Parasitic Light Sensitivity Limits in Stacked Backside Illuminated Global Shutter Image Sensors
Stacked BSI global shutter sensor parasitic light sensitivity depends on silicon depth, metal shield alignment, and active illumination wavelength.
Chief Ray Angle
Optical Characteristic
Ray bundles passing through an optical system reach the image plane at a specific inclination determined by the aperture stop position relative to the focal plane. Lens designers define chief ray angle as the angle formed between the central ray of an oblique beam and the optical axis at the sensor plane. Solid-state sensors rely on uniform light entry, yet steep angles degrade quantum efficiency toward the array edges.
Sensor Degradation
Pixel microlenses direct incident rays into active silicon areas, but high angular divergence causes light to strike adjacent metallization layers instead of the photosensitive region. When the chief ray angle of an optical lens exceeds the angular acceptance curve of the image sensor array, spatial shading and crosstalk increase across the periphery. Crosstalk alters color fidelity in color filter arrays through spectral contamination between adjacent pixels.
Micro-lens shifting offsets the physical position of upper optical layers relative to underlying photodiodes to match expected ray angles.
Qualification Standard
Bench testing uses collimated light sources mounted on goniometric stages to measure relative response as illumination angle varies. Measurement systems sweep incident angles in increments while recording output amplitude at central and corner pixel groups. These profiles generate an angular response envelope, indicating the maximum allowable mismatch before image degradation occurs.
Verification procedures compare recorded efficiency profiles against nominal optical prescriptions supplied by lens manufacturers.
Operational Boundary
System matching remains limited by chip size and optical format.