# Optical Design and Stray Light Analysis of Underwater Spectral Radiometer
## 1 Introduction  
The ocean is the origin of life on Earth. About 71% of the Earth's surface is covered by water, and the ocean, as the largest part, harbors rich natural resources, especially fisheries resources. For a long time, obtaining food from the ocean and lakes has been one of the crucial means of sustenance for humanity. Since the 20 th century, human activities and global climate change have led to ecological degradation in the water bodies of many countries. Inland and nearshore marine water quality has generally declined, with a substantial reduction in the distribution area of seagrass beds and coral reefs. Additionally, the widespread issue of algal blooms is showing a global trend. Aquatic economic species are experiencing a decrease in age and size, and there is a noticeable trend of desertification on the seabed [1]. Water plants and large algae, similar to terrestrial plants, play the role of primary producers of water. Through photosynthesis, they contribute 50% of the Earth's fixed carbon, playing a crucial role in the global carbon cycle. Light serves as the primary energy source for aquatic ecosystems, influencing the photosynthesis of phytoplankton and, consequently, algal content and primary productivity [2, 3]. To protect water resources, improve water quality, and ensure the healthy development of aquatic ecosystems, it is essential to focus on and maintain the lighting conditions in the water. Underwater spectral measurements are of significant importance for monitoring water quality, the health of ecosystems, and changes in marine and freshwater environments. Water optical property measurement techniques are the driving force behind the development of marine optics [4]. In the mid-19 th century, as attention to the quality of water and environmental conditions increased, scientists began seeking a convenient methodto measure the transparency of water. Consequently, the transparency disk emerged, becoming a portable and easy-to-use tool for monitoring water quality in field environments. In the 1930s, the advent of photomultiplier tubes brought about a significant revolution in the development of underwater optical technology, providing valuable experiential foundations for understanding the interaction between light and seawater. In the 1980s, the development of water color satellite remote sensing technology sparked widespread interest in the optical properties of the oceans. The progress in ocean optical measurement techniques has propelled research and development in marine ecosystems, marine environmental conservation, carbon cycling, and global climate change [5]. The variation and distribution of marine water color are influenced by substances in the upper water layer. To gain a thorough understanding of the optical properties of seawater and their impact on light transmission, it is necessary to employ high-performance optical measuring instruments for water color remote sensing research. However, marine water color remote sensing instruments, primarily relying on remote sensing, are subject to interference from atmospheric Rayleigh scattering, aerosol scattering, and other factors. This leads to disparities between the apparent optical parameters of water bodies observed by satellites and the actual apparent optical parameters of the water. Additionally, due to factors such as cloud cover and low light, optical remote sensing is unable to observe high-dynamic water bodies for a significant portion of the time. In contrast, in-situ measurements, as a method for water monitoring, can overcome these challenges [6]. The in-orbit data from satellite water color remote sensing instruments needs to be verified with on-site measurement data that possesses sufficient accuracy, repeatability, and traceability [7]. There are three methods for on-site observation in water color remote sensing: above-water method, sky-light obstruction method, and underwater profile method.