Mirrors Replace Lenses in UAVs

Unmanned aerial vehicles (UAVs) carry optical measuring equipment that is required to operate free of chromatic aberrations across a wide spectral range – from ultraviolet to infrared. Conventional lens systems comprised of several lens elements are of limited use because when required to image a wide spectral range the image quality suffers from color fringing and becomes blurred.

Traditionally, specific lenses have been used for each different spectral band, but because UAVs can only carry a limited amount of weight this approach proves difficult.

Researchers from the Fraunhofer Institute for Photonic Microsystems IPMS have now designed an all-reflective zoom objective with deformable mirrors – making it possible to capture images free from chromatic aberration in a number of spectral ranges using this single system. This would have the advantage of prolonging the battery’s life and increasing the aircraft’s endurance.

“We’ve come up with a design for a new objective in which we’ve used mirrors instead of standard lens elements. The objective is comprised of four mirrors, carefully arranged to avoid obscuration – this produces a higher-contrast image,” said Dr. Heinrich Grüger, group manager of IPMS. “Two deformable mirrors take care of the triple zoom range – with no loss of image quality. The new design eliminates the need for elaborate mechanical guides within the lens barrel.”

Although IPMS scientists have already developed deformable mirrors, they have not yet managed to achieve the size and degree of variability required for the mirror zoom objective. Optical simulations have shown that the mirrors would need to be at least 12 millimeters in diameter in order to produce a zoom objective with a sufficient f-number.

Nevertheless, the researchers have already been able to demonstrate the optical performance of the objective: they built three identical setups with three different focal lengths in which the deformable mirrors were replaced by conventional rigid mirrors.

“Both the automation technology sector and the automobile and equipment engineering sector would profit from this type of objective,” Grüger said, noting that deformable mirrors will have to be created – conventional optical mirrors are rigid. “For the zoom function, we need mirrors that will permit flexible actuator control of the radius of curvature.”

For more information, visit: www.fraunhofer.de