Chapter 10.3.1 - Resolution Limits of Confocal Scanning Laser Imaging Systems

10.3.1   Resolution Limits of Confocal Scanning Laser Imaging Systems

In a confocal scanning laser imaging system, the effective PSF is computed
as the product of the ingoing PSF with the convolution of the outgoing PSF
and the confocal aperture. When the confocal aperture approaches a size
equaling the radius of the Airy disk of the collection path of the system, the
effective PSF is simply the product of the ingoing and outgoing PSFs. Under
such conditions, the resolution is equally determined by the input PSF and
the outgoing PSF, and the lateral resolution can exceed that of conventional
imaging by about 40% [as assessed by the full width at half maximum
(FWHM) of the PSF]. If the confocal aperture is large, then image quality is
governed only by the ingoing PSF; effective optical sectioning disappears and
lateral resolution approaches that of conventional imaging systems.

Axial resolution in confocal SLOs can be defi ned and computed in several
ways. The standard way to determine axial resolution is to measure the
detected intensity that is recorded from a planar surface as a function of its
axial location from the focal plane of the SLO [25]. The full width (axial
distance) at half maximum of the resulting intensity distribution is a measure
of the axial resolution of the SLO. For small confocal pinholes, the intensity
measured from a plane as it is moved through focus (i.e., the axial resolution)
is computed with the following steps:

• Compute the product of the ingoing and outgoing PSFs of the system in
  three dimensions (for a range of defocus levels).
• Integrate the intensity of each optical slice of the squared three-
  dimensional (3D) PSF [26] and plot the intensity as a function of axial location.
• Measure the FWHM of the resulting curve to get the axial resolution.

Using the minimum pinhole size and a 6.3-mm pupil, the axial resolution in
the eye gets as low as 30 μm. When larger confocal pinholes are used, which
is normally the case, more sophisticated methods have to be used to compute
axial resolution [27]. Experimental measures of axial resolution in an AOSLO
report axial resolutions as low as 71 μm (see Fig. 10.7 for comparisons with
other instruments) [28].

10.3.2   Basic Layout of an AOSLO

This section illustrates and describes the basic components of an adaptive
optics scanning laser ophthalmoscope (AOSLO). The schematic for the
AOSLO is shown in Figure 10.4 [29].

10.3.1   Resolution Limits of Confocal Scanning Laser Imaging Systems

In a confocal scanning laser imaging system, the effective PSF is computed
as the product of the ingoing PSF with the convolution of the outgoing PSF
and the confocal aperture. When the confocal aperture approaches a size
equaling the radius of the Airy disk of the collection path of the system, the
effective PSF is simply the product of the ingoing and outgoing PSFs. Under
such conditions, the resolution is equally determined by the input PSF and
the outgoing PSF, and the lateral resolution can exceed that of conventional
imaging by about 40% [as assessed by the full width at half maximum
(FWHM) of the PSF]. If the confocal aperture is large, then image quality is
governed only by the ingoing PSF; effective optical sectioning disappears and
lateral resolution approaches that of conventional imaging systems.

Axial resolution in confocal SLOs can be defi ned and computed in several
ways. The standard way to determine axial resolution is to measure the
detected intensity that is recorded from a planar surface as a function of its
axial location from the focal plane of the SLO [25]. The full width (axial
distance) at...