IMAGE COMPRESSION


Compressing 16-bit images from sCMOS sensors


Martin Schwarzbauer, at PCO, and Bernd Jähne, at HCI, Heidelberg University, discuss the need for quasi lossless image data compression from 16-bit images to 8-bit or less


F


or state-of-the-art CMOS image sensors a digital resolution of 8-bit is no longer sufficient and, especially for scientific


CMOS (sCMOS) image sensors, nearly 16-bit is necessary. Te reason for the larger dynamic range is, on the one hand, the very low standard deviation of the dark noise, with just about 1 electron, while, on the other hand, the full-well capacity is in the order of 30,000 electrons or higher. In the near future image sensors will be available with the same low dark noise but an even higher full-well capacity, so the quantisation of 16-bit would no longer be sufficient. Higher resolution requires more storage space, the need for faster data interfaces caused by higher bandwidth, and image processing algorithms may require more processing time, which has an impact on real-time processing applications.


Parameter Direct (uncompressed)


After compression and decompression


System gain corrected


Quantum efficiency


0.454 0.385


0.455


Figure 1: Standard deviation (σg) of the temporal noise for the PCO Edge 5.5 sCMOS camera measured in fast-scan mode


The compression algorithm Image sensors with such a low dark noise are dominated by photon noise. Terefore, the variance of the noise is about equal to the mean number of accumulated charge units. Te standard deviation of the noise consequently increases with the square root of the mean digital value. Any equidistant quantisation is a clear mismatch in this situation. Te quantisation must be fine


Standard deviation of dark signal (DN)


3.91 4.36


4.36


Standard deviation of dark signal (e-


)


1.97 1.86


2.21


1.975 2.334


1.975


96.32 96.15


96.15


System gain (DN/e-


)


enough to resolve the small random fluctuations at low signal levels, but it is much too fine at high signal levels. As an illustration, the standard deviation of


the noise of a PCO Edge 5.5 sCMOS camera is shown in Figure 1. It covers more than two orders of magnitude. It is about four for the dark image, but more than 300 close to saturation. Tus, the quantisation is more than two orders of magnitude too fine in the bright


Dark signal (DN)


Maximum


signal-to-noise ratio


185 172


172


Table 1: EMVA 1288 measurement results for the PCO Edge camera. Upper row: original linear 16-bit results; middle row: results from 16-bit signal after compression to and decompression from non-linear 8-bit signal; lower row: after correction for system gain according to additional quantisation noise


22 Imaging and Machine Vision Europe • December 2017/January 2018 @imveurope www.imveurope.com


Dynamic range


12,570 11,530


11,530


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