29.06.2006

page 3 of 8

light sensor. Figure 4 shows the spectral

sensitivity

of

a

standard

silicon

photo

transistor, an OSRAM ambient light sensor

of the first generation and the human eye (V-

lambda curve).

Figure

4:

Spectral

sensitivity

of

a

standard Si-detector and an ambient light

sensor (SFH 3410) compared to the

human eye

(V-lambda)

Because the IR portion of the spectral

sensitivity of the ambient light sensor is

greatly reduced compared to a standard Si-

detector (see Figure 4), it is less sensitive to

the effects of different lamps. Figure 5

shows the signals of the ambient light

sensor SFH 3410 received from different

lamps of the same brightness compared to

the signals of a standard Si-detector.

Figure 5: Signals received by a standard

Si-photo detector and the ambient light

sensor

SFH

3410

for

different

light

sources at the same brightness (500lx)

The difference of the signals indicates the

accuracy of the brightness measurement. In

the case of the standard Si-detector the

signals vary by more than a factor 8

between light bulb and fluorescent lamp.

This factor is reduced to 3 for the ambient

light sensor, which therefore provides a

much better accuracy for the brightness

measurement.

Measuring ambient light levels

(brightness)

Ambient light sensors are photo detectors.

They yield a photo current which is related to

the

illuminance.

In

most

cases,

the

correlation

between

photo

current

and

illuminance is linear

photo current – illuminance relationship for

the ambient light photo transistor SFH 3410.

The efficiency of the sensor describes the

amount of photo current the sensor yields for

a certain illuminance. In the example of

figure 6, the ambient light sensor yields a

photocurrent of 300

µA at 1000lx. Hence the

efficiency of the sensor is 0,3

µA/lx.

The efficiency of a photo detector depends

on

the

illuminance

under

which

it

is

operated. A change of the efficiency results

in a deviation from the photo current -

illuminance correlation. The linearity of a

detector describes the magnitude of this

deviation. Figure 7 shows the linearity for the

ambient

light

sensor

SFH

3410.

The

deviation from the linear correlation is < 5%

within a brightness range of 30lx … 100klx.

In lower light levels a correction might be

necessary.

sensor SFH 5711 with logarithmic output.