Light sensors power new products

Light sensors power new products
With the development of the microprocessor and the digital signal processor, highly sophisticated instruments have become part of our daily lives. While these ubiquitous devices are entirely digital, the world we live in is analog. Even though digital systems are becoming more intelligent and sophisticated every day, they must receive information from the analog "real" world in order to be useful. Sensors are the bridges between all these different real-world signals and the growing digital world. Light sensors or optoelectronic sensors are one such bridge making up this growing sensor market.

Over the last decade or so, an enormous amount of interest has been focused on fiber-optic high-speed communications and, of necessity, on the sensing of these fast-moving light pulses. Impressive strides have been made in the capabilities of these high-speed light sensors. But another world exists for light sensing, a world that does not require the processing of gigahertz pulses, and a world that is slower but just as important and even more pervasive. This is a world that is enabled by light-sensing technology: fax machines, copiers, scanners, printers, garage door openers, liquid-crystal displays, colorimeters, spectrometers, automobiles and medical diagnostics, to name a few.

An optoelectronic sensor is a device that is capable of producing an electrical signal proportional to the amount of light incident on the active area of the device. There are a number of devices that meet this definition. But none is more prevalent than the semiconductor photodiode. The photodiode resulted from the discovery, early in the development of semiconductors, that when light is incident on a p-n junction, electrons and holes are generated. Over the years, this two-terminal device has become the mainstay for light sensing.

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Today, however, the market is asking for more then just a light-to-current transducer. It is looking for more functionality integrated around this semiconductor workhorse. The market is asking for a more complete solution to light sensing not only because of the value in having an integrated solution, but also because it is capable of providing improved performance and lower system cost.

Consider the cell phone market, for example. Vendors are looking for ways to differentiate their products from those of the competition. One way is to offer added functions to the cell phone's capabilities. Currently, not only can we receive voice and data messages with our cell phones, we also can get stock quotes and check e-mail. In the near term, we will start to see functions such as bar code scanning, optical-character recognition (OCR), digital cameras and even the ability to monitor our physical health as part of the standard cell phone repertoire. Highly integrated optoelectronic sensors are enabling all of this.

The scanning of bar codes to capture business card information and OCR will be accomplished using a CMOS linear pixel array with integrated analog-to-digital converter and preprocessing circuitry configured as a contact imager. A CMOS digital camera-on-a-chip will be used to capture digital images and, with the availability of high-speed cellular networks like 3G, these images can be transmitted to family and friends around the world. The monitoring of pulse and blood oxygen levels can be accomplished using an optically enabled pulse oximeter.

As the cell phone evolves into a true mobile multimedia platform, great demands are going to be made to display ever-increasing amounts of data with high quality. Color LCD screens will become the norm and light sensors will be necessary to monitor ambient lighting conditions so that the best possible picture can be provided under varying lighting conditions. Not only will this type of device help provide an aesthetically pleasing display, it will also help manage battery life by controlling the display backlight under varying ambient conditions.

While the cell phone market is relying on optoelectronic sensors to help make its "third generation" a reality, many other diverse markets are demanding optoelectronic sensors that can actually "see" in color. Just as the world is not digital, it also is not black and white. Color sensors have two primary applications: determining a specific color; and filtering out unwanted light in order to perform a sensing application more efficiently.

Color detection applications include:

• Eliminating the dentist's subjectivity in matching a new cap for your teeth;
• Identifying the exact color of the paint on your bedroom walls so the local paint store can provide a precise color match;
• Determining your skin tone for matching your favorite cosmetics;
• Reading color codes for the control of industrial processes;
• Monitoring and controlling white-light generation using red, blue, green LEDs; and
• Detecting certain diseases in their earliest stages by checking for changes in skin color.

All of these applications are currently being enabled by the availability of cost-effective sensors that integrate detector and color filters in the same package or, lately, on the same piece of silicon.

Optoelectronic sensors are now important for safety and comfort features in automobiles. Some examples are the detection of sun load to control the air conditioning; monitoring the level of carbon monoxide; or occupancy detection for the deployment of airbags.

We see two definite emerging trends: Optoelectronic sensors will continue to evolve from a niche to a mainstream enabling technology; and as the need for increased integration continues, the market will demand optoelectronic systems-on-chip.