
47-6 Optical
shall be backed by a black diusing material having a reection density not less than 1.5. is procedure
is specied to improve reproducibility, especially for thin samples.
It wi
ll
be ob
vious
th
at
a re
ection
de
nsitometer
ca
nnot
be “ze
roed”
by ta
king
a re
ading
wi
th
no
sa
mple
in pl
ace
. In
stead,
as pr
escribed
by th
e
st
andard
[1
d],
ca
libration
sa
mples
th
at
ha
ve
be
en
me
a-
sured
on ot
her
pr
imary
in
struments
ar
e
us
ed
bo
th
to ze
ro
an
d
sl
ope
th
e
de
nsitometer
. Fo
rmerly,
co
atings
of Mg
O
2
or BaSO
4
were used as reference “whites,” but these layers are dicult to use. An
ASTM standard [10] discusses the preparation of reference whites from pre ...