## 4 Imaging System Characterization and Performance
### Photonic to Digital Relationship  
CMOS (or CCD) sensors are based on metal-oxide semiconductor technology, which means that field effect transistors are employed to gate the charge coming from the semiconductor, i.e., the photosensitive area. The photo-site material (semiconductor), used in most modern cameras operating in visible light, is silicon. Silicon is used because its valance to conduction energy gap, or bandgap, of 1.1 eV is near ideal for wavelengths in the visible or near infrared. This means that when a photon of higher energy (above 1.1 eV, or below 1127 nm) hits the surface of the silicon, that photon is absorbed in the silicon and generates a charge. The silicon is doped with different materials with positive and negative charges to create a diode-like structure. The pixel charge is related to the amount of photons hitting the photosensitive area, and being converted by the semiconductor. The conversion efficiency is generally referred to as quantum efficiency (QE). The fill factor is another key information to assess overall photonic conversion efficiency; it represents the fraction of the photosensitive area compared to the effective pixel area. Microlenses are commonly used to collect the light signal and direct it onto the photo-site, effectively improving the fill factor. CMOS sensors read the charge out under the form of voltage or current directly on the pixel, next to the photo-site. This allows parallel readout, as opposed to the sequential readout used in CCD sensors, a major advantage when it comes to speed. Several transistors are employed to perform the necessary operations to read and move the charges, as well as commonly available features in recent cameras such as gate transfer or global shutter. Global shutter means that all pixels are exposed at the same time, which is important in high-speed applications [16], including the current study. After the charge has been converted to voltage, it is further amplified, and sent to the on-chip analog-to-digital converters (ADC). Each photonic-derived voltage coming from a pixel is converted into digital format based on the level and ADC bit-depth (e.g., 8-bit). Readout rate is in principle related to the number of ADCs, as well as their operating frequency. The camera sensor used in this work features 64 ADCs, all of which can be operated at rates up to 600 MHz, for parallelized and fast conversion. The digital information is then transferred to the computer or directly saved to a high-speed storage system. As commented earlier, the high readout rate of the camera used in this study (18.9 Gbit/s) requires advanced hardware and data handling methods to be sustained. To relate the photonic energy measured at the probe volume to a digital level, and estimate the light losses through the microscope objective, fundamental characteristics of the sensor are needed. The camera sensor is specified to feature a full well depth (charge) of 13.5 ke-, a sensitivity of 4.64 V/lux.s, and a conversion factor of 0.075 bit/e\({}^{-}\) (for 10-bit digitization). Other parameters such as dark noise, dark current, and fixed pattern noise are insignificant compared to the light-induced charge or the measured digital quantities at the current camera settings. The various noise sources are included as a global noise level under the current camera settings and reported in Figure 6. The absolute (or effective) QE combines the fill factor (including the microlens) and the conversion efficiency of the sensor from photon (light) to charge, i.e., the QE. The absolute QE of the sensor at the LED illumination wavelength (455 nm) is on the order of 43%, whereas the peak QE is near 47%, at 550 nm wavelength. As expected for visible camera sensors, the QE is much lower in the near-infrared wavelength range of the VCSEL, with values on the order of 16% according to the sensor specification document. Based on the digital resolution of the system, the area of a pixel at the object plane is 6.25 um\({}^{2}\).