VTL5C2SLASH2 datasheet(29/76 Pages) PERKINELMER

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Application Notes—Photoconductive Cells

LED Light Sources

APPLICATION NOTE #4

Spectral Matching of LEDs and

Photoconductive Types

Since light sources and light detectors are almost always used

together the designer must take into consideration the optical coupling

of this system or the ability of the detector to “see” the light source.

In order to have good optical coupling between the emitter and the

conductor the spectral output of the light source must, to some degree,

overlap the spectral response of the detector. If the design involves the

use of a light source with a broad band spectral output the designer is

assured that the photocell will have good response to the light. This

may not be the case when an LED light source is employed. LEDs emit

their light within a very narrow spectral band so that they are often

considered to be emitting at only on (peak) wavelength.

Spectral matching factors were calculated for a number of different

LEDs and the photoconductor material types manufactured by

PerkinElmer. Each matching factor was derived by multiplying the

detector response curves by the LED spectral output curve and then

measuring the resulting area.

The LED/photocell matching factors listed are independent of power

output from the LEDs. In order to get a real feel on how well any LED/

photocell pair couple together, the power output from the LED at a

particular forward drive current must be considered.

Normalized LED/Photocell Matching

The intensity of the light being emitted by visible LEDs is often given in

units of millicandela. Millicandela is photometric unit of measure which

assumes the human eye as the detector. For most detectors other than

the human eye the most convenient system for measurement is the

radiometric system. Listed below is the typical light power output of

some LEDs measured at two different forward drive currents. Note that

LEDs of a given type can show a 5:1 manufacturing spread in power

outputs.

LED Type

Color

GaP

GREEN

569 nm

GaAsP/GaP

YELLOW

585 nm

GaAsP/GaP

ORANGE

635 nm

GaAsP/GaAs

RED

655 nm

AIGaAs

RED

660 nm

GaP/GaP

RED

697 nm

GaAIAs

INFRARED

880 nm

GaAs

INFRARED

940 nm

LED Type

P (nm)

Type Ø Material

Type 3 Material

GaP

569

39%

40%

GaAsP/GaP

60%

52%

GaAsP/GaP

635

49%

38%

GaAsP/GaAs

655

31%

27%

AIGaAs

31%

27%

GaP/GaP

697

47%

31%

GaAIAs

880

—

GaAs

940

—

LED Type

Color

P (nm)

Power Output

If = 1 mA

If = 10 mA

GaP

GREEN

569 nm

1.2 µW

24.1 µW

GaAsP/GaP

YELLOW

585 nm

0.3 µW

26.2 µW

GaAsP/GaP

ORANGE

635 nm

3.2 µW

101.9 µW

GaAsP/GaAs

RED

655 nm

6.2 µW

102.1 µW

AIGaAs

RED

660 nm

33.8 µW

445.1 µW

GaP/GaP

RED

697 nm

54.3 µW

296.2 µW

GaAIAs

INFRARED

880 nm

76.8 µW

1512.3 µW

GaAs

INFRARED

940 nm

35.5 µW

675.0 µW

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