Filter Transmittance Measurements in the Infrared

Area of Science:

• Optical Metrology
• Infrared Spectroscopy

Background:

• Accurate measurement of filter transmittance is crucial for optical systems.
• Existing heterodyne techniques have limitations in accuracy and dynamic range.

Purpose of the Study:

• To develop and validate a novel direct detection system for precise filter transmittance measurements.
• To enable independent verification of heterodyne transmittance measurement techniques.
• To achieve high accuracy (0.5% uncertainty) over a wide attenuation range (5 decades) in the 9-11 μm spectral region.

Main Methods:

• Utilized a modulated source (30 MHz) and a high-accuracy lock-in amplifier.
• Employed a direct detection approach, adapted from a heterodyne system apparatus.
• Implemented detector scanning and corrections for non-ideal detector and attenuator characteristics.

Main Results:

• Achieved relative combined standard uncertainties as low as 0.5% per decade.
• Demonstrated reliable transmittance measurements over an attenuation range of at least 5 decades.
• Successfully suppressed thermal background radiation through high modulation frequency and narrow bandwidth.

Conclusions:

• The novel direct detection system provides a highly accurate method for filter transmittance measurement.
• The system validates the accuracy of heterodyne techniques.
• Systematic error analysis and correction methods enhance measurement reliability.