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To characterize the water column, the diffuse attenuation coefficient of downwelling irradiance, K_d(z,λ) (m⁻¹) is one of the most important optical properties of seawater. The purpose of this research was to determine the downwelling diffuse attenuation coefficient of water around Roatan Island, Honduras. In situ K_d analysis showed low attenuation coefficient values in green and blue and increased exponentially after 570 nm. The blue, green and red portion of the spectrum showed a K_d value of 0.138, 0.158, and 0.503 m⁻¹, respectively. Error analysis revealed a significantly high uncertainty in the red region (600–700 nm) and, as expected, low estimation uncertainty in blue and green. When compared with IKONOS derived K_d (490 nm), it was observed that the differences were negligible, being 0.0084 and 0.0054 m⁻¹ for station #1 and #2, respectively. Based on the fact that 90% of the diffused reflected light from …