SDP8475-201
Low Light Rejection Phototransistor
Chart A. Low Light Rejection Phototransistor vs. Standard Phototransistor
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
0.000 0.125 0.250 0.375 0.500
Source intensity - mW/cm
2
Light current - mA
Min. Light
Current Slope
Max. Light
Current Slope
Min. Light
Current Slope
Standard
Max. Light
Current Slope
Standard
Designing with the Low Light Rejection
Phototransistor:
The Low Light Rejection detector is tested at different
incident light levels to determine adherence to the
specified knee point and light current slope. This method
assures proper functionality vs. standard
phototransistors, and guarantees required light current
output.
The light current slope is the change in light current
output at two given source irradiances divided by the
change in the two source irradiances.
(Formula # 1)
I
L
Slope = [I
L 1
(@ H
1
) - I
L2
(@ H
2
)] / [H
1
- H
2
]
Where:
• • I
L
slope is the light current slope in mA/mW/cm
2
• I
L
is the light current output in mA
• H is the source intensity in mW/cm
2
Chart A shows the specified limits of light current slope
for the low light rejection phototransistor which begins its
slope at the typical knee point, 0.125mW/cm
2
. To make a
clear distinction between this device and a standard
phototransistor, light current slopes for high and low
sensitivity standard phototransistors are also shown.
Note that for phototransistors of the same gain, the
slopes of the two products are parallel.
The knee point, the source irradiance needed to increase
I
L
to 50uA, is a necessary parameter for circuit design.
All variation in the knee point will be offset by the
internally guardbanded light current slope limits. The
appropriate formula for circuit design is the following:
(Formula # 2)
I
L
= I
L
slope
MIN.
* (H
A
- H
KP
)
Where:
• I
L
is the light current output in mA
• I
L
slope
MIN.
is the minimum limit on the light current
slope (i.e. 4.0mA/mW/cm
2
)
• H
A
is the source light incident on the detector for the
application
• H
KP
is the specified level of source light incident on
the detector at the typical knee point (i.e. 0.125
mW/cm
2
)
Example :
To design a transmissive sensor with two of Honeywell’s
standard components, the SEP8505-002 and the
SDP8475-201, it is first necessary to determine the
irradiance level in mW/cm
2
that will be incident on the
detector. The application conditions are the following:
139
Honeywell reserves the right to make
changes in order to improve design and
supply the best products possible.
