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Light detectors adjust LCDs to match driver needs

Silicon-based light detectors have helped to increase ergonomics for the driver of a car. By measuring the ambient light conditions the brightness of LCDs can be adjusted.

But the downside of silicon is that, if untreated, it will show a spectral sensitivity that differs greatly from that of the human eye. If the brightness control of an automotive display is based on information from this type of source, irrelevant wavelengths can cause mistakes.

Historically the first step of improvement was to position the sensor’s P-N junctions higher up in a transistor chip. As the penetration depth of varying wavelengths differs, this helps to reduce the mismatch in sensitivity to the human eye. If you add additional filters to the surface of a diode you get an even better match, but there is still a notable difference.

To overcome this situation, Osram Opto Semiconductors has developed a new type of infrared (IR) ambient light sensor, based on a visible emitter chip that produces a weak signal when operated as a diode. An integrated circuit amplifies the signal and compensates for temperature changes. As a logarithmic output steps up the signal, the IR sensor, called SFH 5711, covers a very wide range of lighting conditions and can be used without additional resistors.

The new IR sensor has its peak sensitivity at 560nm just the like the eye, which is claimed to be an industry first among semiconductor manufacturers by Osram.

The sensor’s spectral sensitivity offers better ergonomics for instance in controlling the brightness of displays. In a very dark environment, over bright displays can be very tiring for the eyes, which is why precise measurement of the ambient conditions is beneficial. Also the energy consumption of display backlighting could be reduced by operating the display according to a power-level-on-demand principle.

Automatically switching on the headlights is another application that asks for a good match between human perception and sensor signal.

For head-up displays (HUDs) a good sensor reading of the light conditions in the directions of the driver’s view is more than a question of ergonomics: to make sure that the driver can read his HUD under almost any ambient light conditions, the backlighting has to be adjusted very precisely and dynamically.

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