There are two solutions to this problem. First, the circuitry can be made smaller and the photo sensors bigger. But there is a limit to this reduction - and an escalation in manufacturing cost - so there will always be space between the sensors. To counteract this, millions of microlenses are used, one over each photo sensor. The microlens covers the photo sensor and the circuitry. Rays of light hitting the edge of the micro lens, which would normally be wasted, are now focused on the photo sensor. The effect is to increase the sensitivity of the sensor unit.