Theory Of Operation - ASD FieldSpec 3 User Manual

www.asdi.com
1.4

Theory of Operation

Fiberoptic Collection of Reflected/Transmitted Light
Inside the FieldSpec® 3 Spectroradiometer
Visible/Near-Infrared (VNIR)
ASD Document 600540 Rev. J
The FieldSpec spectroradiometer measures the optical energy that is reflected
by, absorbed into, or transmitted through a sample. Optical energy refers to a
wavelength range that is greater than just the visible wavelengths, and is
sometimes called electromagnetic radiation or optical radiation.
In its most basic configuration, the spectroradiometer views and detects the
form of radiant energy defined as radiance. With accessories, various set-ups,
and built-in processing of the radiance signal, the FieldSpec
spectroradiometer can measure:
• spectral reflectance,
• spectral transmittance,
• spectral absorbance,
• spectral radiance, and
• spectral irradiance.
Optical energy is collected through a bundle of specially formulated optical
fibers, precisely cut, polished, and sealed for extremely efficient energy
collection. The fibers themselves are of low OH composition providing the
maximum transmission available across the wavelength range of the
instrument.
The fiber cable delivers the collected optical energy into the instrument,
where it is projected onto a holographic diffraction grating. The grating
separates and reflects the wavelength components for independent
measurement by the detectors.
The Visible/Near-Infrared (VNIR: 350-1000 nm wavelength) portion of the
spectrum is measured by a 512-channel silicon photodiode array overlaid with
an order separation filter. Each channel (or detector) is geometrically
positioned to receive light within a narrow (1.4 nm) range. The VNIR
spectrometer has a spectral resolution (full-width half maximum of a single
emission line) of approximately 3 nm at around 700 nm.
Each detector converts incident photons into electrons. This photo current is
continually converted to a voltage and is then periodically digitized by a
16-bit analog-to-digital (A/D) converter. This digitized spectral data is then
transmitted to the instrument controller for further processing and analysis by
the controlling software.
7
Chapter 1 Introduction
FieldSpec® 3 User Manual