Newport 1815-C, Operator'S Manual

Page: 25 / 34

12
3.3.4 Ambient and Stray Light
Ambient and stray light striking the detector will be measured by the Model
1815-C, and should be considered when making careful measurements. Ambi-
ent light can be distinguished from dark current (or the detector/meter noise
floor) by either turning off or blocking the source
and covering the detector
face with opaque material such as a piece of black metal. Using the human
hand to cover the detector is not advised because it both emits a significant
amount of infrared radiation, and because it radiates a temperature signifi-
cantly different from ambient. With the detector covered, a reading of the
dark current may be made. Next, remove the material which is covering the
detector and take another reading. The difference is the ambient light level.
The effects of ambient light are greatly reduce when using a fiber-
connectorized signal input to the detector. If free-space beam measurements
are desired, using an attenuator (
Low-Power detectors have an OD3 attenua-
tor included) will reduce stray light and often improve the source signal to
ambient signal noise level. Wavelength-specific filters, such as optical cutoff,
bandpass, or spike filters can also be used if the signal wavelength spectrum
permits. Other techniques to reduce stray light include using apertures,
placing the detector in a box or other housing to shield the surface from light
(or air currents when using Newport’s
High-Power disk thermopile detectors)
which is not coming from the source, and turning off room and other lights.
Note
Changes in ambient light levels can occur from such factors as
turning room lights on or off, or by moving people or equipment.
Remember, if you can see your detector element, then your detector
can see the light bouncing off your shirt!
Basic measurement techniques for using the Model 1815-C are covered in the
following sections. Also included are methods of background correction and
common measurement errors. In general the absolute measurement accuracy
is limited by the accuracy of the detector calibration and environmental
factors affecting the detector. See the appropriate detector manual for
specific information on a particular detector model.
3.4.1 Making a Power Measurement
The following process describes the procedure for making a basic optical
power measurement while properly removing influence of ambient light and
other drift effects.
1. Set up the meter calibration factor to reflect the detector at wavelength
of light you will be making your power measurement at. See Sections
2.4.4.1 and 2.4.4.2.
2. Cover or otherwise block the source that you will be measuring and
adjust the Range knob until the ambient signal is displayed to three
significant figures.
3. Use the ZERO knob to remove the ambient signal by zeroing the display.
4. Uncover the source and adjust the Range knob until the source power is
displayed to three significant figures of accuracy.
5. Record the display value and the appropriate units from the Range
knobs position. This reading is your optical power.
Performing Basic
Measurements
3.4