Powerful LED flash for smartphones

November 26, 2010
The new Oslux provides brighter and more uniform light across the entire illuminated surface. With its compact dimensions it is perfect for modern low-profile cell phone and smartphone designs. Press picture: OSRAM

Researchers from Osram Opto Semiconductors have developed an LED chip that makes it possible to offer smaller pocket projectors, vehicle headlights, and cell phone camera flashes. In combination with a new package the new UX:3 chip is 50 percent brighter than the precursor package. In combination with an optimized lens, the light is much better distributed. The chip is used in the Oslux LED, which is therefore considerably more efficient at high currents than previous LEDs and is impressive for its very high luminous efficiency over a small area.

At a distance of one meter, for instance, such an LED flash evenly illuminates a diagonal of 90 cm. That is sufficient for capturing sharp images even under unfavorable light conditions. At 150 lux, the LED with the UX:3 chip is 50 lux brighter than its predecessor. As a result, high-quality images can be taken even with very flat cell phones or smartphones.

Normally, when taking photos at night with a camera phone, the flash is capable of relatively bright illumination of the middle of the image area, but the corners appear somewhat dark. This is because the of the LED itself is too low — it just can’t produce enough light — and the lens doesn’t distribute the light evenly enough. This creates a bright circle with dark edges, an effect that occurs especially under very unfavorable light conditions. To change this situation, the researchers from Osram rearranged the internal layout of the chip.

The chip consists of a metallic lattice and two semiconductor layers. The lattice conducts the current to the upper layer, from where the electrons move to the lower layer and release energy in the form of light. With conventional LEDs, however, the metallic lattice is positioned above both layers and thus diminishes the light. The effect is similar to what would happen if you place a dark cloth over a light bulb. The researchers at Osram therefore moved the lattice all the way to the bottom, enabling them to increase the “wall plug efficiency,” which describes the relationship between the radiant flux of the chip and the electrical power that flows through it.

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4 / 5 (1) Nov 26, 2010
Lux is a measure of how much light falls onto an area, which means that you can make an LED brighter in Lux by narrowing the projection angle of the lens without actually making it output more light.

So you can't make a direct comparison between LEDs like that. The competitor may offer less light per area, but also illuminate a bigger area. You may get more even light within the area, but less light outside of it.

An ordinary point&shoot camera will, at the wide angle end of the lens, see a much greater area at 1m distance than a 90cm diagonal square, which means that if the light of the LED is concentrated mainly in that area, you get dark edges unless you zoom in.

And again, as with all LEDs, you can cheat by omitting wavelenghts that the eye isn't sensitive to, but which are still required for good color reproduction. This gives you a higher lumen/lux figure with the same power which looks good on the brochure, but the colors in the photo will be off.
3 / 5 (1) Nov 26, 2010
Imagine for example, that you have a lamp that outputs very monochromatic light at 445nm, 545nm and 580nm. These are approximately the peaks of the human color vision sensitivity curve, so the light appears quite brilliant and white and will therefore have a good luminous efficiency as measured in lumens or lux per watt. This makes it very good for purposes like building a television backlight, because TVs need only those wavelenghts in different proportions to simulate all the different colors.

But, if you use the lamp to illuminate an object that reflects light at, say mostly 500 nm, it will look strangely colorless and possibly very dark.

That's because the eye, or a camera, "triangulates" any light it sees based on how much it affects the differently tuned sensors, so in the absence of a specific wavelenght, an object cannot bounce it to the eye and you can't see its color correctly.

The color rendering index may vary. That's also why direct comparison between LEDs is difficult.
5 / 5 (1) Nov 26, 2010
The point is, that while the article makes this new LED sound much better than its predecessor, it omits all the real details about how they actually compare and what limitations the new LED has.

I find this a lot in reports about LED efficiency and applications, and I find it sad that they have to restort to these kinds of "half truths" in marketing them.

It's very much alike the electric car situation where the company touts a range of a 100 miles and conviniently forgets to tell that you can only achieve that by driving slower than 40 mph.

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