Nikon S-kt - Page 7

3.
I TTlJ]t|I]{ATION
Resolution
and
image
contrast
are
greatly
affected
by the
illumination
method.
(1)
Condenser
lris
Diaphragm
Stop
down
the
condenser
iris
diaphragm
and
slide it in
a radial
direction
from
center
to edge.
The
more
the iris
dia-
phragm
is off-center,
the
higher
the
con-
trast and resolution.
Details
of the object
are
distinguished
by increased
and
unsym-
metrical
shadows
at the
boundaries
(Fig.
7,a,bl.
When
the iris diaphragm
is
positioned
so
as to let
the light bundle enter
the object
at an angle of inc.idence
equal
to the
aperture angle of
the objective,
the
reso-
lution reaches
the maximum
and twice
as
much as the resolution
by central
illumi-
natron.
lf the diaphragm is further
decentered to
such an extent
as
to
prevent
the
light
bundle entering
the objective,
oblique
dark
field
illumination
will
be
obtained
(Fig.
7,
c). lf the iris
diaphragm
is opened
wide, images
by
illumination
at
various
angles
are
obtained.
An illumination
angle
unfavorable for
the object may
be in-
cl
uded.
In
central illumination
maximum resolu-
tion is
obtained when the
opening of the
iris
diaphragm
just
corresponds
to the
aperture angle of the objective.
In this
case
excessive outer rays
as used
for
dark
field
illumination
are cut off
and
flare
is
minimized. lf the
opening
ismade
small-
er,
contrast is enhanced,
although the
resolution
is lowered.
But
if
the iris
diaphragm is large enough
to cover
60-700/0 of the objective
aperture, the
decrease
of
resolution
will not
be
pro-
nounceo.
lf
the diaphragm is
stopped down to the
minimum
for
admitting only very
small
light
bundles, the
effects
of diffraction,
reflection, refraction,
etc., may
be ex-
aggerated
so
that fringes
may be
seen at
the
image
edges which may likely
induce
misinterpretation of
the image, but it
(a)
Central
illumination
(b)
Oblique
illumination
(c)
Dark field illumination
Fig.7