A sensor is a grid of light-sensitive pixels. Photons free electrons that accumulate in each pixel's well during the exposure (charge proportional to light). At readout, an amplifier and ADC convert each well's charge into a digital number (ADU). Limits and noise enter here: full-well capacity caps the brightest recordable signal, read noise is added every readout, quantum efficiency sets how many photons become electrons, and bit depth sets quantization fineness. This pipeline — photon → electron → voltage → ADU — is where signal and several noise sources originate.