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The quality of camera phones is continually improving — more megapixels, better lenses, improved image-processing software, anti hand-shake features. The one area that has been lagging behind is the power and energy of the flash for taking pictures in low light. It is often in low light environments where people want to take photos to record the occasion, such as in restaurants, bars or nightclubs. Many cell phones have compromised by providing a low-current LED photo light or flash, which provides insufficient light energy for an acceptable photo in low light, as shown in Figure 1.
Figure 1: Photo taken in very low ambient light using a low current LED and 1W flash power. The girl is 2m from the camera. Note the color reference-chart beside the girl and how the colors are barely apparent.
Now, two solutions are emerging to provide a good photo flash in low ambient light:
- High-current LEDs supported by a supercapacitor
- Xenon
This article will explore the limitations of existing LED flash implementations without a supercapacitor, and go on to compare LED flash with supercapacitor and xenon flash solutions. The comparison will be across multiple dimensions including:
- light power and energy
- shutter requirements
- ease of circuit implementation
- safety
- size
Light power vs light energy
The light power of a flash determines how bright it appears. Naturally, this is what draws most people's attention. However, what is important to a pixel in a camera sensor is the total amount of light it receives while it is capturing data. This is the light energy. For a flash pulse, this is the area under the curve of light power over time. If the light power is constant during the flash pulse, as is the case for LED flash, then the light energy is light power (lux) x the flash pulse duration (seconds), and the unit is lux second.
- Xenon flash has up to several hundred thousand lux, but a very short pulse duration, typically 50 - 100μs
- An LED Flash, with the support of a supercapacitor, can now generate upwards of several hundred lux with a flash pulse of up to ~100ms
This means the xenon needs to have 1,000 to 2,000 times the power of the LED Flash to deliver the same light energy. A major restriction on the light energy delivered by a xenon pulse is the size of the electrolytic 330V storage capacitor.
In this article we show the results of light power over time for:
- Three xenon camera phones, with varying size storage capacitors, the largest of which has an external xenon flash accessory
- Existing low-power LED flash
- A supercapacitor-based high-current LED Flash solution.
Integration of the area under the curves gives light energy available to fill pixels in the camera sensor and enables the relative merits of the two solutions to be objectively compared.
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