Using CMOS Cameras for Light Microscopy
to provide comparable image quality to the CCD under these conditions. Te trend becomes more apparent when SNR values from the two technologies are graphed side-by-side with error bars corresponding to the standard deviation of the SNR across the thresholded cell images, as shown in Figure 3. Te differences in SNR between images from the two sensor types fall within the variation in SNR across each image, indicating almost indistinguishable SNR performance under these conditions. Another important parameter for scientific imaging
Figure 3: Bar graphs showing the increase in Figure 2 SNR values with exposure time. SNR values with error bars appear statistically indistinguishable for scientific grade CMOS and CCD images under these conditions.
sensors with larger pixels (>3.8 microns) would no longer adequately sample, leading to lost resolution and aliasing and thus should not be used. Performance comparisons using SNR are useful because
they provide quantitative numbers for camera evaluation under various realistic lighting conditions. However, the SNR needed to produce the image quality necessary for further data analysis or publication may not be known ahead of time. As a result, it is advantageous to compare additional complementary information, such as the quality of side-by-side images. Image quality is improved when the SNR is high, and the side-by-side images of CMOS and CCD cameras under these conditions demonstrate this correlation in Figure 2. It is apparent in Figure 2 that the image quality improves
with longer exposure times because the number of photons of light increases with collection time, producing higher signal and thus higher SNR. In terms of image appearance, it should also be noted that the cells in the 40× CMOS images appear slightly larger than in the 60× CCD images. When both sensors are used at the different magnifications where they still adequately sample, because the CMOS is a smaller pixel sensor that incorporates more pixels per square micron and both images have been scaled to approximately the same size, the images appear as displayed. Additionally, it is noticeable that the image quality is similar for the two image sets for the two camera technologies, in spite of several factors. One factor is that to achieve faster frame rates, CMOS pixels employ additional electronics to each pixel, which one might expect to reduce light collection efficiency. In both front-illuminated CCDs and some CMOS sensors, the fill factor (percentage of total pixel area that is light-sensitive) of each pixel is oſten improved through the addition of microlenses, micron-sized lenses on each pixel of the chip that collect light that would otherwise be blocked by electronic components and redirect it to the photosensitive pixel area. Although additional electronics are added to CMOS pixels, the implementation of increased pixel aperture and optimization of microlens design can compensate for what would be perceived as a potential loss in fill factor. Also, the CMOS has a significantly smaller pixel size. However, these factors are offset with the lower magnification and lower read noise of the Rolera Bolt scientific-grade CMOS (~3 electrons)
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is the dynamic range. Tis defines the ability of a camera to quantitatively image dim and bright signals in a single image. It is a function of a pixel’s ability to respond to incoming light prior to saturation and its read noise. Many commonly used scientific CCD cameras (based on Sony CCD sensors) have dynamic range of approximately 2000:1 (the full well capacity divided by the read noise, for example, 16000/8). It should be noted that the new Rolera Bolt camera has a dynamic range of approximately 4500:1 (~16000/3.5). Tus due to clever pixel design and low noise electronics, new scientific-grade CMOS sensors compete very well with standard CCD devices in terms of dynamic range.
CMOS Random Telegraph Noise What is also noticeable in the short exposure images in
Figures 2 and 4 is the difference in the noise in the CMOS versus the CCD. Te noise in the CMOS images is salt-and- pepper in nature with both bright and dark speckles. Tese speckles result from random telegraph noise, a noise unique to CMOS sensors and included in the read noise. Telegraph noise results from certain pixels on the CMOS sensor that are noisier than average as their signal fluctuates high and low (salt and pepper, respectively) around the average signal [5, 6]. Because of this noise, a number of pixels have noise levels that fall outside the expected Gaussian distribution seen in scientific CCDs. Tis difference in noise type can affect the behavior of the noise when operations such as averaging are applied.
Figure 4: Zoomed-in expansion of CMOS image, highlighting random telegraph noise apparent at short exposures.
www.microscopy-today.com • 2011 July
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