Light sensors are ubiquitous across a broad range of everyday products now
These systems and applications use reflected light with optical detection
for position sensing, like bar code readers, laser printers and
auto-focusing microscopes. Other applications, such as digital cameras, cell
phone and notebook PCs, use optical sensors to gauge the amount of ambient
light. In notebook PCs, for example, they adjust the screen’s backlight to
comfortable levels for the viewer. The range of the comfortable levels
depends on the room’s light. This group of products and applications has the
potential for saving power by applying sensors intelligently.
Ambient light sensors now are included in notebook computers to sense the
ambient (or ‘encircling’) light, allowing for adjustment of the screen’s
backlight to levels that can be considered comfortable for the viewer. The
range of “comfortable levels” is dependent on the room’s light and the
sensitivity of the human eye. 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 — for comfortable viewing and also
to save battery life. The human eye’s response breaks the received light
into one of three regions: low-light (as in the car or home), medium-light
(as in an office setting) and full daylight. The best and most optimal
ambient light sensors will incorporate the brightness versus illumination
information to maximize resolution and at the same time will save power. In
laptop design, ambient light sensors are typically placed next to the
speakers where the case has an opening for light. These portals are commonly
covered by a crosshatch pattern to protect the speakers. Because of this
protection (and the fact that 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 to be measured, requiring a solution with low-light
A second common example is an ambient light sensor used in a cell phone
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 a factor of 4 (assuming the backlight would
remain on full power without any feedback
from a light sensor). Devices like the ISL29000 from Intersil sense ambient
light intensity and outputs a proportional current.
The process technology is able to place a photo diode and transimpedance
amplifier in one die. This combination allows for lead length reduction and
minimum parasitic capacitance on the amplifier inputs and is the optimal
condition for minimum noise, high frequency and convenience. The active
circuitry allows the extension of sensitivity down to 1 Lux while preserving
the upper limit of 100 K Lux. The power drawn is still dependent on the
amount of light sensed, reaching 0.9 mA for 1000 Lux.
To conserve power, a power-down pin is included making the device suitable
for many kinds of applications, including such diverse products as digital
cameras and automotive navigation systems.
Typically, these types of active solutions, which integrate a
phototransistor or a photo diode with a current amplifier, are the best
choice for advanced applications that require higher resolution, low-light
capability, power supply rejection or a disabling function.
There are various optical sensors available in small packages at reasonable
prices. They are available now to save power in a wide range of products
that consumer use every day. By taking advantage of the latest optical
sensor technology, battery life can be extended significantly.
By Tamara Schmitz, Intersil Corporation
About the Author:
Tamara Schmitz is a principal application engineer with Intersil Corp., Milpitas, Calif. She also is a former professor of electrical engineering at San Jose State University, Calif. She has a BSEE, MSEE, and PhD in RF CMOS design from Stanford University.