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Chapter 5 – Measuring with the
Photodiode Head
Photodiode Head
This chapter provides an overview of photodiode heads and instructions for
taking measurements with the photodiode head. Topics include:
taking measurements with the photodiode head. Topics include:
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Overview of Photodiode Heads
When a photon source, such as a laser, is directed at a photodiode head, a current
is created proportional to the light intensity and dependent on the wavelength.
is created proportional to the light intensity and dependent on the wavelength.
Ophir photodiode heads have a unique dual detector head containing two
identical detectors, connected back to back. When a uniform signal, such as
background room light, falls on the detector head, the signal from the two
detectors cancels.
identical detectors, connected back to back. When a uniform signal, such as
background room light, falls on the detector head, the signal from the two
detectors cancels.
Conversely, when a laser beam falls on the head, it illuminates the first detector
only and is detected. In this way, the head subtracts most of the background
while still detecting the desired signal. In general, 98% of the background signal
is eliminated. This means that the detector can be used in ordinary laboratory
lighting conditions.
only and is detected. In this way, the head subtracts most of the background
while still detecting the desired signal. In general, 98% of the background signal
is eliminated. This means that the detector can be used in ordinary laboratory
lighting conditions.
The instrument amplifies this signal and indicates the power level received by
the head. Due to the superior circuitry of the Ophir instruments, the noise level is
very low, and Ophir photodiode heads have a large dynamic range, from pico
Watts to Watts.
the head. Due to the superior circuitry of the Ophir instruments, the noise level is
very low, and Ophir photodiode heads have a large dynamic range, from pico
Watts to Watts.
Since many low power lasers have powers ranging from 5 to 30mW, and most
photodiode detectors saturate at about 2mW, Ophir photodiode heads have a
built in filter to allow the head to measure up to 30mW or more without
saturation. When the additional filter is applied, the maximum power can range
from 300mW to 3W. Photodiode heads saturate when the output current exceeds
1.3mA so the exact maximum power depends on the sensitivity of the detector at
the wavelength used. For a more detailed description of the principles of
operation of Ophir thermopile heads, refer to the Ophir Optronics catalog,
available for download from
photodiode detectors saturate at about 2mW, Ophir photodiode heads have a
built in filter to allow the head to measure up to 30mW or more without
saturation. When the additional filter is applied, the maximum power can range
from 300mW to 3W. Photodiode heads saturate when the output current exceeds
1.3mA so the exact maximum power depends on the sensitivity of the detector at
the wavelength used. For a more detailed description of the principles of
operation of Ophir thermopile heads, refer to the Ophir Optronics catalog,
available for download from
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StarLab User Guide
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