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Making sense of light sensors

Posted: 01 Jan 2007 ?? ?Print Version ?Bookmark and Share

Keywords:light sensors? light sensor? optical sensors? sensor resolution? optical sensor?

As electronics seamlessly weave their way into our lives, sensors play an increasingly important role. Light sensors are among the simplest and cheapest sensors, allowing their inclusion into numerous consumer products ranging from night lights to cameras.

There are a many ways to sense light. This article looks at the array of available light sensors, covering trade-offs in resolution, dynamic range and cost. The use of IC with light sensors enables on-chip calibration, filtering and increased resolution. These advances are discussed with respect to two applications: an ambient-light sensor for a laptop and one for a cellphone display.

Types of ambient-light sensors
A spectrum of optical sensors is shown in Table 1.

They are arranged from left to right, in order of complexity; at the same time, in reasonable order quantities, they are also arranged by cost. A discussion of the trade-offs will uncover what advantages can be bought with a few extra pennies per unit.

The simplest optical sensor is a photoresistor, and it can be identified by the meandering channel between two terminals. The low-end versions are made with cadmium sulfide; their more-expensive counterparts are made with gallium arsenide (GaAs), which allows the inclusion of a photoresistor in an IC. The small bandgap of GaAs (1.4V at 300K) allows the low-energy photons in infrared light to free electrons into the conduction band. The data from the reference part is shown from 1lux to 100lux, but various resistance values are available.

Photodiodes are the next step in complexity. Photons that bombard the junction produce current. For best use, the diode should be reverse-biased. The amount of bias directly translates into quality of operation, as larger reverse bias enhances speed and linearity while also increasing dark current and shot noise. Light will create forward current, subtracting from the reverse-bias current. External circuitry can be added to linearize the diode's I-V curve, to amplify the signal and to allow a disable function.

A phototransistor exhibits the same general characteristics as the photodiode, with the addition of amplification. It requires more bias current, but the noise associated with the current forces a shift in the sensitivity of the sensor to a higher lux range, of 1klux to 100klux (instead of 7klux to 50klux). Response time is similar and can be varied using the bias. Current will also vary with the detected signal level. A phototransistor is capable of determining coarse environmental light levels like indoor/outdoor, day/night, and bright light/shade. External circuitry is still needed to calibrate the output signal and include an enable.

Table 1: The array of available sensors offers a wide choice of performance and cost tradeoffs. (Click to view full image)

Shrinking device sizes have allowed the creation of a hybrid device such as the EL7900. It places a photodiode and transimpedance amplifier in one package, as shown in Figure 1.

Figure 1: Block diagram of EL7900 ambient light sensor with integrated current amplifier.

This combination allows for lead-length reduction and minimum parasitic capacitance on the amplifier inputs. This, of course, is the optimal condition for minimum noise, high frequency response and convenience. The low-noise characteristics extend the sensitivity of the sensor down to 1lux, while keeping the upper limit of 100klux. The power drawn is still dependent on the amount of light sensed, reaching 0.9mA for 1,000lux. To conserve power, a power-down pin is included. This device is suitable for many situations, not just digital cameras. But before discussing these applications, there is one last device to discuss.

The ISL29001 family of devices, Figure 2, is representative of a packaged solution for light sensing and calibration.

Figure 2: Block diagram of ISL29001 ambient-light sensor with ADC, enable and filter.

The temperature-compensated light sensor is a PIN diode. The output from the sensor is calibrated and fed through a current amplifier before entering a high-pass filter to eliminate 60Hz noise. After the filter, an ADC and I?C interface deliver the output signal.

Two major benefits of using an ADC are constant power usage and 15-bit resolution. In fact, the current draw is less than for all of the other active devices in the table. The ADC has an internal 327.6kHz clock that sets the device response time to 100ms. Even with the increased delay, the serial 15-bit output signal allows the sensor to be suitable for a much wider range of applications.

Applications for ambient-light sensors
Light sensors are ubiquitous in modern society. Some applications use reflected light with optical detection for position sensing; these include bar code readers, laser printers and autofocusing microscopes. Other applications, such as digital cameras, cellphones and laptops, use optical sensors to gauge the amount of ambient light. It is this second group that we'll investigate further.

Ambient-light sensors are included in laptops to adjust the screen's backlight to comfortable levels for the viewer. The range of the comfortable levels is dependent on the room's light, with the relationship shown in Figure 3.

Figure 3: Graph of the desired brightness with respect to ambient illumination (Click to view full image).

Understandably, a screen's brightness needs to increase as the ambient light increases. What is less obvious is the need to decrease the brightness in lower-light conditions, both for comfortable viewing and to extend battery life.

In laptop design, ambient-light sensors are typically placed next to the speakers where the case has an opening for light. These audio portals are commonly covered by a cross-hatch pattern to protect the speakers. Because of this protection, and because the light sensor is next to the speaker instead of on top of it, the light is obstructed. The obstruction reduces the amount of light available to be measured, thus requiring a solution with good low-light accuracy. For the accuracy needed in low-light conditions, the best sensor choice is the integrated photodiode with an ADC, Figure 4.

Figure 4: Block diagram of light-sensing system in a laptop.

The inclusion of a high-pass filter minimizes power supply noise from coupling into the backlight illumination.

Another common application is an ambient-light sensor used in a cellphone, where every mA-hour saved translates into longer battery life and happier customers. The enable/disable function is equally important for the battery-saving, power-down feature. The extension of battery life is remarkable. With the light sensor to adjust the backlight illumination, battery life is increased by at least a factor of 6, assuming the backlight remains on full power without feedback from a light sensor.

Figure 5 shows a complete automatic backlight control circuit for cellphones.

Figure 5: Automatic white-LED backlight intensity control circuit (Click to view full image).

The EL7900 senses ambient-light intensity and outputs a current proportional to this intensity. Equation 1 shows the relationship between light intensity (E) and output current (Iout):

The light sensor output current is injected into the feedback input of the white LED driver. In a bright environment, the light sensor sources more current into the feedback node; as a result, it reduces the white LED's output current and output light intensity. The relationship between ambient-light intensity and white-LED output current is shown in Equation 2:

Conclusion
There is a wide variety of optical sensors available in small packages at reasonable prices. Passive solutions have been serving consumers for decades in night lights and digital still cameras. Active solutions have increased the range and usefulness of ambient-light sensors. Typical active solutions integrate a phototransistor or a photodiode with a current amplifier. When greater resolution, low-light capability, power supply rejection, or a disabling function would be useful, devices such as those in the ISL29001 family extend the usefulness of typical ambient-light sensors.

About the authors
Mike Wong
is the director of application engineering focusing on high-speed analog applications at Intersil Corp. He has previously worked on power supplies at Astec. He graduated from UC Davis with a BSEE in 1989.
Tamara Papalias is a principal application engineer for analog applications. She is also a full-time professor of electrical engineering at San Jose State University. She has a BSEE, MSEE and PhD in RF CMOS design from Stanford University.




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