Specialized optics increases depth of field in digital images
Systems integrators faced with using available digital cameras in machine-vision and scientific image-processing systems must carefully evaluate factors such as resolution, speed, types of output, and camera cost. Even after such factors have been determined, other camera factors such as lighting, field of view, and depth of field also play important roles in deployed systems.
For example, an automated packaging system may require a large depth of field to image data on curved surfaces. Traditionally, systems integrators increase the depth of field in such systems to increase the amount of illumination on the object to be examined and to stop down the aperture of the camera.
In a new approach, engineers at CDM Optics (Boulder, CO) are extending the depth of field of camera-based systems. Known as extended depth-of-field imaging, this method uses modified optics and signal processing to obtain a larger depth of field, or depth of focus, than is possible from traditional optical systems without stopping down the camera`s aperture or increasing its f-number.
"We have designed an optical/ digital system that delivers nearly diffraction-limited imaging performance with a large depth of field," says Ed Dowski, CDM vice president of engineering. "In operation," says Dowski, "standard optical components are modified by a phase mask that alters or codes the received images so that their optical system`s point-spread function and optical transfer function do not appreciably change as a function of misfocus." Digital filtering of the intermediate image then produces a combined optical/digital image with an extended depth of field.
To show how this approach can benefit machine-vision applications, Dowski and his colleagues used an ES-1 1024 ¥ 1024 ¥ 10-bit MegaPixel MegaPlus 1.6 camera from Eastman Kodak (Rochester, NY) with the company`s CPM 127-60 rectangularly separable cubic phase mask and a custom-designed lens system. After digitizing images using a PCI-Bus interface board and a Snapper Dig-16 frame grabber from Active Imaging (Maidenhead, Berkshire, England), a MatLab-based digital filtering software package was used to filter the images.
"Traditional optical systems may not produce images with a large depth of field," says Dowski, "and stopping the aperture down to say a factor of six will result in an image with a signal-to-noise ratio nearly 36 times smaller than that of the full aperture image." However, after modifying the optics with a special-purpose phase mask or optical filter, an intermediate image appears that is uniformly blurred. After digital filtering, however, such images have the same depth of field as stopped-down images, but with a higher signal-to-noise ratio.
At present, CDM Optics does not offer complete optical systems. Rather, the company has chosen to make the technology available to OEMs through the sale of phase masks for high-volume applications and modified lens systems with extended depth of field for moderate or low-volume applications. These phase masks can be plano/cubic optical elements or incorporated onto the surface of a lens for high-volume applications to reduce cost. In addition, four development kits that contain two different phase masks and holders are available.
On its Web site (www.cdm optics.com), CDM Optics also offers a MatLab-based EDF calculator program that answers "what-if" questions concerning depth of field and depth of focus for traditional clear-aperture incoherent imaging systems and CDM`s optical/digital extended depth-of-field system.