A few weeks ago I mentioned that Photometrics had released a new camera, the Prime95B, featuring a back-illuminated sCMOS sensor with 95% peak QE. I got a chance to play with it last week, and I’m pleased to say that it performs as well as you would expect. We compared it to an iXon 888 EMCCD mounted on our CSU-W1 spinning disk confocal. We had purchased this EMCCD for imaging those samples that were too dim to get good images with the Zyla 4.2 sCMOS camera we also have mounted on the confocal. (You can see a sketch of how everything is configured in this previous post). For testing the Prime95B, we replaced the Zyla 4.2 with the Prime95B, allowing us to directly compare the Prime95B and the iXon 888.
Before I get to the data, however, what performance do we expect? To get a sense of what to expect I wrote a Matlab script that calculates the theoretical performance for a number of different cameras, using their quantum efficiency, read noise, and excess noise factor (for EMCCDs). You can get the script here. You can read more about how to calculate the signal-to-nosie ratio for a camera in this Hamamatsu white paper. Here, I’m ignoring the different pixel sizes of the various cameras by assuming that they all receive the same photon flux per pixel, as if the magnification had been adjusted to produce the same effective pixel size at the sample.
Theoretical performance of different cameras. Ideal is a theoretical ideal camera with QE=1 and no read noise. EMCCD assumes a high EM gain, ~200x; 82% QE sCMOS is a Flash4.0v2 or Zyla 4.2 Plus; 72% QE sCMOS is a Flash 4.0 or Zyla 4.2; ICX285 is a Coolsnap HQ2 or similar interline CCD camera.
As can be seen here, the Prime95B is expected to outperform all other cameras at high light levels. In particular, it even should outperform an EMCCD camera for light levels greater than about 2 photons/pixel. This calculation does not take into account fixed pattern noise or dark current. Fixed pattern noise is not so easy to quantify, but my sense is that it is a significant contributor to the noise of some sCMOS cameras. Finally, it’s interesting to see how much better all of these cameras are than the ICX285 cameras, which were state of the art only a few years ago.
Now on to the data. We ran a number of tests on different samples, comparing the Prime 95B to the iXon 888 EMCCD. In all cases, we saw what you would expect from the graph above – it’s very difficult to find a regime where the EMCCD outperforms the Prime95B. Here are some example images from a test slide with our laser launch at 0.1% power and a 50 msec exposure time. For comparison, the white level was chosen to saturate the top 0.01% of pixels in each channel and the black level was set to the mode of the background peak. All the images below are scaled like this.
EMCCD at 0.1% laser power; click to see full size image.<a href="http://nic.ucsf.edu/blog/wp-content/uploads/2016/07/EMCCD_light_50ms.tif"> Download the 16-bit tiff here.</a>
Prime95B at 0.1% laser power; click to see full size image. <a href="http://nic.ucsf.edu/blog/wp-content/uploads/2016/07/Prime95B_light_50ms.tif">Download the 16-bit tiff here.</a>
I think the Prime95B image looks better than the EMCCD, but this is a pretty bright image, even with our laser power at 0.1%. To further reduce the sample brightness, I inserted an OD1 filter in the filter turret, so that it attenuated both excitation and emission light, resulting in a 100x attenuation of the signal. This let us reduce the signal to where we were unable to see an image on the EMCCD camera.In the next set of measurements, we also adjusted the exposure times to compensate for the different pixel sizes of the two cameras. The iXon 888 EMCCD has 13 µm pixels; the Prime 95B has 11 µm pixels. The different in area is (13/11)2 = 1.4, so we used a 1.4x longer exposure time for the Prime95B to collect the same number of photons per pixel on each camera.
EMCCD, attenuated signal, 50 ms exposure. <a href="http://nic.ucsf.edu/blog/wp-content/uploads/2016/07/EMCCD_1xMag_50ms.tif">Download the 16-bit tiff here.</a>
Prime95B, attenuated signal, 70 ms exposure (= 50 ms * 1.4). <a href="http://nic.ucsf.edu/blog/wp-content/uploads/2016/07/Prime95B_70ms.tif">Download the 16-bit tiff here.</a>
Finally, we did one additional test. The 13 µm pixel size of the EMCCD is not quite Nyquist sampling with the 100x/1.4 lens we used to acquire these images, so we used a 2x magnifier to reduce the effective pixel size to 6.5 µm and then increased the exposure time on the EMCCD so that the photon flux per pixel was equivalent between the EMCCD and the Prime95B. This is probably the most favorable test case for the EMCCD.
EMCCD, attenutated signal, 2x magnification, 143 ms exposure time. <a href="http://nic.ucsf.edu/blog/wp-content/uploads/2016/07/EMCCD_2xMag_143ms.tif">Download the 16-bit tiff here.</a>
Prime 95B, attenuated signal, 50 ms exposure. <a href="http://nic.ucsf.edu/blog/wp-content/uploads/2016/07/Prime95B_50ms.tif">Download the 16-bit tiff here.</a>
Here it’s harder to decide which image is better; I would say they’re about equivalent. This was essentially what we found for all our testing, which included real experimental samples in addition to these test slides. The only times we could see the EMCCD outperform the Prime95B was when the image was barely detectable. This is consistent with the theoretical graph shown above, where the crossover between the EMCCD and Prime95B occurs at at SNR just greater than 1.
If you’d like to see more data from our demo, email me. I have a number of other demo images I can send you.
I’m very impressed with this camera. We did not see significant fixed pattern noise, and although the tests presented here are not quantitative, they are consistent with the theoretical expectations that this camera should outperform existing cameras for most signal levels. The improvement in camera performance over the last decade is quite striking, and there is not much more improvement that one can hope for. Manufacturers can continue to beat down the read noise, and perhaps increase QE by a percent or two, but that’s about it. The one thing I would like to see would be a version of this camera with more, smaller pixels for Nyquist sampling on low magnification objectives, and the ability to capture their full space-bandwidth product.
In conclusion, if you’re looking for a high performance camera, the Prime95B is definitely worth checking out. It lives up to the hype!