Small Systems -
High Resolution

The Fraunhofer IOF presents a new approach in which many microscope objectives are arranged in parallel in order to increase the imaged object area or, alternatively, to enable the resolution of very small structures.

Array Camera Technologies at Fraunhofer IOF

Array-Microscope with Sub-Micrometer Resolution

Fig. 1: An array microscope inside a commercial camera's housing.
© Fraunhofer IOF
Fig. 1: An array microscope inside a commercial camera's housing.
Measured imaging contrast vs. modulation frequency of a single channel (blue curve) compared to simulation (gray curves) with example images shown in the insets.
© Fraunhofer IOF
Measured imaging contrast vs. modulation frequency of a single channel (blue curve) compared to simulation (gray curves) with example images shown in the insets.

Optical imaging of small structures is applied in almost all scientific and technical disciplines. Conventional microscopes, which are well-known tabletop devices, provide a limited field of view depending on the magnification of the objective. Increasing the respective field of view requires large, complex optical systems and/or scanning of the object. By contrast, a novel method using miniaturized objectives enables a parallel arrangement of multiple microscopes in one array. In such an approach, the field of view equals the size of the image sensor and larger areas can be accessed without increasing the optic’s complexity and/or scanning. However, the numerical aperture, as well as the detector’s pixel size, limit the resolution of respective systems. Alterna-tively, magnifying array microscopes enable the resolution of small objects, but necessitate a number of scanning steps that is proportional to the square of the magnification.

Figure 1 shows one example of a 0.3 NA, 10x magnification array microscope consisting of 160 identical channels. The distance between the front lens array, visible in the photo, and the image sensor is in the range of 10 mm only. Therefore, the whole imaging optics is integrated completely into a commercial camera housing. In addition, a two-band spectral filter enables fluorescence imaging. The optical system has been replicated on both sides of one wafer with high lateral and axial precision and ultimately allows a purely passive integration of the overall system. The measured data shown in Figure 2 illustrates the contrast of the image, demonstrating clearly resolved 1 μm wide stripes (500 lp/mm). Further advancing this approach will enable microscopic imaging of large areas by means of a small, handheld system.

This project has been funded by the Thüringer Aufbaubank under contract 2015 FE 9115.

 

Authors: Norbert Danz, Rene Berlich, Bernd Höfer, Peter Dannberg, Jens Dunkel

Compact Multispectral Array-Camera

Prototype of the compact multispectral camera.
© Fraunhofer IOF
Prototype of the compact multispectral camera.
Simultanous recording of 66 parallel imaging channels with different transmission wavelengths.
© Fraunhofer IOF
Simultanous recording of 66 parallel imaging channels with different transmission wavelengths.
Test scene and merged multi-spectral-image together with a pseudocolor image evaluating the NDVI.
Test scene and merged multi-spectral-image together with a pseudocolor image evaluating the NDVI.

The particular challenge of multispectral cameras is the simultaneous detection of spectral and spatial information with high resolution. Scanning systems which measure either one spatial dimension or the spectral information in a time sequential mode are typically used. To overcome the associated problems, new developments have to operate in real-time or so called snap-shot mode. A second trend goes towards miniaturization, e.g. for applications such as  precision farming based on UAVs (unmanned aerial vehicle) for plant monitoring.

In the project "MIRO", an innovative multispectral camera system was realized that combines both goals by using a multiaperture approach with 66 simultaneously working imaging channels each seeing its individual wavelength. The concept is based on a special microlens-array, which results in a very thin imaging optic of approximately 7 mm. For spectral selection, a linear variable bandpass filter is used. The definition of discrete spectral transmission bands for each channel is possible by aperture structures placed between the filter and the microlens-array. An additional crosstalk module is integrated to ensure the channel-separation on the image sensor. In conclusion, this system enables the recording of multispectral images ranging from 450 nm to 850 nm with a sampling of about 6.0 nm and a spatial resolution of 400 x 400 pixels.

The evaluation of the multispectral images is carried out by software developed in-house. Key features are the merging of sub-images and the extraction of local spectral information. Small imaging errors, as well as the angle  dependency of the used bandpass filter, are corrected. Furthermore, selected spectral channels can be combined for the calculation of classification indices such as the NDVI (normalized differences vegetation index) and shown as a pseudocolor image. The presented project is funded by the Fraunhofer Society within the framework of the research project "MIRO".

 

Authors: Robert Brüning, René Berlich, Christin Gassner, Martin Hubold, Robert Brunner