Single-Pixel Sensor Revs Up Mirrors

Brent D. Johnson, Senior News Editor

Gentex makes about 7 million auto-dimming mirrors each year for major auto manufacturers. Despite these impressive numbers, the company is always looking for "major improvement in system cost and manufacturing," said Joe Stam, the company's lead engineer for imaging products. In an industry where gas tank caps and plastic reflectors are negotiated to the nearest penny, contractors must keep a close eye on their budgets.

Auto-dimming rearview mirrors use sensors to determine when another driver's headlights appear in a mirror. A single-pixel sensor can reduce the mirror's cost.

Gentex's continuously variable reflective mirror comprises an ambient light sensor directed toward the front of the car and a glare sensor directed backward. A microprocessor calculates the ratio between the two sensors and, when the glare exceeds the ambient threshold, current runs through a transparent electrochromic material sandwiched within the mirror, reducing reflectance to 4 or 5 percent.

Traditional photoresistors require analog-to-digital converters, which adds to the cost of manufacturing, Stam said. In investigating ways of eliminating the converters, the company studied complementary metal oxide semiconductor (CMOS) detectors from Photobit.

CMOS light sensors can read the circuitry directly without extra components and, in high volumes, they are very inexpensive to produce. "We can get many thousands of these parts in one CMOS wafer," Stam said.

Working with Photobit, Gentex engineers designed a light sensor with a very different pixel function, compared with photodiodes and photoresistors: "We wanted one big pixel," Stam said.
The light-sensitive area of the pixel is only about 100 µm in diameter, and the entire chip measures only 0.5 mm². Stam said he likes the optical performance of the single-pixel architecture.

"You can build all kinds of logic around it," he said. "In a CMOS fabrication process, you can put that all on one chip."

Although the company is not publicly announcing its specifications as yet, he said that the device can measure light levels down to a few hundredths of a lux and up to many thousand lux.