Technical information

5
2 How a metal hal
i
de lam
p
work
s
Contact plate
UV-Filter
Quter bulb
(Quartz)
Molybdenum foil
Arc tube
(Quartz)
Discharge arc
Heat reector
Lead-in wire
Molybdenum foil
Base
Getter
Metal halides
Mercury
Electrode
Fi
g
. 1: An example o
f
how a metal halide lamp works
b
ase
d
on a
d
ou
bl
e-en
d
e
d
l
am
p
w
i
t
h
a
q
uartz arc tu
b
e.
S
imilar to hi
g
h-pressure mercury lamps or hi
g
h-pres
-
sure so
di
um
l
amps, meta
l
h
a
lid
e
l
amps a
l
so
b
e
l
on
g
to
the group of discharge lamps. Low pressure discharge
lam
p
s include
f
luorescent lam
p
s and com
p
act
f
luores-
cent
l
amps
.
I
n
di
sc
h
ar
g
e
l
amps,
ligh
t
i
s
g
enerate
d
b
y a
g
as
di
s-
char
g
e o
f
particles created between two hermetically
sealed electrodes in an arc tube. A
f
ter ignition, the
part
i
c
l
es
i
n t
h
e arc are part
i
a
ll
y
i
on
i
ze
d
, ma
ki
n
g
t
h
em
e
l
ectr
i
ca
lly
con
d
uct
i
ve, an
d
a
“
p
l
asma
”
i
s create
d
.
I
n
high
i
ntens
i
ty
di
sc
h
ar
g
e
l
amps, t
h
e arc tu
b
e
i
s usua
ll
y
e
n
closed
i
n
a
n
e
v
acua
t
ed
ou
t
e
r
bulb
w
hich
isola
t
es
t
he
hot arc tube thermally
f
rom the surroundings, simila
r
to the
p
rinci
p
le o
f
a thermos
f
lask. But there are also
some
di
sc
h
ar
g
e
l
amps w
i
t
h
out outer
b
u
lb
s, as we
ll
as
lamps with
g
as-
f
illed outer bulbs. In contrast to low
-
pressure
di
sc
h
ar
g
e, t
h
ere
i
s
high
pressure an
d
a
high
temperature
i
n a
di
sc
h
ar
g
e tu
b
e.
I
n an arc tu
b
e,
g
as
di
sc
h
ar
g
e wor
k
s t
h
rou
gh
exc
i
tat
i
on
of the luminous additives
(
metal halide salts
)
and the
mercur
y
is excited b
y
the current
f
low. Visible radia
-
tion characteristic
f
or the res
p
ective elements is emit
-
t
ed
. Th
e
mixt
u
r
e
of
th
e
vi
s
i
b
l
e
r
ad
i
a
ti
o
n
of
th
e
d
i
ffe
r
e
nt
e
l
ements resu
l
ts
i
n t
h
e
d
es
ig
ne
d
co
l
our temperature
Outer bulb
Hg = Mercury
Me = Metals
Hal = Halids
Arc tube
Electrodes
Mo-Foils
supply voltage
Alternative: ECG
ignitor
ballast
Fi
g
. 2: Tasks of the metals [sodium (Na), thallium (TI), indium (In), tin (Sn), lithium (Li), rare earths; d
y
sprosium (D
y
),
h
olmium
(
Ho
)
, thulium
(
Tm
)]
and colour renderin
g
f
or a particular lamp. In the op
-
erat
i
n
g
state, t
h
e mercury evaporates comp
l
ete
l
y.
Th
e
other elements involved are present in saturated
f
orm
at t
h
e
gi
ven temperatures,
i
.e. t
h
ey on
l
y evaporate
i
n
p
art; the rest is in li
q
uid
f
orm at the coolest
p
oint in
the arc tube. The
f
raction o
f
the
f
illin
g
that has evapo-
rated depends on the temperature o
f
the coolest point
o
n th
e
a
r
c
t
ube
w
a
ll
a
n
d
a
l
so
v
a
ri
es
fo
r th
e
d
i
ffe
r
e
nt
fillin
g
components.
C
han
g
es to the temperature of the
arc tube wall can chan
g
e the composition o
f
the metal
halides in the dischar
g
e, thus also chan
g
in
g
the colour
p
ro
p
erties o
f
the lam
p
.