Alexander Koppelhuber

Towards a transparent, flexible, scalable and disposable image sensor using thin-film luminescent concentrators

Most image sensors are planar, opaque, and inflexible. We present a novel image sensor that is based on a luminescent concentrator (LC) film which absorbs light from a specific portion of the spectrum. The absorbed light is re-emitted at a lower frequency and transported to the edges of the LC by total internal reflection. The light transport is measured at the border of the film by line scan cameras. With these measurements, images that are focused onto the LC surface can be reconstructed. Thus, our image sensor is fully transparent, flexible, scalable and, due to its low cost, potentially disposable.

A transparent thin-film sensor for multi-focal image reconstruction and depth estimation.

We present a fully transparent and flexible light-sensing film that, based on a single thin-film luminescent concentrator layer, supports simultaneous multi-focal image reconstruction and depth estimation without additional optics. Together with the sampling of two-dimensional light fields propagated inside the film layer under various focal conditions, it allows entire focal image stacks to be computed after only one recording that can be used for depth estimation. The transparency and flexibility of our sensor unlock the potential of lensless multilayer imaging and depth sensing with arbitrary sensor shapes--enabling novel human-computer interfaces.

A classification sensor based on compressed optical Radon transform

We present a thin-film sensor that optically measures the Radon transform of an image focussed onto it. Measuring and classifying directly in Radon space, rather than in image space, is fast and yields robust and high classification rates. We explain how the number of integral measurements required for a given classification task can be reduced by several orders of magnitude. Our experiments achieve classification rates of 98%-99% for complex hand gesture and motion detection tasks with as few as 10 photosensors. Our findings have the potential to stimulate further research towards a new generation of application-oriented classification sensors for use in areas such as biometry, security, diagnostics, surface inspection, and human-computer interfaces.

Enhanced learning-based imaging with thin-film luminescent concentrators

LumiConSense, a transparent, flexible, scalable, and disposable thin-film image sensor has the potential to lead to new human-computer interfaces that are unconstrained in shape and sensing-distance. In this article we make four new contributions: (1) A new real-time image reconstruction method that results in a significant enhancement of image quality compared to previous approaches; (2) the efficient combination of image reconstruction and shift-invariant linear image processing operations; (3) various hardware and software prototypes which, realize the above contributions, demonstrating the current potential of our sensor for real-time applications; and finally, (4) a further higher quality offline reconstruction algorithm.

Multi-exposure color imaging with stacked thin-film luminescent concentrators.

We present a fully transparent, scalable, and flexible color image sensor that consists of stacked thin-film luminescent concentrators (LCs). At each layer, it measures a Radon transform of the corresponding LCs spectral responses. Color images are then reconstructed through inverse Radon transforms that are obtained using machine learning. A high sampling rate in Radon space allows encoding multiple exposures to cope with under- and overexposed cases in one recording. Thus, our sensor simultaneously measures multiple spectral responses in different LC layers and multiple exposures in different Radon coefficients per layer. We also show that machine learning enables adequate three-channel image reconstruction from the response of only two LC layers.

LumiConSense: a transparent, flexible, and scalable thin-film sensor.

The LumiConSense sensor employs a thin luminescent-concentrator film, which allows lensless multifocal imaging and depth estimation at interactive rates.

Computational imaging, relighting and depth sensing using flexible thin-film sensors

We wrap a thin-film luminescent concentrator (LC) - a flexible and transparent plastic foil doped with fluorescent dye particles - around an object to obtain images of the object under varying synthetic lighting conditions and without lenses. These images can then be used for computational relighting and depth reconstruction. An LC is an efficient two-dimensional light guide that allows photons to be collected over a wide solid angle, and through multiple overlapping integration areas simultaneously. We show that conventional photodetectors achieve a higher signal-to-noise ratio when equipped with an LC than in direct measurements. Efficient light guidance in combination with computational imaging approaches, such as presented in this article, can lead to novel optical sensors that collect light in a structured way and within a wide solid angle rather than unstructured through narrow apertures. This enables flexible, scalable, transparent, and lens-less thin-film image and depth sensors.

Thin-film camera using luminescent concentrators and an optical Söller collimator

This article reports our investigation of the potential of optical Söller collimators in combination with luminescent concentrators for lens-less, short-distance, and shape-independent thin-film imaging. We discuss optical imaging capabilities and limitations, and present first prototypes and results. Modern 3D laser lithography and deep X-ray lithography support the manufacturing of extremely fine collimator structures that pave the way for flexible and scalable thin-film cameras that are far thinner than 1 mm (including optical imaging and color sensor layers).

Toward a Flexible, Scalable, and Transparent Thin-Film Camera

This paper summarizes our progress toward a first fully transparent, flexible, and scalable thin-film image sensor and reviews lensless imaging approaches. Combining sensing and imaging in multiple layers has the potential to enable entire thin-film camera systems that make the capturing of images, light fields, and depth information possible.

LumiConSense: a transparent, flexible, scalable, and disposable image sensor using thin-film luminescent concentrators

Our sensor [Koppelhuber and Bimber 2013] consists of a thin, transparent polycarbonate film, referred to as luminescent concentrator (LC), that is doped with fluorescent dyes. Light of a particular wavelength sub-band that penetrates the film is emitted in longer wavelengths, while wavelengths outside the sub-band are fully transmitted. The example shown in figure 1(a) absorbs blue and emits green light. The emitted light is mostly trapped inside the film by total internal reflection (TIR), and is transported with reduced multi-scattering towards the LC edges while losing energy over transport distance. The bright film edges indicate decoupling of the light integral transported to each edge point from all directions inside the LC.