Frequently
Asked
Questions

Especially on CMOS sensors, each pixel active area is surrounded and surmounted by circuitry and metal connections responsible for the image readout: this reduces the amount of light that can be successfully detected.
If the light rays are not perpendicular to the sensor surface, they could even be reflected by near interconnections on the metal layers of the sensor chip.
Almost all modern image sensors are coated with an array of micro-lenses. These lenses gather incident light and focus it on the sensitive area of the pixel, thus increasing sensitivity.