Andrew Wilson, Editor, [email protected]
With the introduction of large-format CCD and CMOS devices with ever-decreasing pixel sizes, optics manufacturers are continually being challenged to develop lenses to support them. Unfortunately, this is not an easy task and, for developers of high-performance imaging systems, one that may necessitate using more expensive, high-performance optical systems.
Indeed, some of the high-resolution CCD imagers now available are already pushing lens designers to accommodate these smaller-pixel pitches. Sony’s ICX432DQ/DQF for example, is an interline-transfer CCD featuring a 2088 × 1550 CCD imager with 2.8-µm square pixels. Such small-pixel imagers are being used by companies such as PixeLink (Ottawa, ON, Canada; www.pixelink.com), which has developed a 2208 × 3000 color FireWire camera based on a CCD device with a 3.5-µm pixel pitch.
While the parameters of these detectors and cameras are impressive, purchasing a lens to resolve images at such a high resolution is no easy feat. If, for example, the system is designed to resolve 125 line pairs/mm, then the pitch spacing of each individual line will be 4 µm. To resolve this, the Rayleigh criterion dictates that a CCD or CCD camera using a detector with a 4-µm pixel pitch must be used. Similarly, a detector with a 2-µm pixel pitch should theoretically be able to resolve images with 250 line pairs/mm.
To resolve such images, lenses must be coupled to the camera body. In choosing a lens for any particular application, the modulation-transfer function (MTF) of the lens provides rapid insight into its resolving power (see photo). At frequencies at which the MTF of a camera is 100%, the pattern is unattenuated and the image retains full contrast, theoretically being able to perfectly discern the darkest and the brightest spots of an image. As the frequency of the image or resolution decreases, the lens is unable to resolve these differences as well.
“In fact,” says Donald Ehinger, vice president of international sales with Navitar (Rochester, NY, USA; www.navitar.com), “there are no low-cost commercially available lenses that can be used to leverage the performance of cameras such as those developed by PixeLink. What makes the issue even more complex,” he says, “is that many color sensors such as Sony’s use the Bayer pattern to capture color images.” To ensure that the proper wavelength is properly focused on the imaging array, specialty materials such as fluorite optical glass must be used in lens designs. At present, a number of companies, including Navitar, are developing lenses for high-resolution cameras.
