The use of phase only spatial light modulators for holographic optical trapping results in the appearance of ghost orders,
creating unwanted traps with uncontrolled intensity and causing variations in the intensity of the desired traps. By
introducing dummy areas in the diffraction plane during the hologram optimization, the intensity in the ghost orders can
be significantly reduced. By directing a variable fraction of the light to the dummy area, the optical power in the traps
can be controlled independently and kept constant also while moving traps to different arrangements. We present and
evaluate an algorithm for hologram generation which utilizes dummy areas and allows arbitrary spot positioning in three
dimensions. The method enables the use of holographic optical trapping for applications requiring precise control of the
intensity in traps, such as optical force measurement.
We have implemented several algorithms for hologram generation, aimed for holographic optical tweezers
applications, using the parallel computing architecture CUDA. We compare required computation time for different
implementations of the Gerchberg-Saxton algorithm and provide guidelines for choosing the best suited version with
respect to the application. We also show that additional calculations, compensating for limitations in the used spatial
light modulator and optical system, can be included in the hologram generating software with little or no loss in computational speed.
We present an experimental demonstration of a method using optical tweezers proposed by Fischer and Berg-Sorensen
for measuring viscoelasticity using optical tweezers. It is based on a sinusoidal oscillation of the liquid in combination
with force measurements using optical tweezers. We verify the method by applying it to measurements in water, glycerol
and polyethylene oxide (PEO).
Optical forces and torques acting on microscopic objects trapped in focussed laser beams promise flexible methods of driving micromachines through a microscope cover slip or even a cell wall.
We are endeavouring to engineer special purpose micro-objects for a range of tasks. Colloidal self assembly of calcium carbonate provides birefringent spheres which can exert considerable torque, while two photon polymerisation allows us to fabricate objects of arbitrary shape that can be designed to exchange both spin and orbital angular momentum. Numerical calculations of forces and torques can allow an optimal design, and optical measurements provide us with certain knowledge of the forces and torques which are actually exerted.
In order to achieve autonomous behavior of UAVs in surveillance missions, it is of interest to embed a vision sensor as an integral part of the control loop. The major difficulty in this type of applications is the lack of realistic simulation environments. Those, if available, can avoid the obvious dangers and cost of extensive field experimentation with visual servoing controllers for airborne platforms. This paper investigates the performance and possibilities of various visual-servoing techniques applied to acquisition and tracking in the case of UAVs. The work is done in SEAVS, a 3D simulation environment for aerial visual-servoing. SEAVS is a simulation software with a graphical user interface including one 2D window for planning of the aerial mission and two 3D windows -- one for the visualization of the ongoing simulation, and one showing the present camera view. The unique feature of SEAVS is the use of orthographic photos which enhances realistic image processing. The paper describes the design of visual-servoing controllers for the purpose of acquisition and tracking tasks. Two types of controllers are investigated: one based directly on the angle errors and one based on the image Jacobian. The algorithms are validated by simulation in SEAVS.
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