LEDs light the way

LEDs have long been favored over incandescent bulbs as light sources in optical sensors. LEDs are much smaller than light bulbs, inexpensive, easy to produce, consume less power, and last longer.

But LEDs have undergone changes as well; from the through-hole mount 5-mm LED with cast lens to the surface-mount variety. However, the LED housing has always limited the size of optical sensors.

The trend toward ever-smaller sensor modules in automation applications called for a rethinking of the way LEDs were built into sensor housings. For instance, Munichbased STM Advanced Technologies installed the semiconductor chips in the sensor housing with powerful optics. But even the dimension and structure of the LED chip itself had become a barrier.

STM has produced a new device that addresses these issues. The new LEDs have a small, circular light-emitting opening, with electrical contacts on the outside that don't interfere with light emission. The smaller opening leads to a smaller scattering angle which means light is not as spread out as on standard LEDs. A more concentrated beam of light lets the beam project further.

The main application for LEDs is in displays which don't require a strong, concentrated light beam. A standard LED's construction is not designed with optosensor applications in mind. At the center is a metal electrical contact pad which emits no light and takes up as much as 30 to 40% of the active front surface. This results in an intensity minimum in the center along the optical axis, producing a dark patch. This only became a problem as optics became smaller.

Although the energy demands on the new LED are lower, there is no compromise in focusing potential. Narrowing the opening angle enables longer beam ranges from the same housing. The opening angle can be reduced from 4 or 5° to below 1.2°. So for instance, at a distance of 100 mm, an angle of 5° gives a light patch with a 17.5-mm diameter. But an angle of 1° gives a 3.5-mm diameter patch of light.

In the 3-mm housing, the old sensor had a maximum distance of 300 mm, while the new version extends to 500 mm. For the reflective sensor, the old distance was 50 mm, and the new is 70 mm.

The biggest advantage is better resolution of light barriers and light switches. This is especially true for light curtains where several light sources operate in parallel with the possibility of crosstalk between light beams. In the past, this interference between light beams was suppressed using electronic circuits. But the smaller angle eliminates interference.