Microlenses make 3-D endoscopes
DUISBURG, Germany – A new image sensor that uses microlenses can transmit sharp 3-D images from inside the human body. The specially designed endoscope enables a surgeon to see minute details during operations, almost as though he were actually inside the patient’s body.
The device’s stereoscopic vision 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.
Researchers at Fraunhofer Institute for Microelectronic Circuits and Systems IMS developed the special microlenses in collaboration with partners from the European Union project Minisurg.
Left: A schematic representation of the 3-D microlens system. Right: New microlenses, the surfaces of which are seen here through phase-shift interference microscopy, make it possible to transmit 3-D images from inside the body. Courtesy of Fraunhofer IMS.
The secret of the 3-D endoscope system lies in the optical design of its CMOS sensor, 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 lens has two apertures that are much like those in the right and left eyes, according to Dr. Sascha Weyers of Fraunhofer IMS.
“These microlenses are used to separate the information for the left and the right eye on one imager chip,” Weyers said. “In addition to focusing the incoming light on the respective photodiode, the microlenses also have to minimize the optical crosstalk between adjacent pixels.”
This means that 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. This results in the CMOS sensor receiving two sets of image data that are processed separately in the way that the brain processes images coming from both eyes. The researchers used a software program to split the incoming data and process each set separately. Depending upon the capabilities of the display system, the surgeon could choose to see the 3-D images directly on a screen, or to see them while wearing polarized glasses.
To ensure that the light rays are focused precisely on the sensor, a special kind of microlens is needed. To manufacture the lenses, the engineers had to calculate the optimum shape by means of simulations. They had to ensure that the lens could separate the right and left visual channels clearly, meaning that no more than 5 percent crosstalk occurred between the channels.
The researchers had to adapt the conventional manufacturing process for microlenses to the requirements they calculated, and they 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.
Weyers and colleagues will continue fitting microlenses to CMOS imagers for specific applications where an optimal adjustment between microlens design and chip/pixel design is crucial. In addition, they will develop specialty imager chips and pixels for applications such as 3-D vision.
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