Opto-Electronics


Silicon ambient light sensor sees light as human eyes do

1 August 2001 Opto-Electronics

Regular silicon detectors have their maximum sensitivity in the IR range, and thus behave considerably differently from human eyes. If they are used as ambient light sensors, ie to adjust display backlighting, one has to cope with inaccurate results due to differences in the IR spectra of various light sources.

Relative spectral sensitivity
Relative spectral sensitivity

Osram Opto Semiconductors' new technology silicon 'sees' the way human eyes do and is not fooled by artificial light from lightbulbs, fluorescent lamps and other light sources. Such lamps are characterised by different emission spectra which then lead to different signals at the standard sensor, even though human eyes do not recognise any difference in brightness.

Spectral emission of different light sources/sensitivity of detectors
Spectral emission of different light sources/sensitivity of detectors

Typical application: LCD display

Normally the sensor is mounted behind the display. The transmission factor of a transmissive or transflective LCD display is roughly 20%. To be on the safe side, the sensor has to function down to a transmission of 10%. 500 lx ambient light (ie office illumination) will result in 50 lx behind the display. This leads to a photocurrent of 30 ma. For negative displays the transmission is too low, unless the sensor is placed behind a window.

The SFH3410 chip is packed into a SMART DIL housing, resulting in a particularly compact sensor (2 x 2,5 x 1 mm). It has a typical application for the sensor to control backlighting of dashboards, audio sets and aircon units in cars as well as for portable devices and consumer applications. It can operate from -40 to +85°C.



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