Wednesday, August 17, 2011

Twice as efficient? Well, sort of

John Wallace
Senior Editor
Laser Focus World

As a former optical engineer, I have carried over that profession's analytical outlook into journalism. Is that a good thing? Well, maybe not always. For one thing, I can't just relax and read science and technical content written for the popular media without getting all bothered by it.

This includes science articles in which the size of things seems to be either "vast" or "tiny," every new lab technique is "special," and scientists are perpetually "peering" through microscopes. Hmph. And too, there are technical press releases containing info that is probably aimed properly at its audience, but to me seems to be overly condensed.

Here's one example: Sony just introduced an innovative small (3 in.) VGA LCD module for digital cameras, which cuts power consumption when compared to a conventional LCD, or conversely increases brightness for the same power consumption. Called "WhiteMagic," the display does this by adding a white (W) subpixel to the usual red-green-blue (RGB) subpixels, resulting in an RGBW layout. Because the W subpixel has no tinted filter to cut down its transmission, the display is more efficient than normal RGB whenever there is white in the image.

Sony's claim for the display is that it is "enabling a reduction in power consumption of approximately 50%* and improved outdoor visibility (around twice* the brightness), suitable for implementation in Smartphones -- *compared with the conventional RGB method (Sony's comparison)." (See

Let's peer at this
So, for the same brightness, Sony says that the power is cut by about half. Now, all you photonics tech people -- think about that for a minute. First, let's assume that the R, G, and B filters each transmit 1/3 of the incoming light through them (in lumens) and that the subpixels all have the same area. Second, let's take two extreme cases: 1) when the LCD is displaying either an R, G, or B fully saturated color; and 2) when the LCD is displaying a totally white image.

In case 1, the W subpixel is off and either the R, G, or B subpixels are all operating. This means that all light passing through the display is filtered to 1/3 of its original intensity (as in a conventional LCD) but, because the W subpixel is totally off, only 3/4 of the total area is illuminated, making the Sony display 3/4 as efficient as a conventional LCD. (Please someone correct me if I'm wrong.)

In case 2, the W subpixel is on and the R, G, and B subpixels are all on too, combining to produce white light. In this case, a conventional LCD would have R, G, and B pixels all on to produce white light, and filtering all but 1/3 of the light in the process. The Sony display is filtered to 1/3 over the RGB subpixels, but is unfiltered over the W subpixel. Doing the math, in this case the Sony display is twice as efficient as a conventional LCD.

A bit frustrating to this ex-optical engineer. What I want to add is, "If you're photographing a polar bear in a snow storm, you'll get twice the efficiency!" Or, "If you're taking a picture of a red chair in a red room, you won't get an efficiency boost." However, most likely no one in the world is fretting about this but me. So be it.

Take that, Voldemort
Maybe someday I'll tell you about some of the "Harry Potter invisibility cloaks are just around the corner" pop-science articles I've seen.

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