e d  w i t h  t h e  sys tem f o r  s e t u p ,  c a l i b r a t i o n .  and d a t a  
d i s p l a y .  The o p t i c a l  head c a n  be  s e p a r a t e d  from t h e  computer sys tem ~y 
30 IU o r  more f o r  o p e r a t i o n  i n  a d v e r s e  envi ronments .  
A d e s c r i p t i o n  o f  t h e  i n s t r u m e n t  is p r e s e n t e d  i n  S e c t i o n  11. I n  S e c t i o n  
IIi i 3  a conpa r i son  o f  measuilements w i t h  t h e o r e t i c a l  c a l c u l a t i o n s  f o r  
g l a s s  beads.  S e c t i o n  I V  c o n t a i n s  t h e  conc lus ion .  
1- 
It. 
- 
11. FAN I DESCRIPTION 
A. Op t i ca l  System 
A schematic o f  t h e  o p t i c a l  head and a s s o c i a t e d  e l e c t r o n i c s  is provided 
i n  Figure  1. A corresponding photograph of  t h e  system wi th  its 
p r o t e c t i v e  covers  removed is  shown i n  , Figure  . 2. A 5mW HeNe l a s e r '  is 
t h e  l i g h t  source  f o r  t h e  ins t rument ,  The system o p e r a t o r  can a d j u s t  
t h e  ou tpu t  beam s i z e  from t h e  l a s e r ' s  nominal 0.8mm beam s i z e  (which 
would be used f o r  maximum ins t rument  s e n s i t i v i t y  and forward s c a t t e r  
angular  r e s o l u t i o n )  t o  e i t h e r  2.4mm o r  8.0mm beam diameter  [ a s  measured 
t o  t h e  exp(-2) i n t e n s i t y  contour]  us ing 3X o r  10X beam expanders which 
screw d i r e c t l y  i n t o  t h e  output  end of t h e  l a s e r .  A f t e r  l e a v i n g  t h e  
beam expander, t h e  output  l a s e r  power is  sampled dur ing  each sampling 
c y c l e  f o r  t h e  phase func t ion .  This  measurement w i l l  a l low t h e  system 
o p e r a t o r  t o  make a b s o l u t e  r ad iomet r i c  s c a t t e r e d  l i g h t  measurements 
should he s o  choose. After  l e a v i n g  t h e  inpu t  beam sampling system, t h e  
l a s e r  beam i s  t ransmi t t ed  a c r o s s  t h e  30.5cm diameter i n  s i t u  sampling 
s e c t i o n .  The beam is then r e f l e c t e d  i n t o  a d e t e c t i o n  system which can 
be used t o  measure t r ansmiss ion  a c r o s s  t h e  sampling s e c t i o n .  S c a t t e r e d  
l i g h t  is  c o l l e c t e d  i n  a p lane  centered about t h e  beam i n  which 100 
g r a d i e n t  index l e n s e s  have been placed i n  a c i r c l e  o f  c o n s t a n t  raa!us. 
The l e n s e s  have f i e l d s  o f  view o f  approximately 1 degree .  Beginning a t  
t h e  t ransmiss ion d e t e c t o r ,  which is def ined as t h e  p o s i t i o n  of  z e r o  
s c a t t e r  ang le ,  and proceeding counterclockwise,  t h e  f i r s t  c o l l e c t i n g  
l e n s  is placed a t  a 1.5 degree  s c a t t e r  angle ,  followed by 11 l e n s e s  
spaced a t  1.5 degree  i n t e r v a l s  up t o  1 8  degrees .  Next, l e n s e s  a r e  
placed a t  5 degree  i n t e r v a l s  from 20 t o  165 degrees  and then  a t  1.5 
degree i n t e r v a l s  from 167.5 t o  176.5 degrees.  Using t h i s  arrangement, 
increased r e s o l u t i o n  is  obta ined i n  t h e  forward and b a c k s c a t t e r  
d i r e c t i o n s .  The l e n s  a r r a y  between 180 and 360 degrees  fo l lows  t h a t  
f o r  0 t o  180 degrees .  (See Figure  3 f o r  forward scatter pos i t ions . )  
Sca t t e red  l i g h t  c o l l e c t e d  by t h e  g r a d i e n t  index l e n s e s  is t r a n s m i t t e d  
t o  t h e  Output Array Block by o p t i c a l  f i b e r s  a s  i n d i c a t e d  i n  F igure  1. 
The s c a t t e r e d  l i g h t  ou tpu t s  from t h e  f i b e r s  a r e  imaged t o  a 1024 
element d e t e c t o r  a r r a y .  Only every t h i r d  d e t e c t o r  i n  t h e  a r r a y  i s  used 
i n  t h e  FAN I. Because t h e  d e t e c t o r  elements must be  p r e c i s e l y  a l igned  
wi th  t h e  elements o f  t h e  output  a r r a y  block, t h e  a r r a y  i s  mount3d on a 
two-dimensional mic ropos i t ioner  t h a t  is  used t o  i n i t i a l l y  a l i g n  t h e  
a r r a y  dur ing  cons t ruc t ion .  The e n t i r e  d e t e c t i o n  system is  housed i n  a 
se l f -conta ined box which is  shock-mounted i n  t h e  main o p t i c s  housing. 
Also included i n  t h e  main housing i s  t h e  ins t rument  c o n t r o l  and s i g n a l  
process ing e l e c t r o n i c s  and a d a t a  b u f f e r  com?uter memory a s  well a s  t h e  
l a s e r  power supply.  Besides making t h i s  p a r t  o f  t h e  FAN I capab le  o f  
opera t ing  i n  a stand-alone mode, t h e  e l e c t r o n i c s  provide  s u f f i c i e n t  
hea t ing  f o r  t h e  o p t i c a l  head dur ing  co ld  weather opera t ion.  
B. S i g n a l  Processing E l e c t r o n i c s  
The s i g n a l  process ing e l e c t r o n i c s  begins  a t  t h e  d e t e c t o r  a r ray .  The 
d e t e c t o r  a r r a y  Ls a two-dimensional s e r i a l l y  scanned o p t i c a l  sensor  
a r r a y  c o n s i s t i n g  o f  1024 s i l i c o n  photodiodes i n  a 32 x 3 2  matr ix .  The 
d e t e c t o r  elements a r e  loca ted  on 100pm c e n t e r s .  The d e t e c t o r  element 
o u t p u t s  a r e  scanned a t  a  nominal r a t e  o f  between 10 and 100 Hz which is 
set by t h e  sys tem o p e r a t o r .  During t h e  time between d e t e c t o r  s c a n s ,  
t h e  o u t p u t s  from t h e  d e t e c t o r s  a r e  c a p a c i t i v e l y  s t o r e d  t h u s  e f f e c t i v e l y  
i n t e g r a t i n g  t h e  s i q n a l  between s c a n s .  The s c a n  ra te  t h e r e f o r e  sets t h e  
i n t e g r a t i o n  time f o r  t h e  d e t e c t o r s .  S i g n a l  i n t e g r a t i o n  is used t o  
a c h i e v e  h i g h  s igna l - to -no i se  r a t i o s  from low l e v e l s  o f  s c a t t e r e d  l i g h t .  
The d e t e c t o r  e l emen t s  have  s o l i d  s ta te  r e ' i a b i l i t y  f o r  rugged f i e l d  
a p p l i c a t i o n s  and t h e  e l emen t s  have  fas t  r ecove ry  times a f t e r  r e a c h i n g  
s a t u r a t i o n .  
The d e t e c t o r  a r r a y  and i n t e g r a t i n g  e l e c t r o n i c s  a r e  c o n t r o l l e d  by a 280 
microprocessor .  The mic rop roces so r  is  used t o  c o n t r o l  t h e  d i o d e  sample 
rate and t h e  ' f rame r a t e '  o r  t h e  r a t e  a t  which t h e  a r r a y  is 
p e r i o d i c a l l y  sampled. S c a t t e r i n g  c o n d i t i o n s  and i n s t r u m e n t  s e t u p  
pa rame te r s  a r e  c o n t r o l l e d  from a computer t e r m i n a l  by t h e  sys tem 
o p e r a t o r .  A s  t h e  d e t e c t o r  s c a n  beg ins ,  t h e  d e t e c t o r  e l emen t s  are 
sampled s e r i a l l y  i n t o  a  12 b i t  ana log  t o  d i g i t a l  c o n v e r t e r .  The f i r s t  
e lement  sampled co r r e sponds  t o  t h e  s c a t t e r  magnitude a t  90 d e g r e e s .  A 
t h r e s h o l d  comparator  c i r c u i t  is  used t o  de t e rmine  i f  s u f f i c i e n t  s i g n a l  
e x i s t s  from a p a r t i c u l a r  s c a n  t o  s a v e  t h e  d a t a .  The s i g n a l  l e v e l  
t h r e s h o l d  is s e t  by t h e  sys tem o p e r a t o r  and can  be  one  o f  256 d i f f e r e n t  
l e v e l s .  If t h e  s i g n a l  o u t p u t  a t  90 d e g r e e s  i s  below t h e  d e s i g n a t e d  
t h r e s h o l d  l e v e l ,  t h e  a r r a y  o u t p u t  is dumped and t h e  d e t e c t o r  a r r a y  i s  
r e s e t  t o  a c q u i r e  d a t a .  When t h e  r e q u i r e d  t h r e s h o l d  is exceeded ,  t h e  
d e t e c t o r  o u t p u t s  a r e  s t o r e d  i n  a b u f f e r  memory con f igu red  on t h e  280 
microprocessor .  If t h e  d a t a  a c q u i s i t i o n  system is  ready  t o  r e c e i v e  
d a t a ,  t h e  b u f f e r  memory immedia te ly  trai~smits t h e  a c q u i r e d  s c a n  t o  t h e  
d a t a  a c q u i s i t i o n  memory which i s  much l a r g e r  t h a n  t h e  Z80 memory. The 
d a t a  a c q u i s i t i o n  memory h a s  been g i v e n  s o f t w a r e  i n s t r u c t i o n s  on how t o  
accumula te  t h e  d a t a  and st'ore t h e  d a t a  i n  f l oppy  d i s k  f i l e s  f o r  p o s t  
experiment  a n a l y s i s .  Should t h e  d a t a  a c q u i s i t i o n  memory b e  busy a t  t h e  
t ime  t h e  b u f f e r  memory h a s  a c q u i r e d  a s c a n ,  t h e  b u f f e r  memory c a n  
a c q u i r e  as many a s  128 s c a n s  o f  d a t a  b e f o r e  i t  becomes f u l l  and must be 
unloaded b e f o r e  d a t a  a c q u i s i t i o n  c a n  con t inue .  The b u f f e r  memory t h u s  
a l l o w s  c o n s i d e r a b l e  f l e x i b i l i t y  i n  how t h e  d a t a  a r e  a c q u i r e d  and 
s t o r e d ,  and a l l o w s  t h e  u s e r  many o p t i o n s  i n  sys tem o p e r a t i o n  t h a t  would 
n o t  be  p o s s i b l e  o the rwi se .  
C. Data A c q u i s i t i o n  System 
The d a t a  ~ c q u i s i t i o n  sys tem s u p p l i e d  w i t h  t h e  FAN I p o l a r  nephelometer  
c o n s i s t s  o f  a computer t e r m i n a l ,  a computer p r i n t e r  w i t h  g r a p h i c s  
c a p a b i l i t y ,  and a microcomputer  having  256K o f  RAM, and two 8 i n c h  
d i ame te r  f l oppy  d i s k  d r i v e s .  The computer memory can  b e  expanded t o  
1Mb. A 12 s l o t  c a r d  cage  i s  provided  w i t h  t h e  computer  t o  a l l o w  e a s e  
o f  memory expans ion  and t h e  i n c l u s i o n  of s p e c i a l i z e d  hardware  shou ld  
t h e  need a r i s e .  ' C '  s o f t w a r e  is used i n  t h e  d a t a  r e d u c t i o n  r o u t i n e s .  
It is  a  language  p a r t i c u l a r l y  s u i t e d  t o  t h e  p o l a r  nephelometer  s i n c e  
one  of i ts major  u s e s  is  i n  t h e  man ipu la t i on  o f  d a t a  a r r a y s .  
Suppl ied  w i t h  t h e  d a t a  a c q u i s i t i o n  sys tem i s  b a s i c  d a t a  r e d u c t i o n  
sof tware .  The b a s i c  f u n c t i o n  o f  t h e  d a t a  r e d u c t i o n  s o f t w a r e  is  t o  
a v e r a g e  t h e  r eco rded  s c a n s  acco rd ing  t o  t h e  u s e r  d e f i n e d  g r o u p i n g s  o r  
ensembles and d i s p l a y  t h e  phase  f u n c t i o n  d a t a  a s  p l o t s  and /o r  listings. 
Cal ib ra t ion  and weighting f a c t o r s  may be app l i ed  t o  t h e  raw d a t a .  
Provis ions  a r e  made f o r  c r e a t i n g  averaged background scans  and 
s u b t r a c t i n g  them from t h e  raw d a t a .  Every scan nay be reduced 
i n d i v i d u a l l y  o r  ensembles can be c r e a t e d  which reduce as much a s  t h e  
e n t i r e  d a t a  f i l e  o n t o  one p l o t .  Ensemble ave.-aging is done on an a n g l e  
by ang le  b a s i s  very  much l i k e  a mul t ichannel  ana lyze r .  I n  p r i n c i p l e ,  
t h e  m u l t i p l e  scan ensamble reduc t ion  r e p r e s e n t s  a second i n t e g r a t i o n  of 
t h e  d a t a .  The f i r s t  i n t e g r a t i o n  occurs  a t  t h e  s i g n a l  d e t e c t o r  an4 
r e p r e s e n t s  t h e  i n t a g r a t i o n  o f  t h e  s c a t t e r e d  l i g h t  as a p a r t i c l e  passes  
through t h e  i n c i d e n t  beam. Th i s  i n t e g r a t i o n  should r e p r e s e n t  a s i n g l e  
o r i e n t a t i o n  o f  an  asymmetric p a r t i c l e .  The second i n t e g r a t i o n  should 
represen t  an  i n t e g r a t i o n  over  numerous p a r t i c l e  o r i e n t a t i o n s  assuming 
t h a t  t h e  p a r t i c l e s  a r e  randomly o r i e n t e d  a s  they  pass  through t h e  
i n c i d e n t  l a s e r  beam, The d a t a  i n  t h e  histogram a r e  d i sp layed  i n  a 
t y p i c a l  phase func t ion  format where r e l a t i v e  histogram magnitudes a r e  
d isplayed a s  a func t ion  o f  a p l e .  With t h e  FAN I, however, i t  must be 
r e c a l l e d  t h a t  t h e  f u l l  360 degree  ~ c a t t e r  a n g l s  is  recorded.  It should 
be expected t h a t  f o r  asymmetric p a r t i c l e s ,  t h e  0 t o  180 phase f u n c t i o n  
w i l l  not  be congruent wi th  t 360 t o  180 degree  phase func t ion .  
Dif ferences  i n  t h e  two phase func t ions  should r e f l e c t  p a r t i c l e  
asymmetry. Therefore,  t h e  two ha lves  o f  t h e  f u l l  360 degree  phase 
func t ion  a r e  d isplayed s e p a r a t e l y  f o r  t h e  u s e r ' s  convenience. 
111. PERFORMANCE 
I n  o rde r  t o  check t h e  performance o f  t h e  FAN I,  s p h e r i c a l  p a r t i c l e s  of  
known r e f r a c t i v e  index and diameter  ( range)  were dropped through t h e  
s c a t t e r i n g  volume. These r e s u l t s  were then compared wi th  Mie 
s c a t t e r i n g  c a l c u l a t i o n  r e s u l t s  f o r  s i n g l e  p a r t i c l e s .  F igures  4 ,  5 ,  and 
6 show t h e  r e s u l t s  f o r  58, 193, and 6 5 0 ~  beads and t h e  t h e o r e t i c a l  
c a l c u l a t i o n s .  The s c a l e  f a c t o r  o f  t h e  c a l c u l a t i o n s  h a s  been a d j u s t e d  
t o  make them co inc ide  wi th  t h e  measurements. The d i f f e r e n c e  i n  
s c a t t e r i n g  between t h e  0 t o  180 and 360 t c  180 measurements can be 
a t t r i b u t e d  t o  t h e  assymetry and imper fec t ions  o f  t h e  beads, w i t h i n  t h e  
l i m i t  o f  t h e  measurement e r r o r s .  Figure  7 shows a s l i g h t l y  d i f f e r e n t  
r ep resen ta t ion  of  t h e  same d a t a  a s  i n  Figure  4 f o r  t h e  58pm beads. 
Here t h e  d a t a  has  been p l o t t e d  wi th  equal  spacing between t h e  d a t a  
po in t s  which emphasizes t h e  forward and backward s c a t t e r i n g  r e s o l u t i o n .  
IV. CONCLUSIONS 
The FAN I is a s ta te-of- the-ar t  ins t rument  capab le  o f  measuring t h e  
r e l a t i v e  s c a t t e r i n g  i n t e n s i t i e s  throughout 360 degrees ,  We have shcwn 
t h e  r e s u l t s  of measurements f o r  s p h z r i c a l  ( o r  n e a r l y  s o )  p a r t i c l e s  and 
have comparedthem t o  M e  c a l c u l a t i o n s  and shown good agreement. 
Figure 1. Schematic of optical arrangement of FAN I nephelometer. 
ORIG!N;i'- ?+?-.:I 13 
OF POOR QUALITY 
\_--. -.--/-- 
Figure 2. Photograph of FAN I nephelometer optical system. 
-2. . 
 re 3. Forward scattering lens positions for FAN I nephelometer. 
oR~~:;q.'/' 7- i-.n.r: :> 
OF POOR QUALITY 


Particle phase function measurements by a new Fiber Array Nephelometer: FAN 1

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