#AMBIENT LIGHT DETECTOR FULL#

Having removed the IR component of light from the ALS’s output, the smartphone designer now has a second problem to solve: How do you limit the angle of view of the ALS/proximity-sensor module without impairing its performance? This is a matter of trading off the ALS against the proximity sensor.įor ambient-light sensing, the ideal angle of view is a (practically impossible) 180°, since this is the angle of view of the display on which ambient light is incident. In most cases, a packaging material that can absorb UV light is sufficient to remove it for the purpose of ambient-light sensing. The sensor is fairly insensitive to UV light, and in any case, the common light sources emit little UV radiation. Subtracting the IR photodiode’s output from the full-spectrum sensor’s output produces a measurement of visible light.

#AMBIENT LIGHT DETECTOR FULL#

One photodiode (shown as Channel 0 in Figure 2) senses the full spectrum, while the other (shown as Channel 1) senses light mainly in the IR portion of the spectrum. Moreover, additional space will be occupied by the IR photodiode, which requires an aperture in the face of the display to allow for the passage of IR light.Ī more streamlined solution to the problem is a dual-diode module, which has been developed by ams. However, this is an inelegant solution-now the design must carry the cost of the optical filter on the ALS as well as a discrete IR photodiode. Of course, the smartphone designer could then add a separate IR photodiode just for proximity sensing. The spectral power distribution of sunlight shows a strong IR element thatâs invisible to the human eye.

But in a smartphone, this same sensor is also typically used for proximity detection (in tandem with an IR LED) to turn off the display and touch controller while the phone is held to the user’s face. An obvious way to remove the IR light is to overlay an optical IR filter on the sensor. 1) and the light from an incandescent lamp. This is true, for example, of both sunlight (Fig. This is because both natural and artificial light contain an IR element. In short, the ambient light will tend to appear brighter to the sensor than to the human eye. If this lux measurement includes UV and IR as well as visible light, the display’s backlight controller will receive a distorted representation of the human, or “photopic,” response to ambient-light conditions. The function of the ALS is to capture a measurement of the illuminance (of which the unit of measurement is the lux) incident on the display of the smartphone. This means that the designer’s first challenge is to remove the IR and UV components from the sensor’s output. A standard silicon photodiode, on the other hand, normally senses light at any wavelength between 3 nm. The human eye is insensitive to infrared (IR) light (a wavelength greater than 780 nm) and ultraviolet (UV) light (a wavelength less than 380 nm). The first difficulty is that photodiodes respond differently to light than the human eye. It also offers ways to overcome these challenges to achieve highly responsive and accurate adjustment of the backlight’s brightness in response to ambient light. This article describes the main challenges in implementing successful ambient-light response in a smartphone. These and other constraints seriously limit the design engineer’s freedom to optimize the design. In addition, it must be able to perform proximity detection (to turn the display off when held to the user’s face) as well as the core ambient-light measurement function. But while this application is simple to describe, in practice it’s difficult to implement successfully-i.e., in a way that achieves worthwhile power savings while remaining unobtrusive to the user.Īn ALS must be mounted behind a display screen in which every square millimeter of area is valuable. This is also easier on the user’s eyes, thus improving the user experience at the same time.Īmbient-light sensors (ALSs) are in fact now widely used in smartphones to provide information about ambient light levels, in support of the backlight LED power circuit. In a relatively dark environment, the brightness of the display can be reduced to save power. Since the backlight of the LCD panel in a smartphone accounts for around 40% of the device’s total power consumption, one can benefit greatly by adjusting the brightness in response to changes in ambient-light levels.

This file type includes high-resolution graphics and schematics when applicable.

Ambient light detector