iExpander: Overcoming Poor Low-Light Images from iPhones
A key enhancement Corr-Tech wanted in its iExpander for the iPhone 4/4S and 5 was a flash capable of crisp photographs and clear videos, even in low light. The eventual solution – a supercapacitor-powered LED flash capable of delivering more than twice the light energy of the traditional iPhone flash – not only made a huge difference in picture and video quality, it also solved an additional problem Corr-Tech didn’t anticipate fixing – red eye.
By extending the torch mode period prior to the flash, iExpander inventor Charlie Corry found that he solved "red eye" problems in the resulting photographs as well.
The lens and camera sensor in an iPhone are very small, and need lots of light to deliver a good image. The standard LED flash in the phone is also constrained by the limited power available from the Li-ion battery, and doesn't perform well in low light. The challenge was to improve the flash, without being able to 1) change the iPhone lens or camera sensor, or 2) access the software controls in the phone for exposure, white balance or flash activation.
Investigating a Solution:
Corr-Tech first experimented with adding a xenon flash to the iExpander case, but found this solution unattractive because the large electrolytic capacitor required to discharge the bulb, charged to ~300V, added both bulk and safety concerns about the high discharge voltage. Furthermore, the xenon flash tube is fragile, and delivers such a short pulse of light (much shorter than the time the CMOS sensor is collecting light) that a mechanical shutter is usually needed to regulate the image exposure. The iPhone uses a rolling shutter and CMOS sensor. For all these reasons, a xenon flash fell short of a rugged, self-contained flash solution, able to operate independently of the iPhone's hardware and software.
Further research revealed the CAP-XX BriteFlash power architecture, which uses an ultra-thin, high-power supercapacitor to drive high-brightness LEDs over an extended flash pulse period, matching the time that the CMOS sensor is collecting light, overcoming the power limitations of the battery, and dramatically increasing the total light energy emitted by the flash.
The Key to Clear Images is Light Energy:
Working with Australia-based CAP-XX, Corr-Tech learned that the total light energy captured by the camera sensor determines the image quality, not the light power of the flash source. Light energy is defined as the total light power (lux) delivered in a specified time (sec) - in this case, the intensity of the flash, integrated over the image-capture time. Light energy is measured in lux.sec: light power (lux) x flash duration (sec) x number of LEDs = light energy (lux.sec).
CAP-XX research has shown that smartphone cameras can achieve good pictures provided their flash delivers at least 10 – 15 lux.sec of light energy. Looking at Figure 1, a comparison of xenon vs LED flash light energy at 1m, a typical low-power LED, driven directly from a battery, delivers only ~1 - 4 lux.sec of light energy. A good quality xenon flash in a mobile phone delivers an average of ~100,000 lux for 100 µsecs, for a total light energy of about 10 lux.sec. A supercapacitor-driven high-power LED flash can deliver ~300 lux for 67 ms, for about 20 lux.sec of light energy. The 67 ms LED flash time = 1/15 sec, and is the light capture time for 1 frame of a rolling shutter running at 15 frames/sec.
The BriteFlash-powered LED solution Corr-Tech implemented (see Figure 2, BriteFlash block diagram) delivers twice the light energy of many xenon solutions, and an order of magnitude more light energy than the low-power LEDs found in most phones – including the iPhone 4/4S and 5.
Triggering the iExpander Flash:
The next issue Corr-Tech needed to resolve was how to trigger the flash without having access to the iPhone camera control software. The company included a high-speed photo diode in the iExpander flash module, facing the iPhone flash in order to sense and activate by the phone's own camera. The iExpander flash module charges up and is ready to go every time the case is attached to the phone, so when the iPhone flash fires, so does the iExpander flash.
The CAT3224 supercapacitor-optimized LED flash driver from On Semiconductor enabled Corr-Tech to elongate the pulse of the flash, or in other words, extend the torch mode before the flash, which solved “red eye” problems.
In the iExpander Flash:
The iExpander flash consists of 2x Philips Lumileds Luxeon Flash LXCL-EYW4 LEDs, a CAP-XX HS206 supercapacitor to deliver the power, the CAT3224 supercapacitor-optimized LED flash driver to regulate the LED flash drive current and charge the supercapacitor (with in-rush current management and an active balance circuit to ensure that the two supercapacitor cells remain at the midpoint voltage), and a microprocessor to control the high-speed photo diode flash trigger.
For more information, visit:
CAP-XX BriteFlash Architecture: http://www.cap-xx.com/news/photogallery.php#BriteFlash