Endoscopic Microlenses Provide Three-Dimensional Images

Researchers at the Fraunhofer Institute for Microelectronic Circuits and Systems (IMS; Duisburg, Germany) developed a complementary-symmetry metal-oxide- semiconductor (CMOS) image sensor that can be used in medical applications. The optical design of CMOS sensors involves a cylindrical microlens that is placed in front of every two vertical lines of sensors in the pixel configuration; a superimposed lens captures the light falling on the microlenses, which focus it on the pixels. The CMOS sensors developed at IMS, however, have an additional feature: an arrangement that incorporates two apertures in the lens, enabling a stereoscopic view.

The two beams of light are captured by the lenses, focused on the contra-directional sensor (left lens to right sensor and vice-versa); the two light rays cross underneath the lens arrangement. As a result, the CMOS sensor receives two sets of image data that are processed separately, similar to the way that the brain processes images coming from the left and right eye. Software splits the incoming data and processes each set separately. Depending on the capabilities of the display system, the surgeon either sees the 3D images directly on the screen or can see them when wearing polarized glasses.

This is made possible thanks to the new microlens, which ensures that the light rays are focused precisely on the sensor. In order to manufacture the lenses, the Fraunhofer IMS engineers first had to calculate the optimum shape by means of simulations. To eliminate external factors, they had to ensure that the lens was capable of clearly separating the right and left visual channels. In concrete terms this means ensuring that no more than 5% of the energy from one light ray is captured by the line of sensors serving the other channel, a condition in signal transmission known as “crosstalk”.

The next task for the researchers was to adapt the conventional manufacturing process for microlenses to the requirements of the calculated lens shape. They also had to fulfill a number of requirements relating to the production of the miniature camera. The resulting chip is so small that it fits into a tube measuring no more than 7.5 mm in diameter. Together with the bundle of optical fibers that serves as the light source, the endoscope measures 10 mm in diameter, adequate for minimally invasive surgery applications.

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Fraunhofer Institute for Microelectronic Circuits and Systems