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In this letter, two methods for characterizing fog attenuation in terms of atmospheric visibility V in free space optics (FSO) communications are analyzed and compared. Both methods measure the fog attenuation based on V using a charge-coupled device (CCD) camera and a laser-diode at 0.55 μm. The methods are evaluated in a laboratory controlled FSO atmospheric chamber operating at individual wavelengths of 0.83, 1.31, and 1.55 μm using a continuous optical spectrum range 0.6-1.6 μm. The CCD technique shows great accuracy for V <; 50 m and the laser technique for the range 100 m, thus allowing enhancement of the characterization of FSO links in thick and dense fog conditions. The experimental results also show a wavelength dependency for V <; 0.5 km where the attenuation decreases from 52 dB/km at visible wavelengths to 46 dB/km at near infrared wavelengths.