VT200 PerkinElmer Optoelectronics, VT200 Datasheet - Page 29

• Manufacturer Part Number: VT200
• Description: Photoconductive Cells and Analog Optoisolators (vactrols)
• Manufacturer: PerkinElmer Optoelectronics

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