Samsung Electronics announced ISOCELL technology for its CMOS sensors earlier this week. Previously Samsung were using Sony’s BSI-CMOS sensors (popularly known as Exmor-R sensors) for many of their flagships. BSI-CMOS is an excellent technology. However Samsung claims that the new ISOCELL “substantially increases light sensitivity and effectively controls the absorption of electrons, resulting in higher color fidelity even in poor lighting conditions.”
Samsung hasn’t mentioned it anywhere but ISOCELL might stand for ISOlated CELLs. The “cell” here refers to the individual pixels on a camera sensor. In simpler terms a 12MP camera has 12 million cells (or pixels). I will discuss about this nomenclature later in the post.
Samsung’s press release mentions 3 key advantages of ISOCELL over BSI:
- Reduction in Pixel Crosstalk by 30%
- Increase in Full Well Capacity (FWC) by 30%
- 20% wider Chief Ray Angle (CRA)
Ideally a “perfect pixel” should have the following characters:
- Lossless collection of light
- Lossless conversion of light into electrical energy
Lossless collection is impossible to achieve in real world because some amount of light will be reflected or absorbed by the structures above the light collecting surface. BSI technology significantly reduced this loss (compared to older FSI sensors) by adopting a different sensor structure, thereby maximizing the amount of light reaching the light collecting surface or photodiode.
Even with BSI the light collection is not lossless because there still will be some loss from the microlens (on-chip lens) and filter. Along with this there occurs crosstalk leading to further loss of light collection.
Pixel crosstalk refers to the interactions between two adjacent pixels. This interaction occurs because of a number of things
- A light ray can strike the sensor at such an angle that it illuminates two adjacent pixels at the same time
- Electrons may “leak” from circuitry of one pixel to the circuitry of adjacent pixels
- Many other technical things that I wouldn’t like to harass you with; so here is a simpler explanation
The need for ISOCELL comes because smartphones are becoming thinner every year and manufacturers want smaller sensors to put inside their anorexic phones. Imagine a sensor as a building with lots and lots of rooms where each room is analogous to a pixel. Now if you and a few other good people were living inside those rooms, your neighbors wouldn’t normally bother you unless the rooms were so tiny that you could hear what your neighbors were talking. This is exactly what happens when pixel size is too small. You won’t be able to sleep if someone is blasting his music system next to your bedroom right?
Unlike rooms in a building, we cannot increase the “wall thickness” between pixels. What we can do is make your room sound proof. This is what Samsung claims to do with ISOCELL. It is like providing better insulation between pixels so that pixel crosstalk is reduced. Hence the name “Isolated Cells”. The pixels in ISOCELL sensor are better isolated than BSI sensor without any increase in pixel size. Samsung claims a 30% reduction in crosstalk. Due to interaction of signals between pixels there occurs:
- Less color accuracy
- Less sharpness
ISOCELL technology claims improved sharpness, color accuracy and less noise in low light/high ISO situations because of 30% reduction in crosstalk.
Full Well Capacity
The building with rooms analogy is not adequate to explain FWC. So I will use a different analogy this time. Suppose there are a number of bowls(pixels) in a tray(sensor). The capacity of the bowl is FWC. If you were to fill one of the bowls with water, you can only fill it up to the brim, after which water will spill and flow into adjacent bowls.
Technically FWC is the amount of charge an individual pixel can hold before saturating. Samsung claims increase the full well capacity (FWC) by 30 percent which leads to greater dynamic range. This means a high contrast scene will have more details in ill lit shadows and bright skies and less of blown highlights. Current cameras achieve this by using HDR mode to improve dynamic range
In the above sample by Samsung, we can see improvement in Dynamic range over BSI
Chief Ray Angle (CRA)
I won’t go into in-depth discussion about CRA as it doesn’t affect image quality directly. CRA is a property of the lens and not the sensor itself. Samsung claims that ‘an imager designed with ISOCELL can feature a 20 percent wider chief ray angle (CRA), reducing the height of the camera module.’
“A ray that starts at the edge of the object, and passes through the center of the aperture is called Chief Ray. The angle it makes with the optical axis is called CRA” – Wikipedia. Here is a self-explanatory diagram which isn’t 100% accurate, but explains the relation between CRA and height of camera module
Samsung is currently working on a 1/4″ 8MP sensor with 1.1 micron pixels and ISOCELL technology. It will be available for sampling in Q4 2013 and we will see the first products with this technology in first half of 2014 hopefully. I would like Samsung to use this technology with larger 1/3″ or 1/2.3″ sensors as well.