This page contains a Flash digital edition of a book.
Choosing a Microscope Camera


Table 1 : Comparison of typical cooled CCD and sCMOS cameras.


Parameter Pixel size


Readout speed Readout noise Dynamic range


cCCD 6.5 µm × 6.5 µm


Number of pixels 1392 × 1040 Peak QE


3,000:1


70% at 600 nm Up to 17 fps 2e


sCMOS


6.5 µm × 6.5 µm 2048 × 2048


72% at 560 nm Up to 100 fps 2.5e


30,000:1


Note: sCMOS has much faster acquisition speeds and a much larger dynamic range, while the cCCD has lower read noise.


Figure 1 : Schematic diagram of a CCD sensor with an array of photodetectors arranged geometrically. For CCDs the charge is read out sequentially one photodetector at a time, which limits the frame rate.


the trade-off s for this increased speed, however, is that there is increased pixel-to-pixel variability since each pixel has its own separate amplifi er circuit. Even just fi ve years ago, choosing between a CCD and CMOS camera was fairly simple. One chose a CCD for high resolution or a CMSO to capture rapid events. With current developments, particularly the advent of scientifi c CMOS (sCMOS) systems, the choice is not as clear-cut. One needs to delve deeper into the specifi cs of each camera system in order to match the system to your needs. Unfortunately, comparisons can be frustrating because there are so many aspects of the


Figure 2 : Schematic diagram of a CMOS sensor. Here the charge is read out and amplifi ed simultaneously for each individual sensor. This parallel approach allows much faster readout.


26


camera response that could be evaluated. In my experience, however, there are really only a handful of features that need to be considered in order to make a rational decision about which camera system best matches your application. Pixel size . One of the key components to consider is the size of the individual photodetectors (pixels). T is determines the spatial resolution limit of the sensor. For a given magnifi - cation, smaller pixel size allows fi ner image features to be captured. However, this is not the only determinant of spatial resolution. As individual pixel size gets smaller, its photon capacity (how many photons before it becomes saturated) decreases. T us, smaller pixels typically have lower signal-to- noise ratios and lower dynamic range [ 4 ]. Cooling the camera below room temperature can help reduce noise in cameras with smaller pixels. Today, most high-resolution microscope cameras, cooled CCD (cCCD), and cooled sCMOS with pixel size around 6.5 µm, will provide full microscope resolution with good dynamic range and signal-to-noise characteristics [ 5 ]. Table 1 compares some key attributes of recent cCCD and sCMOS cameras with a sensor size of 6.5 µm 2 . Recently some newer cCCD cameras have been introduced with a sensor size of 4.5 µm and reasonable signal-to-noise ratios. In general, systems using CMOS technology have a little more noise than CCD systems of equivalent sensor size. In practice, however, I have not found the diff erence to be very noticeable. Of course, specifi c tests always should be run to determine the highest spatial resolution the sensor is capable of obtaining under your specifi c conditions. Resolution is oſt en stated as the total number of megapixels or the pixels per inch of a system. I fi nd these parameters much less useful than the individual pixel dimensions. Megapixels and pixels per inch do not provide the critical information regarding the smallest object in your sample that can be eff ectively captured. T e number of megapixels is not totally useless; it does indicate how large a fi eld of view can be captured, but it provides little information about the spatial resolution within that fi eld of view. A pixel’s per inch value is derived from the pixel size, so you can calculate sensor size from pixels per inch and the number of pixels along one direction of the sensor. But why not just look at the sensor size directly for comparing maximum achievable spatial resolution between diff erent camera systems?


www.microscopy-today.com • 2017 September


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76