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Let's See: Light Sensor Approximates the Human Eye

The manufacturer says . . .

ChipCenter's Alex Mendelsohn says . . .

TAOS, Inc., Announces Industry's First Ambient-Light Sensor to Convert Light Intensity to Digital Signals Ambient-Light Sensor with SMBus Interface Helps Reduce System Power by Automatic Adjustment of Backlight Levels on Portable Display Screens Plano, Texas--The industry's first ambient-light sensor that converts light intensity to a digital signal output capable of direct SMBus interface was announced by Texas Advanced Optoelectronic Solutions, Inc. (TAOS). The TSL2550 Ambient-Light Sensor is designed to help reduce overall system power by measuring the ambient—or available—light as a means of controlling display-screen backlighting in portable equipment such as laptop computers, cell phones, PDAs, camcorders, and GPS systems. Display-screen backlighting, which can account for 30–40 percent of total platform power, now can be adjusted automatically with the help of this new single-chip light sensor from TAOS. "Our goal with the TSL2550 is to help portable-equipment users realize significant power savings by adjusting display brightness to ambient conditions," said Carlo Strippoli, TAOS vice president for marketing and sales. "Research shows that portable-equipment power consumption is directly proportional to display luminance. Dimming can reduce display power by as much as 70 percent. In fact, a minimum brightness setting can extend battery life for more than an hour. The TSL2550 is taking Lumenology—the TAOS light-sensing technology—in an exciting new direction." While the TSL2550 is designed primarily for use in portable-communications equipment, it can also be used to monitor lighting conditions for the control of commercial and residential lighting. The TSL2550 uses an all-silicon technique that combines two photodetectors to measure ambient-light brightness as perceived by the human eye. One of the photodetectors is sensitive to both visible light, or light the human eye can perceive, and infrared light, which the eye cannot perceive. The second photodetector is sensitive only to infrared 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 infrared component of ambient light, thereby approximating the response of the human eye. This information can be used to calculate the light intensity in the commonly used units of lux. This technique eliminates the need to use an expensive photopic filter. Additionally, the ratio of infrared light to visible light can be used to determine the type of ambient-light source (incandescent, fluorescent, or sunlight, for example), thereby allowing more intelligent control of display contrast and/or color. The TSL2550 uses a companding A/D converter to provide light measurements over an effective 12-bit dynamic range. The integrating conversion technique used by the TSL2550 effectively eliminates the effect of flicker from AC-powered lamps, thereby increasing the stability of the measurement. A single-supply operation (2.7 V to 5.5 V) makes the TSL2550 well-suited for use in 3 V systems. Moreover, a powerdown mode, which shuts off the device on command when not in use, ensures that the TSL2550 consumes as little power as possible. The TSL2550 Ambient-Light Sensor with SMBus Interface comes in an 8-pin SOIC package and is available from TAOS, Inc. and its authorized distributors. The TSL2550 has a suggested resale price of $1.59 in 1,000-piece quantities. In addition, two versions of a TSL2550 Evaluation Module are available from TAOS, Inc. and its authorized distributors. Both the TSL2550-RS232 EVM and the TSL2550-PCMCIA EVM have a suggested resale price of $110.

Texas Advanced Optoelectronic Solutions (TAOS) isn't a company with a name that's a household word, but it nonetheless makes intriguing optoelectronic products. In addition to opto-couplers, TAOS produces and sells a raft of solid-state devices that combine mixed-signal circuits and photo-detectors on one substrate. TAOS's opto-electronic products include light-to-frequency and light-to-voltage converters. It also makes CMOS linear sensor arrays, color and color-reflective sensors, and ambient-light sensors such as the TSL2550. Although the company does indicate that its SOIC-packaged 8-lead TSL2550 is primarily for portable equipment and notebook PC displays, and can additionally be used to monitor room lighting, there should be other applications for such a direct-converting device. When you factor-in its attractive price tag, design-in opportunities will likely suggest themselves. The fact that the sensor's capability of measuring the ratio of infrared to visible light can be used to determine the type of light source should suggest some unusual applications, too. A 12-bit Dynamic Range Primarily intended for broad-wavelength ambient-light detection, this power-managed device will most likely be used in at systems where adjustments are best made to a display's brightness or contrast based on the brightness of the light as perceived by the human eye. To do that the CMOS chip combines two photodiodes and its companding A/D converter mentioned in the press release. Together, these blocks permit the device to make light measurements over an effective 12 bits of dynamic range. However, most silicon detectors respond strongly to infrared light, which the human eye doesn't see. This can lead to significant error when the infrared content of the ambient light is high, such as with incandescent lighting. That's due to the difference between a detector's response and the brightness perceived by the human eye. In contrast, one of the TSL2550 chip's photodiodes is sensitive to visible light and infrared; the second photodiode is sensitive to infrared only. As such, the first detector is fed to the A/D converter to compensate for the effect of the infrared component of ambient light on the device's second digital channel. click for full-size image The outputs of the two A/D converter channels are then used to obtain a value that approximates the human-eye response (in lux). This plot shows the spectral response of the internal diodes. An exciting feature of this device is that its integrating conversion technique eliminates the effect of flicker from AC-powered lamps, increasing the stability of a measurement. Both 50 Hz and 60 Hz ripple are rejected, making it useful for places where fluorescent lights are in use. Fluorescent-generated light has a high harmonic content, but because the TSL2550 integrates ambient light over a 400 ms interval per channel, ripple from fluorescents is typically reduced to less than ¼ LSB. SMBus Connectivity On the I/O side, TAOS is clearly attuned to market needs, as evidenced by the inclusion of register-based System Management Bus (SMBus) connectivity in the TSL2550 sensor. Moreover, the chip can be used in SMBus v1.1 and v2.0 systems. For those of you not familiar with the SMBus, it's a two-wire communications link that's usually used between a battery pack and a microcontroller-based charger in a notebook or similar portable product. The scheme was proposed by Intel a few years ago, and is now endorsed by most notebook makers and smart-battery vendors. Significantly, SMBus is based on the popular I²C serial-communication bus developed by Philips for talking to 8051 microcontrollers. Today, many EEs consider it a de facto controller/peripheral communication standard. As it evolved, SMBus was augmented by a protocol set not only equipped for smart batteries, but also for the related chargers, system hosts, and even unusual SMBus peripherals such as this light sensor. On the TSL2550, the SMBus interface makes it straightforward to use it in laptops and other portables. The device lends itself to existing designs running standard SMBus controller chips. However, the TSL2550 can also be interfaced with other microcontrollers that don't include dedicated SMBus ports, thanks to the similarity between SMBus and I²C. Eval Modules, Too Briefly mentioned in TAOS's press release are the company's pair of $110 evaluation modules for this sensor. Their functions are encrypted in the TSL2550-RS232 EVM and TSL2550-PCMCIA EVM model numbers. As you might guess, the difference between the two eval products is in their PC interfaces, with a choice of either RS-232 serial or PCMCIA PC Card communication. Regardless of which you choose, the modules are shipped with Windows-based software. While you can use the TSL2550 to dim a display and save power as a function of the display's luminance, there should be many other applications for this device as well, especially when you consider its responsivity, small size, wide voltage range, and built-in power management. That's what TAOS calls "Lumenology."

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