High quality imaging is of paramount importance in a variety of application areas. Imaging performance is assessed on three major benchmarks: image resolution, amount of volume imaged, and imaging speed. In terms of these benchmarks high resolution volumetric imaging at high frame rate speed is the holy grail of imaging. My research interest are in the development and improvement of high resolution, high speed, volumetric (3D) optical imaging technology.

Tomographic imaging
Tomographic imaging is based on the reconstruction of the structure of an object based on projections of emission or transmission through the object. Based on the analysis of the optical imaging geometry we develop methodology to acquire optimal tomographic images, i.e., the image with the highest spatial resolution. Moreover, we develop algorithms for tomographic image reconstruction based on analytical models and iterative reconstruction techniques.

Fig. 1. Tomographic image reconstruction of a GFP labeled zebrafish. Original reconstruction (middle) and deconvoluted reconstruction (right).

When performing optical tomography through scattering media, the measured projections are strongly distorted. We are developing novel low-coherence based imaging methods to measure projection based only on the ballistic light transmission. In this way we obtain undistorted projections and make high quality tomographic images.

Image resolution and deconvolution in optical tomography
J. van der Horst and J. Kalkman, Optics Express 24, 24460 (2016)
Transmission optical coherence tomography based measurement of optical material properties
A.K. Trull, J. van der Horst, J.G. Bijster, and J. Kalkman, Optics Express 23, 33550 (2015)

Optical coherence tomography

Optical coherence tomography is a successful optical imaging technique reaching micrometer resolution up to a few millimeters deep. Although OCT is already well established, we work on improving OCT to obtain higher quality images and extracting more information from OCT data.

When imaging the retina optical aberrations induced by the eye limit the lateral OCT resolution to about 20 micron. Adaptive optics is a technique to shape the wavefront in such a way as to correct for these aberrations. We develop more efficient, better performing, and lower cost adaptive-optics OCT systems (collaboration with Delft Center for Systems and Control). For this we develop models and algorithms for the OCT signal to enable sensorless adaptive optics image improvement.

Fig. 2. OCT image (top) and sensorless adaptive optics optimized image (bottom).

We study dynamic phenomena in rheology with OCT (collaboration with Academic Medical Center). To quantify particle dynamics we developed a model based on the OCT autocorrelation function. Based on this principle we can determine particle diffusion, lateral flow and longitudinal flow locally at the micrometer scale.

We use OCT for the study of both the the layer stratigraphy of works of art (collaboration, J. Dik faculty of 3mE). A commercial OCT system (Thorlabs Ganymede II) is available for high resolution imaging of samples. We develop OCT signal and image analysis algorithms for high resolution structural imaging of layers of paintings.

Model-based sensor-less wavefront aberration correction in optical coherence tomography
H.R.G.W. Verstraete, S. Wahls, J. Kalkman, and M. Verhaegen, Optics Letters 40, 5722 (2015)
Localized measurement of longitudinal and transverse flow velocities in colloidal suspensions using optical
coherence tomography
, N. Weiss, T.G. van Leeuwen, and J. Kalkman, Physical Review E 88, 042312 (2013)