Instrument self-shading in underwater optical measurements: experimental data

Area of Science:

• Ocean optics
• In-water optical measurements
• Radiometry

Background:

• Self-shading is a significant error source in in-water optical measurements.
• Accurate estimation of self-shading is crucial for reliable radiance and irradiance data.
• Existing theoretical models require experimental validation for various conditions.

Purpose of the Study:

• To experimentally quantify self-shading errors in upwelling radiance and irradiance measurements.
• To compare experimental results with theoretical predictions.
• To assess the applicability of narrow-field-of-view correction schemes to wider fields of view.

Main Methods:

• In-water optical measurements of radiance and irradiance were conducted just below the water surface.
• Fiber optics with 1°, 18°, and 2π optics were used, housed within a disk simulating instrument size.
• Measurements were taken in lake waters at 500, 600, and 640 nm, varying instrument size, absorption, Sun zenith angle, and atmospheric turbidity.

Main Results:

• Self-shading errors varied from a few percent to tens of percent, depending on instrument size, absorption coefficient, Sun zenith angle, and atmospheric turbidity.
• Experimental and theoretical error computations showed good agreement, with absolute differences below 5% for radiance and 3% for irradiance.
• No significant difference in self-shading error was observed between 1° and 18° fields of view.

Conclusions:

• Experimental validation supports the accuracy of theoretical models for estimating self-shading errors.
• Correction schemes developed for narrow-field radiance measurements may be applicable to wider fields of view.
• Accurate self-shading error correction is vital for improving the quality of in-water optical data.