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Eye of the Beholder



Backlit displays have become ubiquitous, and there’s hardly a place you go today where you don’t see one. In fact, it’s practically gotten to the point where we judge a product primarily on the merits of its display. While this is particularly true for cell phones, thanks to the iPhone, it is also true in other cases. At point-of-sale displays, customers’ feelings about a product are influenced by how it looks on the display. Selection not just of TV sets but of other household appliances, such as refrigerators and coffee makers, are being influenced by how good the display looks. And in industrial applications, display performance is often not just a matter of preference, but of safety.

Most designers spend a lot of time selecting a display based on its brightness and resolution, but another aspect of a display system performance demands equal consideration: under what ambient lighting conditions will it be seen? Rarely is a display viewed under a constant ambient light; the use of a display outdoors or indoors with natural light from windows or with changes in interior lighting considerably affects the display’s readability. To optimize perceived display performance, designers have to adjust the amount of light emitted by the display so as to harmonize it with surrounding light conditions.

Fortunately, there are numerous devices -- called ambient light sensors or ALS -- available today to help with this choice. Some provide a direct analog voltage or current output that is proportional to the amount of sensed surrounding light, while others go a step further, digitizing the output and providing an I²C interface to ease integration with digital logic. Further, a wide variety of device package options make it easy for designers to find a device that will fit the size, manufacturing, and performance requirements of their products.

Exactly how the ALS responds to ambient light can be extremely critical in some applications. This information can usually be found in the device data sheet as a graph of relative spectral sensitivity. The graph’s x axis is typically calibrated in wavelengths ranging from UV through visible to IR, and the y axis is usually the normalized device response (peak output = 1). In some cases, manufacturers will also include the normalized response for the human eye along with the normalized device response, so the designer can easily see how well the device’s performance mimics human perception. In some cases it can also useful to compare the device response to the output wavelengths of typical light sources, such as incandescent and fluorescent lamps, such as when a display will only be used in a specific lighting situation.

Using response information, designer can fine-tune display performance to meet the needs of their applications. In some cases, continuous brightness adjustment may be desirable, while in others step adjustments may be perfectly acceptable. But in any instance, a good ALS can help ensure product acceptance.

To see some of the ALS available today, visit ALS.

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