Point spread function measurements for underwater imaging: an analysis of wavelength-specific behavior for image deconvolution.

Underwater optical imaging, especially in coastal waters, suffers from reduced spatial resolution and contrast by forward scattered light. With the increased number of hyper- and multi-spectral imaging applications, the effect of the point spread function (PSF) at different spectral bands becomes increasingly more relevant. In this work, extensive laboratory measurements of the PSF at 450, 500, 550, 600 and 650 nm in different turbidity have been carried out. Measurements were performed in deionized water with added scattering agent Maalox and in selected natural waters. The experimental setup and measurement routine, including a HDR camera calibration allowing PSF measurements at more than 6 orders of magnitude, is described in detail. Measured PSFs are compared and discussed in terms of wavelength-specific slope behavior, showing significant shape differences between color channels with increasing Maalox concentration. Image deconvolutions of a test target with correct and wrong color PSF indicate the relevance of wavelength-specific PSF consideration, especially in correcting channels of longer wavelengths. The results also indicate, that PSFs derived from models that are based on single wavelength PSF measurements can be used to effectively restore images at different wavelength when acquired over long scattering ranges and being scaled properly.