Light sensors pack in greener features
Ambient-light sensors help smartphones conserve display power and improve battery run time.
By Margery Conner, Technical Editor, EDN -- Test & Measurement World, 9/1/2009 2:00:00 AM
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See other articles from our September 2009 issue. |
ALSs (ambient-light sensors) have been around for years, but they are now seeing increased use due to the success of smartphones, such as Apple’s iPhone. The iPhone uses an ALS to reduce power demands and eke out battery life by adjusting the display lighting for ambient-light conditions. The iPhone also uses a proximity detector, a close cousin of the ALS, to reconfigure itself in response to user actions (Figure 1).
Today, smartphones, with their trademark large LCDs, are the main users of ALSs, but future applications can realize energy savings and increased ease of use. For example, a large-screen LCD TV must adjust its LED backlighting to the ambient light. Similarly, a room with SSL (solid-state-lighting) illumination can change its lighting based on natural lighting or to suit its occupant’s mood. Automobile lighting can accommodate day or night driving or reflect the brightness of streetlights, saving power and providing a better user experience.
 Figure 1 . Light and proximity sensors are often located next to a handheld device’s speaker because both the sensors and the speaker require access to the outside world. The proximity sensor in a 3G iPhone is within the red circle, and the ambient-light sensor is the green part to its left. The iPhone’s speaker is the gray, mesh-colored oblong. (See “iPhone puts proximity detection in your face.”) Courtesy of iFixit. |
At its most basic, an ALS consists of a photodiode or a phototransistor. A simple light-sensitive semiconductor is insufficient, however, because the ALS must be “photopic,” meaning sensitive to the same frequency spectrum as the human eye (Figure 2). Incandescent and HID (high-intensity-discharge) lights emit 50 to 60% of their radiation in the nonvisible IR (infrared) range as heat. According to Oleg Steciw, product-marketing manager for ALS products at Intersil, you should use the HID with the best spectral response you can find. Otherwise, he said, “You’ll be in a room, and, suddenly, the backlight will go haywire because there’s some external light source that you can’t even see, wreaking havoc.”
Werner Mashig, application engineer on Arrow Electronics’ lighting team, explained, “[Some] manufacturers put IR-filter [compounds] into the epoxy to filter out the IR light so that the sensor will respond like the human eye.”
Another approach is to use multiple photodiodes in the ALS. “One photodiode is a broadband one that sees everything from 300 to 1100 nm,” said Carlo Strippoli, VP of marketing and sales for TAOS (Texas Advanced Optoelectronic Solutions). “The second diode is a dedicated IR photodiode and serves to monitor the IR reaching the sensor and then subtracting it from the light received at the broadband photodiode.”
Fluorescent-light sources, which are more efficient than incandescent or HID lights, emit almost none of their radiation in the IR range, but they may exhibit a 60-Hz flicker that can cause an ALS to trigger when it’s not supposed to. The newer digital ALSs integrate ADCs that convert the photocurrent to a digital signal to interface to a digital-communication bus. The ADC can serve double duty by filtering out optical noise, such as 60-Hz flicker, through high-resolution sampling.
Rohm’s BH17xx series integrates a 16-bit ADC that produces 1-lux resolution over a range of 0 to 65,000 lux. Two measurement-resolution levels allow selection between sampling time and performance. In the high-resolution sampling mode, the ADC filters out optical noise. The lower-resolution mode with its shorter sampling time suits applications such as GPSs (global-positioning systems), in which the light-level changes are dynamic: A GPS system will probably operate in an automobile’s interior or in natural light. The ideal ALS exhibits uniform light sensitivity regardless of the light source.
 Figure 2 . The human eye is sensitive to wavelengths of approximately 380 to 780 nm, peaking at approximately 555 nm. The wider red curve shows the sensitivity of a representative ambient-light sensor. |
“Digital is the direction ambient-light sensing is going,” said TAOS’ Strippoli. “It allows you to put multiple sensors on a single two-wire bus,” such as the I²C. This feature is especially important for flip phones. A digital bus minimizes the number of wires at the hinged interface where the cellphone flips up.
An analog interface requires at least two wires for every sensor. Analog ALSs are still good fits for some designs, such as those in which the voltage or current output of the ALS directly drives the lighting subsystem, those lacking a microcontroller or an available ADC input, and those low-end designs in which price is the dominating feature (Figure 3).
In the past, ALSs could vary from part to part in the amount of current a given amount of light produces. Such variability makes it difficult to design for a tight sensitivity range. “The manufacturers are [now] doing a great job of binning the components to give more consistency across the design so there’s not as much variation of the photocurrent,” said Arrow’s Mashig. He suggests looking at the specification for photocurrent versus brightness to check the tightness of manufacturers’ binning.
 Figure 3 . Although digital ALSs are now available, analog sensors are still popular for many applications. Microsemi’s Best Eye processing provides a nearly perfect photopic light-wavelength-response curve. The sensor output feeds into a wide dynamic-range compression amplifier that provides accurate resolution over five decades of ambient light. Courtesy of Microsemi. |
A low-power lighting system is especially important for battery-powered devices, and this requirement includes the ALS itself. In general, both analog and digital versions of ALSs have a shutdown or sleep mode, during which the sensor operates at approximately 1 µA. Because of the relative simplicity of analog ALSs, they require less power than their digital counterparts.
For example, a representative digital ALS draws 190 µA in active mode and 1 µA in power-down mode due to the integration of the ADC; an analog equivalent of the part draws 97 µA and 0.4 µA, respectively. The total power consumption, however, is comparable to or a little less than that of an analog ALS with a separate ADC.
Integrating proximity detectors
In addition to an ALS, smartphones often use proximity detectors. Apple’s integration of a proximity detector in the iPhone prompted a move toward making handheld consumer devices more intelligent when interacting with their users (see “iPhone puts proximity detection in your face”). Because of the close links in both technology and usage between ALSs and proximity detectors, ALS vendors are starting to add proximity detection to the list of integrated features in ALSs. “The ISL29011 drives an external IR LED so that it synchronizes the transmittivity of the LED and then captures the reflection off the object in front of it,” said Intersil’s Steciw. “You want the sensing range to be within about 3 to 5 cm.”
The placement of the IR LED outside the sensor package gives designers more flexibility in where they place the device or what they’ll place it behind, said TAOS’ Strippoli. “The iPhone puts [the proximity detector] behind a glass that blocks about 95% of visible light,” he added. “So if you use a device that gives you just a [fixed] single output, you get a very low signal.”
For discrete proximity sensors, it’s still common to keep the IR-radiating LED inside the sensor package. Avago recently introduced the APDS-9120 proximity sensor, which combines a built-in signal-conditioning IC, an emitter, and a detector into a package that offers both analog- and digital-output options. Like Steciw, Strippoli views the power-saving requirements of portable devices driving the trend in packaging proximity detectors along with the ALS but sees it as part of the move toward greener products. He believes that Asian countries in particular are likely to mandate the ability to tell when a viewer is using a large screen or monitor by monitoring proximity.
ALSs in smartphones detect light intensity but provide no information about the color spectrum. A recent development in ALSs is the ability to perform RGB sensing, a necessary feature for large-screen LCDs. For the best viewing experience, these displays must match their backlighting to the color temperature of the ambient lighting (Ref. 1).
The LCD controller uses the RGB ALS output to tune the RGB HB (high-brightness) LEDs to match the ambient lighting: Backlighting for a fluorescent-lit room has a different color temperature from that of a natural- or incandescent-lit room. In addition, as RGB LEDs age, their color changes slightly, calling for an additional RGB ALS in the backlight itself to sense and give feedback to drive the compensation for the LEDs’ color change. Intersil, TAOS, and Rohm all offer RGB sensors.
SSL is an emerging application for RGB ALSs. In this application, color sensors provide feedback to a room’s lighting-control system to adjust the light intensity, color, and color-temperature output of the HB LED-based luminaires. Lighting-control information is more complex than the simple on/off-light-switch information that room lighting currently uses, and lighting designers must be familiar with communication protocols. The DALI (digital-addressable-lighting-interface) protocol, which theatrical lighting has used for years, is one possible approach (see “Taking advantage of light sensors with microcontrollers running DALI.”).
Automotive lighting also needs ALSs. Night-driving applications have for years used simple photosensors to turn lights on and off, but more complex ALSs optimize cabin lighting for safe driving and for aesthetics, such as colored lighting and light-intensity variation. Like most other automotive components, ALS specifications must include operation over the wider temperature and vibration range.
A version of this article appeared in the August 6, 2009, edition of EDN.
| Reference |
| 1. Conner, Margery, “The direction of light: Electronic and thermal improvements bring advances to lighting technologies,” EDN, February 5, 2009. p. 26. |
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