Spot scanning increases microscope imaging fidelity

Converting microscope images into digital data for storage, analysis, and viewing presents system designers with several challenges. While CCD cameras are the most popular devices for digital microscope imaging, they suffer from low sensitivity and slow frame rates as the resolution increases. Using higher illumination can overcome low sensitivity but can damage delicate microscope samples. Worse, the color registration achieved with single-chip color cameras might be unacceptable as different c

Jan 1st, 1998

Spot scanning increases microscope imaging fidelity

ANDREW WILSON

Converting microscope images into digital data for storage, analysis, and viewing presents system designers with several challenges. While CCD cameras are the most popular devices for digital microscope imaging, they suffer from low sensitivity and slow frame rates as the resolution increases. Using higher illumination can overcome low sensitivity but can damage delicate microscope samples. Worse, the color registration achieved with single-chip color cameras might be unacceptable as different color pixel elements are not spatially aligned. Although laser-scanning microscopes can overcome these problems, they have even slower scanning rates and can be prohibitively expensive in many applications.

To overcome these limitations, Gravely Research (Raleigh, NC) has developed a digital imaging microscope that combines a spot-scanning system with color frame rates fast enough to examine live samples. According to Ben Gravely, company president, the illumination levels used are 1000 times less than those needed for CCD cameras and eliminate the possibility of sample degradation.

Dubbed Cosmic, the PC-based color scanning microscope produces images that are limited in resolution only by the microscope optics and not by the electronic components of the system (see Fig. 1). Based on spot-scanning techniques developed more than 40 years ago, the system uses a specially designed CRT developed by Moraine Displays (Big Bend, WI) that has a white raster pattern on its screen. A microscope relay lens and an objective lens image this raster pattern onto the sample placed on the microscope stage. The one-to-one mapping of the scanning spot from the CRT screen onto the sample plane creates a white-light probe that scans over the sample (see Fig. 2).

Light transmitted through the sample is filtered into RGB components and digitized by a custom stand-alone scan-converter/display controller from Poynting Products (Oak Park, IL). Here, the three signals are digitized into three 1280 ¥ 1024 math buffers. Math functions such as averaging and summing images are performed directly on the images using these buffers. During processing, images are transferred to three display buffers and converted to RGB for display on a 1280 ¥ 1024 monitor.

For storage and retrieval, the host system controller reads the image data from the math buffers over a SCSI port. For transferring images to a local computer or over a network, the system controller also provides an Ethernet port.

"By changing the scanned area on the sample," says Gravely, "the optical magnification of the Cosmic system can be changed instantly up to 300%." In an optical system, the brightness and focus of the image must be adjusted for each magnification power. "But," says Gravely, "by using electronic zoom, brightness and focus do not change with magnification, saving the operator time and effort."

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