Fraunhofer Scientists Invent Microlenses Suitable for 3-D Minimally Invasive Endoscopy

Researchers at the Fraunhofer Institute for Microelectronic Circuits and Systems (IMS; Duisburg, Germany) have developed an image sensor that transmits perfect 3-D images from inside the body thanks to the use of microlenses. The new technology will improve doctors' ability to perform surgery using endoscopic techniques.

Together with scientists in the  EU project dubbed 'Minisurg,' the Fraunhofer researchers have succeeded in incorporating CMOS image sensors into endoscopic devices. Whereas the charge-coupled-device sensors available in the past could provide only low-resolution images, the CMOS sensor are similar to those used in single-lens-reflex cameras.

"To make this possible, we developed special microlenses," explains IMS project manager Sascha Weyers. The secret lies in the optical design of the 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. In this arrangement, the lens has two apertures which capture two beams of light. For example, light arriving from the left passes through the left aperture and is focused on the right-hand vertical line of sensors, and vice versa. The two light rays cross underneath the lens arrangement.

As a result of this configuration, 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 can see the 3-D images directly on the screen or through polarized glasses.

A special kind of microlens is required for the light rays to be focused precisely on the sensor. To manufacture it, the Fraunhofer engineers first had to calculate the lens's optimum shape using simulations. And to eliminate external factors, they had to ensure that it can clearly separate the right and left visual channels. Thus, the scientists had to ensure that no more than 5% of the energy from one light ray is captured by the line of sensors serving the other channel.

The researchers' next task was to adapt the conventional microlens manufacturing process to the requirements of their special lens shape. They also had create a suitable 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, making it suitable for minimally invasive surgery.