Product Focus: Increasing sensitivity of scientific ccd cameras improves image capture

Scientific CCD cameras offer greater sensitivity and spatial resolution and lower noise than their broadcast-quality counterparts.

By Andrew Wilson,Editor at Large

In microscopy, medical, and astronomy applications, captured images are often weak or of low contrast. In such applications, the increased sensitivity, spatial resolution, dynamic range, and low noise of scientific-quality CCD cameras make them ideal for gathering such data. Although more expensive than broadcast-type devices, the choice of camera is driven mainly by frame rate and resolution required.

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RIGHT. FIGURE 1. Using the Texas Instruments TC237 frame-transfer CCD, the Pegasus camera from Patterson Electronics operates in progressive scan mode providing 12-bit dynamic range and a resolution of 658 x 496 CCD. With up to 1-frame/s readout, the camera includes Microsoft Windows software that allows the host memory to be used as an image buffer and the host VGA for image display.

In applications such as microscopy and astronomy, where images to be captured move slowly, frame rate may be less important than resolution. For such applications, large, often greater than 1k x 1k imagers, may be used with 16-bit or greater resolution. However, in applications such as medical imaging, where image motion may be an important factor, higher-frame-rate, lower-resolution cameras are used.

Increasing performance

In scientific camera systems, the CCD is generally one of three types. In applications where integration times can be long, area-array CCD-based cameras are generally used. In such cameras, the CCD is used to collect photogenerated charge and transport the charge into a horizontal CCD for charge transfer. Such area arrays, also known as full-frame image sensors, use an external shutter to remove incident light before any charge transfer begins. In such devices, the time between exposures is limited by the time required to read out each frame.

Using a KAF-1400, 1317 x 1035-pixel full-frame imager from Kodak (Rochester, NY), the CCD PentaMAX Camera from Princeton Instruments (Trenton, NJ) can collect 12-bit images at a readout rate of 2.5 or 5 million pixels per second. Designed for microscopy applications, the camera's CCD features pixel sizes of 6.8 µm and a readout time of 0.3 s at 5 MHz. To increase resolution and speed, the camera can also be configured with 4k x 4k devices and can sustain up to 150 frames/s with binning, the ability to clock multiple pixel charges in both the horizontal and vertical direction into a single larger charge, albeit at the expense of resolution.

To increase speed without sacrificing resolution, many manufacturers offer scientific cameras that use frame transfer or interline transfer image sensors. While frame-transfer imagers are similar to full-frame imagers, they feature an additional optically isolated frame-storage region. In operation, these devices first turn off vertical CCDs within the sensor and open an external shutter. At the end of the integration time, the sensed image is transferred to the isolated frame-storage region. The vertical CCDs are then turned off and the external shutter opened to acquire the next image. At the same time, the image in the storage region is clocked out of the imager.

While the detection area of frame-transfer arrays are usually smaller than their full-frame counterparts, light collection is continuous, and cameras built around such devices are less likely to miss transient events. However, such continuous illumination can lead to smear, where photogenerated charge originating below the space-charge region of a photosite diffuses into the adjacent CCD. But because readout and light collection are simultaneous, frame rates are higher for a given resolution and pixel rate, although the detection areas of such devices are generally smaller than full-frame imager-based cameras.

Using the TC237 frame-transfer CCD image sensor from Texas Instruments (Dallas, TX), the Pegasus digital, cooled CCD camera system from Patterson Electronics (Tustin, CA) operates in progressive scan mode providing 12-bit dynamic range and a resolution of 658H x 496V CCD (see Fig. 1). With up to 1-frame/s readout, the camera includes Microsoft Windows software that allows the host memory to be used as an image buffer and the host VGA for image display. Additional functions allow automatic dark frame subtract of the acquired image, fast focus mode, and auto generation of dark or bias frames.

Adding fiber

In some scientific cameras, fiberoptic bundles coupled to the CCD are used to increase the amount of light that can be collected. Such fiberoptic bundles are often used to couple light from x-ray or neutron scintillator screens, image intensifiers, or streak tubes. To couple light from a fiber bundle to the CCD, both CCD and fiber bundle are bonded to minimize the distance without sacrificing CCD performance.

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RIGHT. FIGURE 2. Designed for lensless, direct imaging of phosphor screens and CRTs, the XR300 camera features a dynamic range of up to 65,536 gray levels. By using a 1024 x 1024-pixel CCD sensor, a spatial resolution of more than eight line pairs per mm is achieved at the phosphor. Capable of digitizing each megapixel image in less than 2 s, the camera interfaces directly to PCI, ISA, or SCSI-based host computers.

Applications requiring the highest possible light-collection efficiency benefit most by using large CCDs to reduce the amount of demagnification required. These include x-ray imaging systems, streak cameras, MCP detectors, and electron microscope scintillators. With an appropriate scintillator, they can also image autoradio- graphic samples. By using a fiberoptic interface designed to fit between the bond wires on an integrated circuit or probes of a probe station, this type of camera can be used for high sensitivity imaging of luminescent semiconductors in operation.

Many of the standard products from Photometrics (Tucson, AZ) are available with imaging fiberoptics. Fiber bundles range in magnification from 1:1 fiber stubs to large 6:1 fiber tapers and in diameters up to 160 mm. Supported CCDs vary from 512 x 512 pixels to 2k x 2k pixels. Fiber bundles are available with absorption fibers to improve contrast and low-thorium glass to reduce background from radioisotopes.

One such camera is the XR300 camera system designed for lensless, direct imaging of phosphor screens and CRTs (see Fig. 2). With a dynamic range of up to 65,536 gray levels, both high- and low-density material can be captured in the same image. By using a 1024 x 1024-pixel CCD sensor, a spatial resolution of more than eight line pairs per mm is achieved at the phosphor. The XR300 system incorporates technology that allows the fiberoptic taper to be directly bonded to the front surface of the CCD sensor, eliminating intermediate fiber stubs or unreliable oil layers between the fiber taper and CCD. With 3:1 fiber magnification, the effective imaging area is 2.3 x 2.3 in. at the front face of the camera. Capable of digitizing each megapixel image in less than 2 s, the camera interfaces directly to PCI, ISA, or SCSI-based host computers.

Cooled cameras

To increase the sensitivity of scientific CCD cameras, imagers can be cooled using thermoelectric or liquid nitrogen. Increased sensitivity is important for low-light-level applications such as chemiluminescent and bioluminescent imaging microscopyand spectroscopy. By cooling the CCD, images can be digitized to a much finer gray-scale resolution, enabling low-contrast features to be distinguished and displayed.

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RIGHT. FIGURE 3. Liquid-nitrogen-based cooled cameras target scientific applications requiring the highest dynamic range, highest sensitivity, and lowest cooling possible. Series 300 camera heads from Photometrics, for example, are available with cooling options ranging from -25°C to -110°C. Liquid-nitrogen cooled heads are available with Dewar sizes ranging from 0.5 to 2.5 liters.

Cooling also offers the potential for weak signals to be detected in the presence of stronger signals that may swamp the image of interest. In addition, if there are weak features of low contrast in an image in the presence of an overall high background signal level, the cooled CCD can be tuned to the particular grey-level region for these weak features only, excluding the large background.

In thermoelectrically cooled cameras, Peltier devices, driven by an electric current, are used to draw heat from the CCD to a heat sink. This heat sink is then cooled either by air or water to lower the temperature of the CCD. In liquid-nitrogen-cooled systems, the CCD operates in a Dewar of liquid nitrogen. Rather than run the camera at the liquid boiling point of -195.8°C, scientific cameras using liquid nitrogen are usually operated at between -120°C to -60°C because the charge-transfer efficiency of CCD devices is too poor at -200°C. For every 4°C-7°C drop in temperature, the dark current of CCD devices is approximately halved, increasing the sensitivity of the CCD and thus the sensitivity of the camera.

In the design of its Spot 2e multipurpose camera, Diagnostic Instruments (Sterling Heights, MI) used the Peltier method to cool a 1315 x 1033-pixel CCD to -12°C. This provides low-light-level color images for applications such as fluorescence, polarized light, darkfield, and high magnification work. According to Diagnostic Instruments, background noise in brightfield images is also reduced. By exposing the CCD three times (once each for red, green, and blue), color CCD images can be captured. Interfaced to a Pentium 333-MHz computer, the Spot2e PC-based software allows image integration time to be increased to 17 minutes per color.

Another example of a Peltier-cooled CCD cameras is the Cryocam from Micro Luminetics (Los Angeles, CA). Offered with CCDs manufactured by Lockheed Martin, SITe, and Texas Instruments, the cameras can be configured with devices ranging from 512 x 512 pixels to 2048 x 2048 pixels and areas of up to 31 mm square. To minimize dark current, the CCD is chilled to as much as 85°C below ambient with a thermoelectric cooler. A precision temperature control holds the CCD within 0.2°C over a temperature range to as low as -80°C. Offered with either a 12- or 16-bit ADC, CCD readout rate can be set to 25k, 50k, 100k, or 200k pixels per second. Slower readout rates yield reduced noise and increased sensitivity.

Liquid-nitrogen-based cooling can reduce the dark current and increase the sensitivity of scientific CCD cameras even further. Usually, these cameras target scientific applications requiring the highest dynamic range, highest sensitivity, and the lowest cooling possible. Series 300 camera heads from Photometrics, for example, are available with cooling options ranging from -25°C to -110°C and passive air, forced air, liquid circulation, or liquid-nitrogen options (see Fig. 3). Liquid-nitrogen cooled heads are available with Dewar sizes ranging from 0.5 to 2.5 liters. For ease of maintenance and even longer hold times, a liquid-nitrogen auto-fill system is also available.

CCD performance is just one of the factors in deciding which scientific CCD camera will best fit a specific application. Others include host interfacing, cooling options, and what types of software development and/or end-user application software are available. While many scientific camera vendors only offer a few models, others offer complete camera system ranges with interchangeable camera heads and cooling options. Because such cameras are relatively expensive, systems integrators should carefully consider all these factors before deciding which camera to purchase.

Product Focus: Increasing sensitivity of scientific ccd cameras improves image capture

Increasing performance

Adding fiber

Cooled cameras

Company Information

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