Sensor converts light to digital signals

"Research shows that portable equipment power consumption is directly proportional to display luminance," says Carlo Strippoli, vice president of sales and marketing at Texas Advanced Optoelectronic Solutions (TAOS; Plano, TX; www.taosinc.com).

Nov 1st, 2002
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"Research shows that portable equipment power consumption is directly proportional to display luminance," says Carlo Strippoli, vice president of sales and marketing at Texas Advanced Optoelectronic Solutions (TAOS; Plano, TX; www.taosinc.com). "Dimming can reduce display power by as much as 70%, and a minimum-brightness setting can extend battery life for more than an hour," he adds.


Developers can purchase the TSL2550 ambient light sensor as part of a $110 PCMCIA-based evaluation module that contains software for evaluating the device under varying illumination conditions.
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Accordingly, TAOS has introduced an ambient light sensor that converts light intensity to a digital-signal output with a system management bus (SMBus) interface, specified by the SMBus Organization (www.smbus.org). While the TSL2550 sensor is designed primarily for use in portable communications equipment, it also can monitor industrial lighting conditions.

Ambient light measured by the device can reduce overall system power by controlling display screen backlighting in portable equipment such as laptop computers. At the recent Sensors Expo show (Boston, MA), TAOS demonstrated the device to control the CMOS LCD power controller drivers of a Dell Computer (Round Rock, TX: www.dell.com) laptop. Under high illumination conditions, the digital output of the sensor increased the power to these drivers; in low-light levels, this power was decreased.

"Display screen backlighting, which can account for 30% to 40% of total system power, now can be adjusted automatically using this light sensor," says Strippoli. To measure ambient light as perceived by the human eye, the TSL2550 combines two photodetectors, one of which is sensitive to both visible light (Channel 0) and infrared (IR) light (Channel 1).

The second photodetector is sensitive only to IR light. The output of both detectors is converted to a digital format and stored in separate registers accessed through the SMBus. The combined information compensates for the effect of the IR component of ambient light, approximating the response of the human eye. Light intensity in lux is then calculated as

Light level(lux)= (Channel 0 counts) × (0.46) × e-3.13R

where R = (Channel 1 counts)/(Channel 0 counts).

According to Strippoli, this technique eliminates the need for an expensive photopic filter. Additionally, the ratio of IR light to visible light determines the type of ambient light source, which allows more intelligent control of the display contrast and/or color. To provide light measurements over an effective 12-bit dynamic range, the TSL2550 uses a companding A/D converter that eliminates the effect of flicker from ac-powered lamps, increasing measurement stability. Operating from a single 2.7- to 5.5-V supply, the TSL2550 features a power-down mode that shuts off the device on command when not in use.

The TSL2550 uses the SMBus interface, a two-wire interface through which various system components can communicate with each other. This provides a control bus for system and power management-related tasks. PC systems can use the SMBus to pass messages to and from devices instead of tripping individual control lines. With SMBus, devices such as the TSL2550 can provide manufacturer information, tell the system what its model/part number is, save its state for a suspend event, report different types of errors, accept control parameters, and return its status.

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