LOFIC stands for Lateral OverFlow Integration Capacitor and is a different way of designing pixel circuitry that allows them to capture more light. Conceptually, it stems from a similar idea to the dual conversion gain sensors we’ve seen over the past 12 years, but the benefits it promises appear even greater.

A drip-fed explanation

Fossum, now a Professor at Dartmouth College, uses the metaphor that capturing light is like collecting rainwater in a series of containers or buckets.

We’re going to slightly complicate the metaphor to encompass the concept of “conversion gain,” which is the relationship between the number of photoelectrons coming from the light-sensitive photodiode in each pixel and the size of the electrical voltage signal that’s then read out (measured in Volts per electron). 

In CMOS sensors, photoelectrons captured in a small bucket (smaller capacitance) give a proportionally bigger signal when read out than the same number would, if captured in a larger bucket (larger capacitance).

Abby Ferguson

So a small bucket is well suited to low light (fewer photoelectrons) but you need a much larger one for capturing a lot of light, so that you don’t overfill the bucket.

In a LOFIC sensor, instead of one, we have both a small and a large bucket. The charge from the photodiode starts to collect in the small bucket, but if it exceeds its capacity, it overflows into the second, larger bucket. (Fossum called this “cascaded integration” in an early patent)

When it comes to readout, we measure the small bucket first. Because the charge was captured in a small bucket (with high conversion gain) we get a large signal that overwhelms any other noise that might get added later in the process. This gives us nice clean shadows and low-light images.

In a LOFIC sensor, we can then connect the two buckets together, combining their capacity (so, lower conversion gain) and adding in any overflowed charge. We then read out this combined signal. If we didn’t capture very much light, we can just use the first readout result. But if the small bucket was full, we can then use the second, combined readout, instead, giving us much more capacity for highlights.

Unlike the dual conversion gain system, Fossum points to the key benefit: “you don’t have to choose in advance.” And, as he points out, there’s no reason this system has to be limited to two buckets.

Why it matters

In practice, this means sensors that can tolerate a lot more light. And its implementation in relatively small, high-resolution sensors, suggests that space constraints aren’t a problem, at least not once you can use stacked sensor fabrication.

In the Osmo 4P we’ve seen DJI claim 17 stops of DR, which is an extraordinary number for a Type 1 sensor. This puts its DR on a par with much larger sensors (though doesn’t mean its image quality will match them). In its implementation, you can see how the additional dynamic range can be used.

The 4P’s camera that uses LOFIC technology is the only one that needs to use the new D-Log2 profile: a profile designed to accommodate more dynamic range. Interestingly, the base ISO of D-Log2 (the setting at which it captures maximum DR) appears to be 1600, suggesting that it’s being exposed to capture two additional stops of highlights compared with the ISO 400 base sensitivity of D-Log profile used for the conventional sensors.

However, the D-Log2 mode also lets you expose this same sensor state all the way down to ISO 100: clipping those additional highlights but using more exposure to capture the remaining tones, and thus boosting tonal quality. 

This gives us an idea of what LOFIC could bring to dedicated cameras: a choice of greater highlight capture or higher quality captures with the same highlight cutoff point as existing sensors.

After nearly a decade of sensors getting faster but not really offering improved image quality, LOFIC technology might be the step forward we’ve been waiting for.

Interestingly, Fossum – who is responsible for inventing the active pixel CMOS image sensor that virtually all modern cameras are based on – invented an early version of this concept, but the successors to the company he’d worked for instead concentrated their efforts on the simpler dual conversion gain system. A team at Tohoku University in Sendai, Japan, independently developed LOFIC as we’re now seeing it, citing Fossum’s Micron patent in their LOFIC patent, but proposing a more modern implementation.