Assessing the Visibility of LEDs in Night Vision
LED Measurement Methods and NVISable Green A LEDs: A Comprehensive Overview
Standardizing the measurement of LEDs, particularly NVISable Green A LEDs, is crucial for aerospace engineers designing illuminated cockpit instruments. Here's a breakdown of the two primary methods for LED light collection, their implications, and the importance of using integrating spheres in measuring NVIS radiance.
Light Collection Methods Compared
Two common methods for collecting LED light emission are integrating spheres and intensity probes. Let's delve into each method, highlighting their advantages and disadvantages.
Integrating Spheres: Light Unification and Accuracy
Integrating spheres are used to collect and homogenize light from all directions, offering a highly uniform optical field at detectors. This uniformity ensures precise radiance measurements since the detector always views the same average irradiance, irrespective of the LED's spatial output pattern. Furthermore, integrating spheres play an essential role in accurately quantifying NVIS radiance, helping ensure compatibility with night vision imaging systems (NVIS) as per MIL-STD-3009 [2][3].
Integrating spheres can also help detect fluorescence by capturing all emitted light (including secondary emissions), though secondary analysis (spectral filtering) is often required to distinguish primary from fluorescent output [2][3].
Tube-based Systems: Limitations and Inaccuracies
Tube-based systems, such as collimated or focused optical paths, collect light primarily along a specific axis. While this is useful for beam profiling or power measurement in a particular direction, it may fail to capture the full spatial or angular distribution of emitted light. As a result, there is a risk of underestimating total NVIS radiance, especially if the LED has significant off-axis emission due to fluorescence or scattering [2][3].
Tube-based measurements may miss fluorescence phenomena that emit light outside the main detection axis, leading to inaccurate NVIS radiance measurements and potentially underestimated or missed fluorescence contributions [2][3].
The Importance of Integrating Spheres for NVIS Radiance Measurement
For NVIS radiance compliance, integrating spheres provide a more complete and accurate assessment by capturing all emitted light, including off-axis contributions that could affect night vision equipment [2][3]. This is crucial because the NVIS radiance must not exceed strict thresholds to avoid disrupting night vision equipment.
Additionally, fluorescence may generate additional wavelengths or off-axis emissions that are only fully detected by an integrating sphere. A tube system could miss these, leading to inaccurate NVIS ratings or unexpected compatibility issues.
In conclusion, using integrating spheres for measuring NVISable Green A LEDs ensures that the entire angular distribution of emitted and fluorescent light is accounted for, leading to more accurate NVIS radiance measurements. Tube-based systems may underestimate off-axis and fluorescence emissions, potentially resulting in non-compliance or unforeseen effects on night vision systems [2][3].
In the context of LED measurement methods and NVISable Green A LEDs, integrating spheres offer superior accuracy in radiance measurements due to their ability to capture light from all directions, which is essential for quantifying NVIS radiance and ensuring compatibility with night vision imaging systems (MIL-STD-3009). On the other hand, tube-based systems, like collimated or focused optical paths, may underestimate total NVIS radiance and miss important fluorescence contributions, especially for LEDs with significant off-axis emission or multi-wavelength outputs, potentially leading to non-compliance or unforeseen effects on night vision systems.
