. A thickness variation of the object results in
defocusing and a sharp drop of the detector intensity 
. With the numerical aperture NA of the
objective lenses and the defocusing 
, can be approximated as:
S. Sinzinger, J. Jahns
Optische Nachrichtentechnik
Confocal microscopy is an imaging technique widely used in scanning microscopes, for
sensing, and optical data storage. A point source is used in combination with a point detector to
yield enhanced lateral resolution as well as depth discrimination [1]. In the conventional
configuration the point source is imaged onto the object by an objective lens. Light reflected or
scattered from the illuminated object point is collected by the same objective lens and imaged via
a beam splitter onto the detector. The detector intensity is influenced by changes of the
reflectivity R of the illuminated object point . A thickness variation of the object results in
defocusing and a sharp drop of the detector intensity . With the numerical aperture NA of the
objective lenses and the defocusing , can be approximated as:
If assembled with conventional optical components and optomechanical alignment a bulky setup
results. In this case size and stability problems limit the variety of applications to which the
confocal concept can be applied. Such problems can be overcome with an integrated version of
the confocal microscope.
Planar optics is an approach to the integration of free-space optical setups and thus can be
applied to the integration of confocal microscopes [2]. To this end all necessary components are
integrated on the surfaces of a glass substrate. Fig. 1 shows the schematic of a fully integrated
confocal sensor head. The optical system consists of two collimating microlenses (L1 and L4)
and two off-axis lens segments (L2 and L3) integrated on opposite sites of a glass substrate.
The optical axis is folded into an off-axis configuration typical for planar optics. The depth
discrimination now depends on both the aperture angle 
of the optical system as well as the off
axis angle 
:
This approach leads to a compact, stable, small size, and light-weight sensor head. The planar optical setup offers high functionality and the potential to build fully integrated systems including optoelectronic and electronic components through hybrid integration techniques.
Fig. 1: Integrated confocal sensor head
References:[1] T. Wilson, C. Sheppard, "Theory and practice of scanning optical microscopy", Academic Press, London (1984).
[2] S. Sinzinger, J. Jahns, "Integrated confocal microscope based on planar optics", OSA Annual Meeting, Portland (1995), paper ThRR7.