Graphical Displays
1919-R User Manual
53
For Photodiode Sensors Measuring Exposure
Background noise that affects power measurement will affect exposure as well. To overcome this,
perform the following steps:
1.
Enter
Power
mode.
2.
Block the laser/light source under measurement and press the
Offset
key. This will activate the
offset feature. When active, the
Offset
key is reverse highlighted and the offset that is being
subtracted is shown in the upper right part of the screen.
3.
Set the
Mode
to
Exposure
.
4.
The Offset stored in the Power screen will be subtracted in order to provide accurate Exposure
measurements.
The legend is highlighted when
Offset
on the right below). The offset that
is being subtracted is shown on the upper left part of the screen (right above the numeric display).
Figure
8-5 Photodiode Bargraph
Figure
8-6 Photodiode Bargraph with Offset
To deactivate, press
Offset
again. If the
Offset
is engaged, and you wish to subtract a new value of
the background, press
Offset
twice. The first press will cancel the old value, and the second will
activate a new value.
If you suspect that 1919-R has a permanent zero offset, the instrument’s internal zero should be
reset. See
For Thermopile Sensors Measuring Single-Shot Energy and for Energy
Sensors
Unlike power, offset subtraction is not necessary to achieve accurate energy measurements.
However, Offset can be used to facilitate comparison between readings. For example, the first
laser pulse is 1 Joule. To subtract this from future readings, press the
Offset
key. If the next pulse
is actually 3 Joules, 2 Joules will be displayed on the screen, thereby indicating the difference
between the two laser pulses.
8.2.4
Measuring Loss Using the dB Offset Function
Since dBm is a logarithmic measurement, the ratio between two measurements will be the
difference between the dBm measurements. For instance, if you want to measure the loss in a
fiber optic cable where the measurement before the cable is 1mW = 0dBm and the measurement
after the cable is 0.1mW = -10dBm. The ratio is then 1:10 = 0.1 and the dB loss is 0 – (-10) = 10dB.
The dB offset function allows you to easily measure this.
