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Cameras for TDI Applications
Princeton Instruments offers several rectangular format cameras that are ideal for TDI applications, which require high resolution in one direction only. This allows for a lower cost system with the same high dynamic range and performance of square format imaging arrays.
How TDI Works
In all TDI applications the charge on a CCD is moved during exposure. This movement is precisely coordinated with the movement of the camera or the camera's subject so that no blurring occurs. As rows of the image are shifted to the edge of the array, they are continuously read out and digitized. The number of lines read out can therefore be much larger than the number of rows on the CCD, and in principle can be unlimited. Although this method of operation is similar to image scanning using a linear CCD array, it offers 2-3 orders of magnitude more sensitivity by integrating light over a much larger number of rows. PI's Model ST-138 Controller provides full support for TDI operation of CCD arrays. It follows the timing provided by an external system, or it can provide timing to the external system. The WinView software acquires and displays images.

TDI Applications
Imaging of conveyor belts is a common industrial application of TDI. The CCD images the width of the belt and successive rows represent positions along the belt. By using TDI, a continuous image can be acquired from a moving belt. PI software breaks the incoming data into chunks and passes these on to other software for real time display and analysis. Extrusion machines produce a continuous flow of product that must be imaged for quality control. X-ray imaging of many types of extruded products allows inspection for internal voids, for example, as the product is manufactured. By using slotshaped X-ray illumination and observation, the ratio of scattered to direct X-rays can be reduced, providing substantially higher contrast images. Imaging of an electrophoresis gel after separation has been completed limits the molecular weight range that can be observed. The smallest molecules observable will be just about to exit the gel and the largest will be limited by incomplete separation. The molecular weight range of an electrophoretic gel system can be extended by imaging in TDI mode. In this mode, the CCD images the bottom of the gel, where bands are best separated. Images are acquired progressively as higher molecular weight bands migrate past the region imaged by the CCD. Maximum Line Rate 50 Hz 900 Hz 970 Hz 970 Hz 970 Hz

Large areas of the sky can be imaged by using TDI synchronized with the earth's rotation. In this configuration, the long axis of the CCD is aligned north-south, and the resulting image is an east-west strip. TDI can be used for imaging from aircraft, with shifting of the rows of the CCD synchronized with the movement of the airplane. In this case, the sensitivity provided by TDI can be useful for night vision or for imaging LIF from the surface. The field of view through a microscope is typically quite limited. By synchronizing CCD readout with stage movement, strip images can be acquired which extend the entire length of a slide. By acquiring multiple strips and overlapping them in software, a high resolution image of an entire slide can be obtained.

Special Applications

CCD Cameras for TDI
The table lists the TDI format CCD arrays supported by Princeton Instruments. In each case, the serial register of the CCD is aligned with the long axis of the imaging area. The maximum line rate is determined by the maximum pixel rate divided by the number of pixels per row. Princeton Instruments' Model ST-138 Controller can also operate conventional (square) CCD arrays in TDI mode. This can be useful when buying a CCD system for multiple applications. All of the CCD arrays described in the table can be provided in configurations coupled to image intensifiers, increasing sensitivity and providing gated operation. Both fiber coupled and lens coupled models are available. For more information, see the catalog section dedicated to intensified cameras. Many non-destructive inspection applications require economical high resolution Xray imaging. Many of the CCD arrays listed in the table can be used for X-ray imaging, with either lens or fiber optic coupling to a suitable X-ray stimulable phosphor. When using TDI cameras for this application, a fast decay phosphor should be used to prevent smearing due to phosphor lag. Tel: 609-587-9797

The Cost Advantage of TDI
CCD arrays made for TDI operation are typically rectangular in format (as shown in the table below). These arrays only need high resolution in one dimension to acquire two dimensional images with high resolution in both dimensions, and these cameras are considerably cheaper than arrays with high resolution in both directions. Manufacturer PI PI EEV Hamamatsu Hamamatsu Pixels 1752 x 532 1100 x 330 1024 x 256 1024 x 128 1024 x 64

Pixel Size, microns 15 x 15 24 x 24 26 x 26 24 x 24 24 x 24

Arrays available in Princeton Instruments rectangular format cameras. Note: Maximum line rates are for front illuminated CCD arrays only. 82 Princeton Instruments