Back Lit Sensors

With Sony's new PXW-Z190 and PXW-Z280 sporting the new Back Illuminated Sensors you'd be forgiven for thinking it's a new technology. In fact, Back Lit sensors have been around for quite a while, around 10 years, it's just that manufacturing has been inconsistent and expensive, until now! The name is a little misleading but makes sense when you know more. Now logic would indicate that the photo sensitive part of a sensor to be at the front, nearest the light; but that's not how sensors work. All the transistors and connections sit in front of the Photo Site meaning the sensitive bit of the sensor is at the bottom of a well. Light is lost when it bounces off the side or doesn't make it all the way. Micro Lenses have been added over time to focus the light and make the sensors more efficient. Why is it built like this? Because the photocathode layer is traditionally one sided. So, simply a back lit sensor is where the backing of the Photo Sensitive later has been thinned so it can be pointed directly at the subject. But it's not that simple! A normal Silicon layer would be 1mm thick. To allow the light through it needs to be 5 to 10 Microns thick. This is the tricky bit that has taken nearly 10 years to perfect. The benefit? Sensors that are about a stop more sensitive and much improved low light performance. e.g. a Sony PXW-X200 has minimum illumination of 0.09 Lux, the PXW-Z190, 0.01 Lux.