A BSI sensor, short for back-side illuminated sensor, is a type of digital image sensor that flips the traditional design so light hits the photosensitive layer directly, without having to pass through a maze of metal wiring first. This simple architectural change lets each pixel capture significantly more light, which is why BSI technology now dominates smartphone cameras, mirrorless cameras, and security systems.
How a BSI Sensor Works
Every digital camera sensor converts light into electrical signals using millions of tiny light-sensitive elements called photodiodes. In a traditional front-side illuminated (FSI) sensor, these photodiodes sit beneath layers of metal wiring that carry electrical signals across the chip. Light has to travel through those wiring layers before reaching the part that actually detects it, and some photons get blocked or scattered along the way.
A BSI sensor solves this by turning the whole structure upside down. The metal wiring moves behind the photodiodes, and light enters from the back of the silicon wafer, hitting the light-sensitive layer first with nothing in the way. The result is a cleaner, more direct path for photons. Peak light-capturing efficiency (known as quantum efficiency) reaches about 70% in BSI designs, compared to roughly 55% in conventional FSI sensors. In practical terms, a BSI sensor at the same pixel size captures about 36% more usable signal before noise becomes a problem.
Why BSI Matters for Small Pixels
The wiring obstruction problem in FSI sensors gets worse as pixels shrink. When pixels were large, the wiring blocked only a small fraction of incoming light. But as smartphone manufacturers pushed toward higher megapixel counts in tiny sensor packages, pixels shrank to 1.75, 1.4, and eventually below 1.1 micrometers. At those sizes, the metal layers in an FSI design block a meaningful percentage of each pixel’s light-gathering area.
BSI technology is what made extremely small pixels viable. It allows pixel sizes down to 1.1 and even 0.9 micrometers while maintaining usable image quality. Without BSI, the 48, 50, 100, and 200 megapixel sensors in modern smartphones simply wouldn’t work in the compact sensor formats those devices require. BSI also accepts light from wider angles (over 35 degrees, compared to about 25 degrees for FSI), which helps when pairing small sensors with the short, wide-angle lenses typical in phones.
Low-Light Performance Gains
Because BSI sensors collect more of the light that hits them, they produce stronger signals relative to background noise. This matters most in dim conditions, where every captured photon counts. Testing at 1.4 micrometer pixel sizes shows BSI pixels deliver about 2 decibels more signal-to-noise ratio than FSI pixels at both low light (20 lux) and moderate light (700 lux). That 2 dB difference translates to noticeably cleaner images with less grain, particularly in evening or indoor shooting.
The improvement is roughly equivalent to jumping up a full generation in pixel size. A 1.4 micrometer BSI pixel performs comparably to a 1.75 micrometer FSI pixel in noise-limited conditions. This means manufacturers can increase resolution (more, smaller pixels) without sacrificing the low-light quality users expect.
Manufacturing Challenges
Building a BSI sensor is more complex than building a traditional one. The process starts by fabricating the sensor normally, then physically thinning the silicon wafer from the back side until only a few micrometers of silicon remain above the photodiodes. The photo-electric conversion layer in a typical sensor is about 3 micrometers thick, though BSI designs can vary from 3 to 10 micrometers depending on the application. Achieving uniform thinning across an entire wafer is one of the most critical manufacturing challenges, since uneven silicon thickness creates inconsistent light sensitivity across the sensor.
Other production hurdles include alignment precision (misalignment between the front and back layers can range from 0.6 to 2 micrometers) and managing extreme temperature gradients during laser annealing, where the back surface may exceed 800°C while components just 4 micrometers away must stay below 400°C to avoid damage. These demands make BSI sensors more expensive to produce than FSI sensors, which is why FSI still appears in some cost-sensitive applications like basic automotive rear-view cameras, where a 15% savings on component costs outweighs the 40% sensitivity penalty.
Stacked BSI: The Next Step
Standard BSI sensors moved the wiring behind the photodiodes. Stacked BSI sensors take this further by layering additional processing components, including a dedicated image signal processor and fast memory, directly into the same silicon package beneath the light-sensing layer. This vertical integration dramatically speeds up how quickly the sensor reads out data from every pixel.
Faster readout enables several practical benefits. A fully electronic shutter eliminates the rolling shutter distortion that warps fast-moving subjects. Burst shooting rates increase because autofocus can recalculate between frames more quickly. Stacked designs now exceed standard BSI sensors in focus speed, accuracy, and subject recognition, which is why flagship smartphones and professional mirrorless cameras increasingly use stacked architectures.
Where BSI Sensors Are Used Today
BSI has become the default technology for most image sensors. Backside-illuminated architectures captured about 45% of image sensor revenue in 2025, with stacked BSI variants growing at over 7% annually. Sony, Samsung, and OmniVision are the dominant manufacturers, competing through increasingly sophisticated stacked BSI designs with built-in AI processing capabilities.
Virtually every smartphone released in recent years uses a BSI or stacked BSI sensor for its main camera. The technology also appears in mirrorless interchangeable-lens cameras, action cameras, security and surveillance systems, medical imaging devices, and automotive sensors. FSI sensors persist mainly in applications where cost is the primary concern and image quality requirements are modest. The concept itself dates back to the mid-1970s, when the first back-illuminated devices were developed to improve the blue-light response of broadcast television cameras, but it took decades of manufacturing advances before BSI became practical for mass-market consumer products.