Ambient Light Sensors (ALS)

In many applications the intensity of visible light has
to be measured. The crucial point is that the word “visible”
introduces a physiological element into the problem. The output
signal of the sensor has to correspond to the subjective
impression of the human eye, e.g. if the subjective impression of
the human eye is “bright”, the sensor signal should also be high.
And for the impression “dark” the sensor should show low
signal.
Standard silicon photodetectors work quite well during the bright
day and at complete darkness some hours after the sun has
gone down. But during dawn a problem may occur (so called
red-sky-condition): when the sun goes down the fraction of
visible light is decreasing drastically, but there is still a lot of
infrared radiation present. As the sensitivity of the silicon
detectors has its maximum in the infrared range the signal of
silicon detectors under “red-sky” conditions is too high compared
to the brightness impression of the human eye. Figure 1 shows the sensitivity curve of the human eye (which is
known as V (λ) -curve), the spectral sensitivity of silicon and the
sensitivity spectrum of an ambient light sensor of EVERLigHT.
Using a standard silicon photodiode as a light sensor for garden
lights, it would be tough to find an adjustment where the light
would switch on during dawn. In most cases it would switch on
after there is complete darkness.
Many of the ambient light sensors are based on a silicon
photodiode chips which are plated by some optical filter layers.
These optical filters absorb the infrared light from the incident
radiation and therefore restrict the sensitivity of the silicon
photodiode chip to the visible area. Due to the small photo
currents at low incident light levels it is convenient to use an
ambient light sensor-IC, which has an amplifier on the same
chip. For higher light intensity levels, also photodiodes and
phototransistors are available as ambient light sensors.
In some special applications it is convenient to use an amorphous
solar cell as an ambient light sensor. The spectral sensitivity of
amorphous silicon does not show essential sensitivity in the
infrared region like crystalline silicon and is quite similar to the
human eye. Since the area of solar cells is usually much larger
than the size of the silicon chips of ambient light sensors, the
photo current is much higher for solar cells. This current can be
used not only to detect the ambient light, but also as an energy
source for the application itself.