29.06.2006

page 5 of 8

Sensitivity Variation

Due to the manufacturing process ambient

light sensors of different production lots will

yield

different

outputs

for

the

same

illuminance. The magnitude of this sensitivity

variation depends on the sensor type. To

account for this, some ambient light sensors

are offered in defined sensitivity bins. These

are described in the datasheets and in the

application notes of the respective sensors.

The

sensitivity

variation

can

also

be

overcome by calibrating the assembled unit

in the production line.

Accuracy

of

the

ambient

light

measurement

Several factors determine the accuracy of an

ambient light measurement:

• Spectral sensitivity:

High detector sensitivity for IR results in

low

accuracy

of

the

brightness

measurement.

• Temperature coefficient:

The output current of photo detectors

varies with the operating temperature.

Large temperature coefficients result in

brightness measurement deviations at

very high and low temperatures.

• Linearity:

Linearity describes the deviation from

the

photo

current

-

illuminance

correlation function.

• Sensitivity variation

• System

errors

such

as

resistors,

calibration, etc.

Each

of

these

effects

contributes

with

different magnitude to the ambient light

measurement accuracy. Table 2 provides an

overview of these characteristics for different

detector types.

Different types of ambient light

sensors

OSRAM OS offers three different types of

ambient light sensors. Table 2 provides a

selection guide for the different types and

gives an overview of these types with their

main criteria.

Phototransistor

Photodiode

Opto Hybrid (Diode +

IC)

Device

SFH 3410, SFH 3710

SFH 2430

SFH 5711

Output signal

high

low

high

Linearity

good

highest

high

3

Temperature

coefficient

high

lowest

low

Sensitivity

variation

Factor 1:2 in

illuminance per

sensitivity bin

+-15%

Factor 1:2 in illuminance

per sensitivity bin

Photo current –

illuminance

correlation

Linear

Linear

Logarithmic

(high accuracy over

entire dynamic range)

Spectral

sensitivity

Low IR contribution

Low IR contribution

perfect V-

λ characteristic

Size

Small

Large

medium

Table 2: Selection guide for OSRAM ambient light sensors. Different types with their main

characteristics.