Graphical Displays
StarBright User Manual
55
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
9-5 Photodiode Bargraph
Figure
9-6 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 StarBright 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.
9.2.4
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.
To measure loss using the dB Offset function:
1.
When measuring the reference value, press the “dB Offset” key.
2.
Now make your second measurement and the value of the difference in dB = ratio in
numerical units will be shown.
Note:
If there is a zero offset in the reference value, you cannot subtract this using the dB offset
function. Instead, before the start of the measurement, press
Offset
and subtract the zero offset.
Then follow steps 1 and 2 above. The zero offset subtracted when
Offset
was pressed will be
saved in the dBm scale and you can now use the dB Offset setting to measure true ratio without
zero offset problems.