Biomedical Engineering Reference
[ 1 - 8 ], the cost and complexity of microscopic imaging tools have relatively
increased, partially limiting their use to well-equipped laboratories.
A different imaging trend has emerged in recent years, where the focus is on
reducing the cost and complexity of optical microscopy devices through the use
of computation [ 9 - 25 ]. These techniques can perform particularly well at specific
tasks such as cytometry, water quality management, and disease diagnostics. If these
devices can be made sufficiently simple, cost-effective, and robust, that could allow
lowering the cost of, for example, medical tests and point-of-care diagnostics while
increasing their availability even to resource-limited settings.
This chapter focuses on this timely opportunity by discussing in greater detail a
family of lensfree on-chip imaging techniques that are based on partially coherent
digital in-line holography and are especially promising for imaging of biochips
toward field use and telemedicine applications [ 9 - 22 ]. This emerging imaging
platform discards most optical components that are found in traditional microscopes
such as lenses and compensates for the lack of physical components in the digital
domain. Widely available image sensors and abundant computational power are
used to digitally process the acquired raw data to recover traditional microscope-
like images with submicron resolution over large sample volumes within biochips.
Though will not be covered in this chapter, other than lensfree on-chip holography,
other approaches to reducing the size and cost of microscopic imaging devices are
also actively pursued. Among those, one can cite optofluidic microscopy (OFM)
[ 23 , 24 ], which is based on microfluidic channels coupled with submicron fabricated
apertures designed to enhance contact imaging by increasing the spatial sampling
frequency, and the CellScope, which miniaturizes the traditional design of an optical
microscope to fit directly onto a cell phone [ 25 ].
Principles of Partially Coherent Lensfree On-Chip
The operating principles of all the lens free holographic imaging devices discussed
in this chapter are based on partially coherent digital in-line holography that is
operated under unit magnification [ 26 , 27 ].
In an in-line holography setup, a single beam or wavefront illuminates the object.
If the wavefront is spatially coherent at the sensor plane, then the light that is
scattered and transmitted through the object can interfere with the portion of the
light that is not scattered, forming an interference pattern (i.e., a hologram [ 28 ]) at
the digital sensor plane that exhibits interference minima and maxima as shown in
Fig. 4.1 b:
R.x; y; z 0 /
s.x; y; z 0 /
R .x; y; z 0 /s.x; y; z 0 /
R.x; y; z 0 /
s.x; y; z 0 /
R.x; y; z 0 /s .x; y; z 0 /