## 3 Design Approach Details
### The Process of Creating a Digital Image  
The fundamental basis of creating a digital image is light. Without light, an image can never be created. Light can synonymously be called \"information\" when it comes to photography. When light from a subject reaches a camera, it has to travel through the glass elements of the lens. These elements decide how much light will enter, the focal length of the frame, and where these rays will be focused. For a sharp image to be produced, a real image must be formed on the surface of the sensor. Image sensors consist of photo sites (pixels) that convert the incoming light into charge/electrons. This charge is accumulated in the pixel well before being transferred for further processing. Depending on the nature of the sensor (CMOS or CCD) [8], the charge is converted into voltage and then amplified using Op-Amps. The voltage obtained here is ultimately the data we need. These data are further processed to become an image, but the image's digital significance starts when a voltage value is assigned to it. When the exposure is concluded, all the electrons in the pixel well are boosted by an op-amp at a factor directly proportional to the ISO value set in the camera. The higher the ISO, the more the amplification occurs. ISO is a measure of the sensitivity of a camera's sensor to light. The term ISO is an abbreviation for the International Organization for Standardization. The higher the number of the ISO, the more light is recorded by the camera's sensor. This increases the brightness of the image. Then, the boosted signal is converted from analog to digital. CPU within the camera then processes this digital image and stores it in the memory card. This process is illustrated in Figure 11.  
However, this does not conclude the process. When the light is being converted to voltage, various disturbances occur in the system; these disturbances are called \"digital noise\". A small amount of current flows in the photosensitive pixel even when no incident photon is hitting it. This current is called a dark current, and it registers as unwanted noise. This noise is the reason to consider a dark frame for calibration in the stacking process. A dark current is the primary reason to have noise in the digital image [9], and it is by virtue of temperature (Figure 12). Figure 11: Illustration of a digital image with a Sony CMOS ISO invariant camera. The signal from dark current = DC * time, and the noise is (DC * time)\({}^{0.5}\). Photons are considered to be discrete. Due to the discrete nature of the electric charge, digital images also suffer from shot noise. The typical read noise in a CMOS is on the order of 6 to 16 electrons. During the process of analog-to-digital conversion, amplification, and imperfections in the electronics, there will be \"readout noise\". CMOS-based cameras can provide a much lower noise floor when compared to CCD cameras. Vital information can be hidden due to the high noise profile, and hence lower noise is required to capture such images. There are two types of reading noise, upstream read noise and downstream read noise [10]. Upstream read noise is the noise that is induced during the hour before the analog amplification process; for example, the noise that is produced due to the sensor heating up during long exposure photography is upstream read noise. Downstream read noise is the noise that is induced after the amplification. Analog-to-digital conversion and image processing, for example, produce downstream read noise. Since upstream read noise is induced during the amplification using higher ISO, it will also increase the read noise and result in a grainy image. However, downstream read noise remains unaffected by ISO amplification. Therefore, it can be seen that if a camera has very low or zero downstream read noise, it would not really matter whether the ISO is increased for amplification while taking a photograph or whether the exposure is increased later during post-processing, since the same upstream read noise is being amplified in both cases.