Roper Scientific's Virtual Chip Mode

A novel CCD readout technique that delivers 16 bit images at frame rates in excess of 600 fps.

In operation, images are continually piped down the CCD at extraordinarily high frames per second (FPS). The mini-frame transfer region is defined by an ROI as illustrated in Figure 1. The charge from this ROI is shifted under the frame-transfer mask, followed by a readout cycle of an ROI-sized region under the mask. Since the ROI is far from the serial register, the stored image is just shifted repeatedly with the readout and the first few images collected will not contain useful data. After the readout period, the next frame is shifted under the mask and another ROI sized frame is read out. The net result is a series of images, separated by spacer regions, streaming up the CCD under the mask. Refer to Table 1 for a listing of virtual chip sizes with their respective readout times and frame rates per second. Masking can be achieved by applying a mechanical or optical mask or by positioning a bright image at the ROI against a dark background on the remainder of the array. The online timing calculator reports the readout time that will result with the current virtual-chip parameter values like subarray x and y dimension, the binning values, the exposure time and the shutter compensation time.

Virtual Chip Readout Speeds for MicroMAX:512BFT
 ROI (NxN)  1MHz (msec)  1MHz (FPS)  100kHz (msec)  100kHz (FPS)
164 29.8 33.6  287 3.5
96  10.9 91.7 102 9.8
84 8.65 115 79.8   12.5
64 5.26 190 47.6 21.0
56 4.22 237 37.4 36.7
47 3.08 324 26.9 37.2
36 1.97 507 16.6 60.2 
30 1.51 662 12.2 82.0 

Figure 1. Virtual chip functional diagram.
 Virtual chip functional diagram

System requirements for the Virtual Chip option :

Tips on masking the CCD
If mechanical masking is used, the mask can be a static one (fixed dimensions) in which case, multiple masks should be made to accommodate a variety of imaging conditions. Alternatively, a more flexible mask can be manufactured by taking two thin metal sheets with a square hole the size of the exposed region of the CCD cut in the center. This would be 512 × 512 pixels at 13 microns per pixel = 6.7 mm × 6.7 mm for the MicroMAX 512 BFT. These masks should be anodized black to prevent reflections in the optical system and they should be very flat. These two sheets can then be slid relative to one another to achieve any rectangular shape required. The sheets should be placed flat in the optical plane and their openings should be centered on the optical axis. Ideally they should be able to move with an accuracy of 2-3 pixels per step (30-45) in the X and Y directions.

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