LBCAST - Comparison Between LBCAST and CMOS Photo Sensors

Comparison Between LBCAST and CMOS Photo Sensors

The main differences between LBCAST and CMOS-based sensors appear to be those given below:

• LBCAST divides the photosites to be read out into two channels by colour, red and blue photosites—making up 50% of the total number of photosites between them, as in CMOS and CCD colour sensors—accumulate using one read-channel, while green photosites—which make up the other 50% of total pixels, as in CMOS and CCD—accumulate in a dedicated channel.

This division is to speed reading, and the separation of green pixels, to which the human eye is most sensitive, reduces noise artifacts which might otherwise be introduced by residual electrical charge in the accumulation circuitry, acquired previously from reading a pixel of a different colour.

Red and blue are significantly less important in human sight, and so presumably a decision was made not to keep separate read channels for the two colours, in order to simplify the circuit design for practical use -- at least in the described version of LBCAST technology.

• Each photosite in LBCAST uses a single JFET transistor in place of two MOSFET transistors used for the separate tasks of photosite read-out selection and signal amplification, in CMOS technology. In total, CMOS uses four transistors per photosite, where LBCAST manages with three.

• Charge to be (re-)distributed between components in the sensor during use are channeled via the LBCAST lower layers, where CMOS channels this charge over the surface layer. This difference is also claimed to reduce the presence of noise artifacts.

• The simpler circuitry required translates into more space for light to be received, since wiring and opaque masking takes less space.

• The simpler circuitry required translates into fewer layers of material in the silicon chip meaning that tangential light requires less correction in order to traverse to the depth at which the photons result in signal, simplifying the need for lensing at each pixel or photosite, and therefore theoretically, attaining greater image uniformity.