The United States Army Uses LabVIEW to Develop Next Generation Night Sky Spectrometer

Author(s):
Roy Littleton - United States Army RDECOM CERDEC, Night Vision and Electrical Sensors Directorate

Industry:
Imaging Equipment, Government/Defense

Products:
PXI/CompactPCI, Data Acquisition, LabVIEW

The Challenge:
Developing a remote spectrometer for next generation low light level electro-optical imaging systems.

The Solution:
Using National Instruments LabVIEW as the basis of a system capable of characterizing the natural spectral irradiance of the night sky at ground level.

At the U.S. Army Night Vision & Electronic Sensors Directorate (NVESD), we specialize in research and development of night vision and other sensor technologies.

The spectral intensity of the night sky is critical for the development, characterization, and deployment of nighttime passive imaging systems operating in the visible (0.4 - 0.7mm), near infrared (0.7 - 1.1mm), and/or shortwave infrared (1.1 – 2.5mm) spectrum. Typical passive low light level imaging sensors are generally signal to noise limited at a given night sky illumination condition ranging from fractional moon, through clear starlight, down to overcast starlight. 

The performance limiting factor for most of these systems is the low amount of available natural scene flux within the operational spectral band of the system and the system's efficiency to convert these low signal levels relative to its fundamental noise floor. The dominant natural sources of night sky irradiation include the moon, airglow (or nightglow), tropospheric thermal radiation and direct and scattered radiation from stars and Zodiacal light. Sources of atmospheric attenuation include absorption, scattering and turbulence. The difficulty in determining night sky spectral irradiance for any given condition is the inherent variation of the individual sources and attenuating mechanisms as well as the angular dependencies relative to the zenith that can also affect the irradiance from individual night sky sources.

Night Sky Irradiance

The atmosphere is divided into six layers that contain various gases and particles, which decrease in temperature and pressure depending on altitude. The first layer, the troposphere, extends from the ground to about 11 km and contains the most significant attenuators, including water, carbon monoxide, clouds, fog, and aerosols. This layer is the dominant source of thermal background radiation at wavelengths > 2 microns. The troposphere has the highest pressure of all the atmospheric layers and thus contains the highest density of particles which in turn produces the most scattering of light. Above the troposphere is the stratosphere, extending to approximately 50 km. The stratosphere contains O₃ (ozone) which is highly absorbent to ultraviolet radiation. Between the next two layers, the mesosphere and the ionosphere, which extend to 90 km and 300 km respectively, OH^- airglow emissions originate, and contribute appreciably to the night sky irradiance. Beyond the ionosphere are the thermosphere and exosphere.

Each of the six atmospheric layers contributes in some degree to the total observable ground level night sky irradiation in terms of transmission, absorption, scattering and even emission. The dominant natural illumination sources on a clear moonless night include airglow, starlight and galactic radiation from space. Airglow is comprised mostly of hydroxyl ion (OH-) emissions which varies with zenith angle but is azimuthally symmetrical. These emissions are due to vibrational and rotational transitions of OH^- at an altitude range of 70 to 110 km, producing energy in several bands. These bands, called Meinel Hydroxyl Bands, contribute significantly to the night sky irradiance, especially above 1 micron. Beyond 2.5 microns the contribution of the hydroxyl emissions is even greater. However, it becomes relatively insignificant compared to the background thermal radiation provided by the troposphere.

Direct and scattered radiation from stars, primarily from the Milky Way, is considered anisotropic across the sky. Galactic radiation, or Zodiacal light, is sunlight scattered by interplanetary dust and therefore contributes more in the visible and near infrared than in the shortwave infrared.

Night Sky Spectrometer

The Night Sky Spectrometer or NSS System uses a grating spectrometer to collect spectral irradiance from a diffuse reflectance panel from 400 to 2,000nm with an effective resolution of <25nm and a sensitivity of ~10^-9 W/cm²/mm. The system is housed in an environmentally controlled enclosure with two solid-state thermoelectric air conditioners that helps keep the interior at nominally 22°C while also reducing any excess moisture. The remote accessibility is possible through a portable satellite system for system control and data transfer and allows the NSS to collect continuous spectral scans over several months unattended.

We developed a custom LabVIEW virtual instrument to be the backbone of the NSS system. The internal computer contains two PCI boards; a NI-PCI-GPIB card and a NI-PCI-6036E Multifunction DAQ card that communicates with each of the hardware components. At the start of each scan, the VI reads the GPS coordinates from the satellite dish to access the current weather conditions from the nearest NOAA weather station and inserts the information along with the temperature and relative humidity from the system’s own sensors into the header of the data. The VI then selects the appropriate detector channel, detector gain, wavelength, filter, and grating for each data point. The VI then acquires the chopper and detector signals and calculates the intensity using a custom lock-in amplifier sub-VI. The VI also includes images from the two external webcams, reads the detector temperature, controls external LED indicators, and even emails the data sets.

The NI software and hardware communicates using two GPIB device addresses, 16 analog input channels, 2 analog output channels, and 8 digital input/output channels which account for every available channel of the PCI-6036E. The VI has three modes that include a Scan Mode, a Noise Measurement Mode, and a Laboratory Re-Calibration Mode.

Author Information:
For more information on this Case Study, contact:
Roy Littleton
United States Army RDECOM CERDEC, Night Vision and Electrical Sensors Directorate
Fort Belvoir, VAUS
Tel: 703-704-0202
roy.littleton@nvl.army.mil

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