Scientists recently said that active optical sensors can dramatically increase the range and usefulness of ambient light sensors, which are now found in a wide range of display applications. But power management strategies are critical in order to reduce total consumption and preserve battery life in portable or cellphone products where optical sensors are used. By applying ambient light sensing in conjunction with LED backlight driving, power consumption can be reduced significantly—while battery life increases. Advances in active solutions are exemplified by an example, an ambient light sensor for a notebook PC.

The process technology is able to place a photodiode and transimpedance amplifier in one die, as shown in Figure 1. 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 low noise characteristics extend the sensitivity of the sensor down to 1lx while keeping the upper limit of 100klx. The power drawn is still dependent on the amount of light sensed, reaching 0.9mA for 1,000lx.

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 photo transistor or a photodiode 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.

Ambient light sensors now are included in notebook PCs 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. The relationship is shown in Figure 2. Of course, 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 as shown in Figure 2: low-light (as in the car or home), mediumlight (as in an office setting) and full daylight.

The very best and most optimal ambient light sensors will incorporate the brightness vs. illumination information to maximize resolution and at the same time will save power. In notebook PC 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 (and the fact that the light sensor is next to the speaker instead of on top of it), light is obstructed. The obstruction reduces the amount of light to be measured, requiring a solution with low-light accuracy