ASD Document 600540 Rev. J 9 FieldSpec® 3 User Manual
www.asdi.com Chapter 1 Introduction
When features for spectrum averaging (or sample count) are turned on within
the application software on the instrument controller, the averaging is
performed at the spectroradiometer.
Dark Current Measurement
Dark Current (DC) refers to current generated within a detector in the absence
of any external photons. DC is the amount of electrical current that is inherent
in the spectrometer’s detectors and other electrical components and is additive
to the signal generated by the measured external optical radiation.
Note: Noise is the uncertainty in a given measurement, one channel at a time. Noise
by definition is random. Noise can be reduced by using more samples and
averaging the signal. Dark Current is different from noise, because it is
relatively stable and can be characterized.
This manual uses DC to refer to all systematic contributions to the detector
signal. DC is a property of the detector and the associated electronics (not the
light source). DC varies with temperature. In the VNIR region, DC also varies
with integration time.
Whenever DC is taken, a mechanical shutter is used to block off the entrance
slit of the VNIR spectrometer so the signal can be measured. This signal is
subtracted from each subsequent spectrum until another DC is taken. The
SWIR spectrometers take and subtract DC on every scan.
The DC measurement can be updated at any time, but should be updated more
frequently in the beginning of a given session while the instrument warms up.
The VNIR spectrometer is fitted with a unique software and hardware
combination feature called Driftlock. Driftlock corrects for DC changes over
time. It automatically updates DC for every measurement by looking at a
series of masked pixels at the front portion of the VNIR array. The Driftlock
feature corrects for the majority of DC variation over time.
White Reference
A material with approximately 100% reflectance across the entire spectrum is
called a white reference panel or white reference standard.
The raw measurement made by the spectroradiometer is influenced by both
the sample and the light source. An independent measure of the light source
illumination on a reference of known reflectance is required to calculate the
reflectance of the sample. The use of a white reference standard with near
100% reflectance simplifies this calculation.
Reflectance and transmittance are inherent properties of all materials and are
independent of the light source.
Reflectance is the ratio of energy reflected from a sample to the energy
incident on the sample. Spectral Reflectance is the reflectance as a
function of wavelength.
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