customers was looking at the circadian rhythm of plants, where every half an hour they would take a 10-minute image. So, in the future, while the majority of cameras will switch to CMOS, I think CCDs will still have a place in life science imaging.’ Dark current on sCMOS is roughly 0.5


electrons per second, which is higher than a CCD camera, Mohindra stated. However, sCMOS can reach one electron of read noise – the minimum amount of signal the sensor can measure – while an optimised CCD will have three to four electrons read noise.


Cell image taken by spinning disk confocal microscopy


performance factor – microscopy techniques such as spinning disk, confocal, super resolution, and single molecule fluorescence all tend to be very low-light applications where the sensitivity really is crucial,’ he added. Te backside illuminated Prime BSI camera


provides about a 30 per cent improvement on top of standard sCMOS in terms of sensitivity, according to Mohindra. It gives 100fps with effectively one electron noise. Mohindra commented: ‘I don’t think


CCDs will disappear entirely, but they are being pushed into niche applications.’ He said that CCDs are relegated to basic microscopy cameras where they are an inexpensive option. Te other place where CCDs have a specific niche is for luminescence imaging, where the camera monitors the natural glow from cells. To image, luminescence needs an acquisition time of minutes, potentially half an hour; here, CMOS technology is less good because the background noise tends to be quite high, according to Mohindra. ‘Tis might not always be the case and CMOS technology could eventually catch up with CCDs, but at the moment CCDs are still better for long image acquisitions,’ he said. ‘Tere are CCD cameras that will do


effectively one electron of background noise per hour,’ Mohindra continued. ‘One of our


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Life in the future Holst noted that CMOS can do things that are not easy to do with CCDs. PCO offers time- of-flight (ToF) CMOS sensors with two or four readout sites per pixel that can be modulated. With an external signal, the charges generated in each pixel can be directed to different readout positions, which means the incoming modulated light can be separated. PCO’s customers use this sensor in the


frequency domain for measuring fluorescent lifetimes. Te sensor gives a double image; the modulation signal is then shiſted and a second measurement made. Tis gives four images corresponding to different time positions in the modulated signal. ‘If you assume a sinusoidal modulation,


Backside illuminated [sCMOS] provides about a 30 per cent improvement [in sensitivity] on top of standard sCMOS


then you get the information of the face angle at zero degrees, 90 degrees, 180 degrees, and 270 degrees, and with that you can reconstruct the sinus and get out the information of the phase angle, as well as the modulation depth,’ explained Holst. ‘From there on you can back-calculate the fluorescent lifetime. Tat’s a relatively new application using these slightly older ToF sensor principles.’ Elsewhere in life sciences,


digital pathology for automated tissue analysis is now more widely accepted (see ‘Automated pathology’), while, in fluorescence


imaging, there has been a push to move further out into the infrared, according to Mohindra – in the infrared there is less phototoxicity and a microscope can image deeper into tissue. Tere is still a limit to what a silicon sensor can image in the infrared, but he noted that ‘with the newer sCMOS cameras we see 50 per cent efficiency at 900nm, which has been quite an improvement.’ O


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