th e a n gle fro m th e sourc e is‘‘ exte n d e d,’’ su c h asin th e m e a sure-
m e nt ofa fluoresc e nt la m p a t dist a n c esc o m p ara ble to orshort er
th a n itsle n gth.In th e la tt erc ase,th e ext e nd e d sourc e pro vid esra di-
a tion fro m m a ny a n gles, all ofw hic h m ust b e pro p erly ‘‘w eig ht e d ’’
asto th eir eff e ctiv e n ess on th e pla n e re prese nte d b y th e se nsor
surfa c e .
In a ctu alpra ctic e,it isdiffic ult t o m a k e a se nsorc o nform to th e id e a l
resp onse ov erth e e ntire solid a n gle of2πstera dia ns. Th e se nsor
h e a dspro vid e d with th e D MXA-
S
eriesa n d D MHA-
S
eriesra dio m e ters
minimize thispro ble m by using a diffusin g m a t eri al(sa p phire) h a vin g
exc elle nt tra nsmission a t th e v arioussp e ctralre gions, whic h ism ount-
e d close to th e surfa c e ofth e se nsorso th a t th e o bliq u e ra ysare not
o bstru c te d.Th e sp e c tralra ng e issele ct e d by providin g a n a p pro pri-
a t e o p tic alint erfere n c e filt erb efore th e se nsorc ell asse m bly.
5.1.2THESENSORCELL
Photo volt a ic O p era tion
Wh e n a p-n jun ction iso p era t e d with no ext ern ally a p plie d volt a g e ,
it isc onsid ere d to b e o p era tin g in th e p hoto volt a ic m o d e .Un d erthis
zero a p plie d volt a g e c o n ditio n a n d lo w le v elsofin cid e nt lig ht,th e
p-n jun ction will g e n era t e a c urre nt pro p ortion alto th e lig ht p o w er
in cid e nt o n th e a c tiv e surfa c e.Thisp hoto n in d u c e d c urre nt,or
p hoto-c urre nt,will divid e b e tw e e n th e dio d e p aralleld yn a mic resis-
t a n c e a nd th e p arallello a d resist a n c e .Th e dyn a mic resist a n c e is
norm ally a hig h v alu e a n d isa n inv erse exp o n e ntialfun ction offor-
w ard volt a g e .Th e dire c tion ofc urre nt flo w will pro d u c e a volt a g e
a cross th e dio d e th a t o p p osesth e b a n d-g a p p o te ntialofth e p ho-
to dio d e jun c tion,thusforw ard biasin g it.Asa result,th e v a lu e ofth e
dio d e dyn a mic resist a n c e (
R
d) dro psexp on e ntially asth e irra dia n c e
in cre asesa n d th e p hoto g e n era te d volt a g e isa qu asi-lo g arith mic
fun c tion ofdio d e irra dia n c e w h e n th e ext ern allo a d resist a n c e is
c onsid ere d.A n oth erm ajordisa d v a nt a g e isth a t
R
dty pic ally h asa
wid e spre a d ofv alu eso v erdiff ere nt pro d u ctio n b a t c h es.
O n e w a y ofa c hie vin g sufficie ntly lo w lo a d resist a nc e a nd elimin a t-
in g th e e ff e ct ofth e dio d e p arallelresist a n c e isto fe e d th e ph o-
to c urre nt into th e virtu algro un d ofa n o p era tion ala m plifier.
Th e o ut put v olt a g e isth e result ofth e p h oto c urre nt b eing driv e n b y
th e a m plifierthrou gh th e fe e d b a ck resistora n d th e in p ut im p e d a n c e
R
in=
R
f/ A w h ere A isth e o p e n lo o p g a in a n d
R
fth e fe e d b a c k
resistor.
Thiscirc uit h asa lin e arresp onse a n d islo w noise d u e to th e alm ost
c o m plete elimin a tion ofle a k a g e c urre nt with th e zero bias. This
resultsin a pro p ortion alvolt a g e b eing prese nte d to th e sign alc o n di-
tionin g se ction ofth e ele ctro nics.
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