Biophotonics_Concepts_to_Applications

6.6 Formation of Speckles

When coherent light illuminates a rough or optically inhomogeneous material, such
as a biological tissue surface, the scattered light forms a random interference pattern
calledspeckle. A generic example is shown in Fig.6.28. This temporally varying
speckle is caused by the interference of a large number of elementary electro-
magnetic waves with random phases when the coherent light is reflected from the
tissue surface or when the light passes through the tissue [ 57 – 60 ].
Speckles have a negative impact on biophotonics measurement techniques such
as optical coherence tomography in which speckle formations limit the interpre-
tation of the observed images. Various speckle-reducing digitalfilters have been
examined to alleviate these distortions [ 57 ]. On the positive side, speckle formation
has been implemented successfully in biophotonics methodologies such as laser
speckle contrast imaging, which is an optical technique used to generate bloodflow
maps with high spatial and temporal resolution. More details on laser speckle
imaging are given in Sect.10.3.

Speckles

Tissue

sample

Fig. 6.28 Example
illustration of a speckle
pattern

S 2

S 1

S 0

Excitation

photons

λ 1 λ 2 λ 3 λ 4

Fluorescence

photons

Non-radiative transitions

Ground state

energy level

Excited state 2

energy level

Excited state 1

energy level

10 -15s

10 -13s

10 -9s

Fig. 6.29 Example of a
typical Jablonski diagram

188 6 Light-Tissue Interactions