EXFO FTB -5240S/5240BP Optical Spectrum Analyzer for FTB 500 (NS-1434) - Manual (Page 215)

Setting Up the Instrument in Spectral Transmittance Mode
Optical Spectrum Analyzer 207
Setting Up Acquisition Parameters
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If you are performing an Averaging type acquisition, enter the
number of scans the unit will perform.
3. Press Input or Output to specify which position to use to store the next
acquisition.
4. Select the wavelength range for your acquisition.
You can select the wavelength range by entering the start and stop values
or by selecting a range on the double slider.
To select the wavelength range using the double slider, move the left and
right handles on the double slider or simply click on any band.