29.06.2006

page 2 of 8

Standard silicon (Si) photo detectors have a

spectral response ranging from 1100nm

right down to 350nm with the peak sensitivity

around

880nm.

Human

eyes,

however,

detect a much narrower wavelength range,

namely from 400 nm to 700 nm with the

peak sensitivity at 560nm (Figure 1).

Figure

1:

Spectral

sensitivity

of

a

standard Si-detector compared to the

human eye

Ambient Light Sensors versus standard

Silicon detectors

Most light sources emit both visible and IR

light. Different light sources can have similar

visible brightness (LUX) but different IR

emissions (Figure 2).

Figure 2: Spectral emission of different

light sources compared to the spectral

sensitivity of human eye (V lambda)

These

differences

in

the

emission

characteristics and the spectral sensitivity of

the detector have to be taken into account

when measuring brightness. Standard Si-

detectors that detect mostly IR radiation

(peak sensitivity at 880nm) can give you a

false reading as to what the real ambient

visible conditions are. In other words, for

light sources with a high contribution of IR

light, the signal received by a standard Si-

detector would suggest a much brighter

situation than our eyes actually see.

Figure 3 illustrates this effect. It shows the

signals a standard Si-detector yields for

different light sources compared to the

signals that a “human eye like” detector

would see. For IR-rich light sources like light

bulbs the Si-detector signals are much

higher than those of the “human eye”

detector. Lighting which is controlled by such

Si-sensors will not resemble the optimum

brightness as felt by humans. To establish a

more suitable dimming or lighting control, it

is essential to find a sensor which emulates

human eyes as closely as possible.

Figure 3: Signals received by a standard

Si-detector for different light sources at

the same brightness (500lx) compared to

a

detector

with

perfect

human

eye

characteristics

Si-Ambient light sensors have a spectral

response ranging from 1100nm right down

to 350nm but with the peak sensitivity

around 560nm. This peak is nearly identical

to

the

human

eye

spectral

sensitivity

maximum. Most ambient light sensors are

also based on Si, but they use different chip

structures and filter layers to shift the peak

sensitivity and to suppress as much IR

radiation

as

possible.

The

degree

of

matching between the sensor’s spectral

sensitivity and the human eye curve is an

indicator of the performance of an ambient