A collaboration between Andor Technology, Fairchild Imaging and PCO Imaging has improved CMOS image sensor (CIS) technology, leading to the development of a next-generation CIS, know as Scientific CMOS (sCMOS). The technology was presented at Laser World of Photonics (15-18 June 2009) and offers parameters suitable for scientific imaging, including low noise, rapid frame rates, and wide dynamic range. The three companies are each developing a 5.5 Megapixel camera containing an sCMOS sensor.
'Scientific CMOS is the first technology that combines extremely low noise with rapid frame rates – at 100fps the sensor will have noise of less than 3 electrons,' according to Dr Colin Coates, market development manager for imaging at Andor Technology. High quantum efficiency, wide dynamic range and a large field of view are also provided, without trade-off.
The rival CCD sensor technology is commonly used in scientific imaging and interline CCDs combine high performance with low cost for applications such as fluorescence cell microscopy and luminescence detection. Speaking to Imaging and Machine Vision Europe, Dr Coates explained that, with CCDs, all the charge is transferred serially through a common output structure and this takes time, especially when maintaining low noise. 'As such, there are certain trade-offs with CCD-based technology, in that sensors with low noise and wide dynamic range will not be able to support fast frame rates,' he said. Furthermore, it is difficult to combine small pixel sizes, which is important for high-resolution optical microscopy, with a wide dynamic range.
Electron Multiplying CCD (EMCCD) sensors go some way to address the mutual exclusivity of low noise and fast frame rates through amplification of weak signal on-chip. However, this comes at a price, and EMCCDs are expensive, especially in large format. The multiplicative noise also reduces the effective quantum efficiency of EMCCDs, and dynamic range can be limited when EM gain is applied.
Demanding bioimaging applications, such as live cell microscopy, often require fast frame rates to follow the dynamics of processes taking place in low light conditions. sCMOS is especially well suited to such conditions. Coates commented: 'We expect the technology to slot in with existing scientific imagers, rather than being an outright competitor technology – EMCCDs are still best suited to extreme low light conditions, where optimising signal to noise at very low photon fluxes is all that matters.'
CMOS has always had a lot of potential for use in scientific imaging, especially where speed is required. In CMOS images, the photon-induced charge is converted to voltage directly on each pixel and the sensor reads out in a highly parallel fashion through many ADC converters, as opposed to CCD sensors, where there are limited ADC converters. However, it has taken time to overcome the final performance hurdles typically present in CMOS technology, namely high read noise, high dark current and non-uniformity of response. 'sCMOS technology has been designed to overcome these barriers and render the technology suitable to high fidelity scientific measurement,' remarked Coates.
Speaking about the collaboration, Fairchild Imaging's Colin Earle commented: 'This announcement is a great moment for all three companies, who have come together in a true spirit of commitment to reach a shared goal.' Dr Gerhard Holst of PCO Imaging added: 'Scientific CMOS (sCMOS) technology stands to gain widespread recognition across a broad gamut of demanding imaging applications, carrying an advanced set of performance features that renders it entirely suitable to high fidelity, quantitative scientific measurement.'
Coates suggested that, aside from scientific imaging, there are other niche applications within machine vision and surveillance where using sCMOS sensors would be beneficial. For instance, photovoltaic inspection using electroluminescence requires sensors that can handle high throughput of panels in low light conditions.
