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Microlenses Make Superior Endoscopes

Photonics.com
Nov 2011
DUISBURG, Germany, Nov. 7, 2011 — A newly developed image sensor transmits perfect 3-D images from inside the human body, thanks to the use of microlenses. The specially designed endoscope enables surgeons to see every detail during an operation, almost as though he were actually inside a patient’s body. The stereoscopic vision the device provides considerably simplifies the work of neurosurgeons and other specialists, who can now navigate a safe path through bodily tissues without the risk of collateral damage.

“To make this possible, we developed special microlenses,” said Sascha Weyers, a project manager at the Fraunhofer Institute for Microelectronic Circuits and Systems (Fraunhofer IMS). The ability to see inside a patient’s body in perfect 3-D is the result of work by Fraunhofer IMS researchers and their partners in the European Union project "Minisurg."


Specially designed microlenses will one day help transmit 3-D images from inside the human body. Their function is to precisely focus the light rays on the sensor. (Photo: Fraunhofer IMS)

The secret of the 3-D endoscope system lies in the optical design of its CMOS sensors, in which a cylindrical microlens 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 special feature of this arrangement is that the lens has two apertures, “rather like the right and left eye,” Weyers said.

In other words, two beams of light are captured by the lenses — the one arriving from the left passes through the “left eye” to be focused on the right-hand vertical line of sensors, 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 in the same way that the brain processes images coming from the left and right eye. A software program splits the incoming data and processes each set separately. Depending on the capabilities of the display system, the surgeon either sees the 3-D images directly on the screen or can see them when wearing polarized glasses.

It takes a special kind of microlens to ensure that the light rays are focused precisely on the sensor. To manufacture the lenses, the Fraunhofer engineers had to calculate the optimum shape by means of simulations. To eliminate external factors, it had to be ensured that the lens could separate the right and left visual channels clearly. In concrete terms, this means ensuring that no more than 5 percent of crosstalk occurred between the channels.

Next, the researchers had 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 fits into a tube 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 — the perfect size for minimally invasive surgery.

For more information, visit: www.fraunhofer.de/en  


GLOSSARY
stereoscopic vision
Vision in depth of three dimensions as a result of the spacing of the eyes. This spacing allows the eyes to see objects from slightly different points of view.
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