Application Notes—Photoconductive Cells
LED Light Sources
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.
LED Type
Color
λ
P
GaP
GaAsP/GaP
GaAsP/GaP
GaAsP/GaAs
AIGaAs
GREEN
YELLOW
ORANGE
RED
569 nm
585 nm
635 nm
655 nm
660 nm
697 nm
880 nm
940 nm
Normalized LED/Photocell Matching
LED Type
λ (nm)
Type Ø Material Type 3 Material
P
RED
GaP
GaAsP/GaP
GaAsP/GaP
GaAsP/GaAs
AIGaAs
569
58
39%
60%
49%
31%
31%
47%
—
40%
52%
38%
27%
27%
31%
—
GaP/GaP
GaAIAs
RED
INFRARED
INFRARED
635
655
66
GaAs
APPLICATION NOTE #4
Spectral Matching of LEDs and
Photoconductive Types
GaP/GaP
GaAIAs
697
880
940
GaAs
—
—
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.
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.
Power Output
LED Type
Color
λ (nm)
P
I = 1 mA
I = 10 mA
f
f
GaP
GaAsP/GaP
GaAsP/GaP
GaAsP/GaAs
AIGaAs
GREEN
YELLOW
ORANGE
RED
569 nm
585 nm
635 nm
655 nm
660 nm
697 nm
880 nm
940 nm
1.2 µW
0.3 µW
3.2 µW
6.2 µW
33.8 µW
54.3 µW
76.8 µW
35.5 µW
24.1 µW
26.2 µW
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.
101.9 µW
102.1 µW
445.1 µW
296.2 µW
1512.3 µW
675.0 µW
RED
GaP/GaP
GaAIAs
RED
INFRARED
INFRARED
GaAs
24
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
