Ophir Optronics Ltd QSR Manuel D’Utilisation

This chapter provides an overview of photodiode heads and instructions for 
taking measurements with the photodiode head. Topics include: 

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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. 

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. 

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. 

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. 

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 

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