SCIENTIFIC HIGH-SPEED CAMERAS
OPTICAL PRODUCT is rapidly written to onboard RAM that exists as a circular memory buffer under the first-in, first-out( FIFO) method. This technique permits users to record up to tens of gigabytes per recording( generally seconds of record time), with available image systems trending toward terabyte-sized onboard RAM.
TECHNOLOGICAL PROGRESSION OF SENSOR THROUGHPUT One of the most critical performance metrics in the high-speed camera market is sensor throughput, which is the product of the max sensor resolution and the max frame rate( at the resolution), expressed in gigapixels per second( Gpix · s-1). To date, high-speed sensor offerings generally range from either 1, 4, or up to 9 Mpix. Over the past decade and a half there has been rapid progress, as shown in Figure 2, where clear improvements in throughput have been made for both 1 and 4 Mpix cameras systems. To give a clearer picture, the 40- and 75- Gpix · s-1 systems can achieve frames rates of ~ 10 kHz at 4 Mpix and ~ 76 kHz at 1 Mpix, respectively. Highspeed sensors can also generally be‘ windowed’ to achieve faster frame rates. A 75 Gpix · s-1 system can achieve frame rates of ~ 1.75 MHz at 41 Kpix( i. e., 1280 × 32).
BSI TECHNOLOGY & SENSITIVITY The high-speed camera industry, in part, has recently transitioned to Backside Illuminated( BSI) sensor technology, over the traditionally implemented Front-side illuminated( FSI), see Figure 3 for comparison. In short, BSI sensors are designed with the photoactive layer un-occluded by the metal layer, enabling more efficient light collection. Going to BSI was recognized as a requisite for the continued development of camera systems with exceedingly fast framing rates( and low
Figure 2: This graph compares the sensor throughput, measured in Gigapixels per second( Gpix · s-1), of two high-speed sensor platforms: one with a 1 Mpix resolution and the other with a 4 Mpix resolution since 2009.
integration periods) due to the fact that as the integration gets shorter there is a direct linear reduction in light-gathering ability( i. e., less signal, lower SNR). Thus, the large improvement in Fill Factor, going from 50-60 % to now upwards of > 90 % with BSI was critically important. Note: Pixel response to incident light is directly proportional to pixel area × quantum efficiency × fill factor. The improvement
Figure 3: Comparison of FSI( Front- Side Illuminated) vs BSI( Back-Side Illuminated) Sensor Architectures. In the FSI architecture( left), photons( represented by orange arrows) must pass through the metal wiring and other layers before reaching the photodiode, which leads to light loss due to obstruction and reflection. In contrast, the BSI architecture( right) allows photons to enter directly into the photoactive region of the pixel, improving light capture efficiency.
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