Biomedical Engineering Reference
In-Depth Information
9.5.3 Resolution and Numerical Focus
The resolution of digital holographic microscopy can be characterized by in-
vestigating calibration test charts. The left panel of Fig. 9.16 shows the recon-
structed amplitude (holographic image) of a negative USAF 1951 resolution
test chart (illumination in transmission), recorded using a
40 microscope
lens (NA = 0.6). The magnified image section of group 9.5 (resolution limit
of the test chart) represents a line width of 620 nm and is resolved clearly.
The comparison with the Abbe criterion shows that the lateral resolution
is diffraction limited (in correspondence to bright field microscopy) and can
be increased by using microscope optics with higher numerical aperture [31].
The right panel of Fig. 9.16 demonstrates the axial resolution for incident
light illumination of a reflective metal surface (nano-structured chromium on
chromium surface) recorded with a
×
5 microscope lens (NA = 0.1). The de-
picted elements represent a height of 30 nm and are resolved clearly in the
reconstructed phase distribution. Because of the phase noise the axial resolu-
tion is determined to be
×
5 nm. The digital reconstruction of different object
planes from a single hologram enables a variable (subsequent) numerical fo-
cus of digital holographic images (“digital holographic multi focus”) without
additional mechanical or optical components. Figure 9.17 demonstrates the
digital holographic refocus for the example of a semitransparent USAF1951
test chart (upper panel) and for the holographic image of a living human
pancreas tumor cell of the type Patu8988S (lower panel).
≈
Fig. 9.16.
(
Left
) Reconstructed holographic amplitude of a negative USAF 1951
test chart (digital hologram recorded in transmission mode), the elements of group
9.6 correspond to a lateral resolution of 550 nm; (
b
) Reconstructed phase contrast
image of a reflective phase object (chromium on chromium sample) with steps of
30 nm in axial direction (digital hologram recorded in reflection mode) [28]
