【文章內(nèi)容簡介】 is aera result in the so called “blind spot”. Except for this region, the distribution of receptors is radially symmetric about the fovea. Receptor density is measured in degrees from the fovea. It can be noticed that the cones are mostly dense in the center of retina (in the center area of the fovea), and the rods increase in density from the center out to approximately 20 186。 off axis and then decrease in density out to the extreme periphery of the retina.Fig. 2 Distribution of rods and cones in the retina Human visionThe perception caused by the external light radiating into the human eyes to stimulate the retina is called vision. Vision is a plex physiological process. External light is focused on the retina through cornea, iris and lens, and stimulates photosensory cell to generate nerve pulse, which is transmitted to the brain through vision nerve to produce vision. Because of the visual temporary retaining phenomenon of human eyes, when the interval of two different lights is very short, the retina can’t distinguish the order of the stimulation by the lights. It only produces a total stimulation perception, causing the vision’s time blending effect. Besides, human eyes also have a limited distinguishing ability, when the minimal region of the retina is stimulated by two different lights at the same time, the retina can’t distinguish two stimulations in the minimal region, only produce total stimulation perception, which causes vision’s space blending effect.Fig. 3. A few MTF curves of the human visionSeveral typical frequency response models have been introduced to represent the characteristics of human visual systems [1317], ., exponent model, Gauss model, Barten model and pound model, which are illustrated in Fig. 3 by curves of modulation transfer function (MTF). From theses curves, it can be noted that,? Gauss model and exponent model are lowpass filters, while Barten model and pound model are bandpass filter. Compared to Barten model, pound model is inclined to low frequency.? When the region is greater than 15 cyc/deg, Barten model and pound model are generally located between mrad gauss model curve and mrad one, but the exponent one is much higher.? Because of the plexity of Barten model and pound model, exponent MTF model or gauss MTF model is mostly used in the previous analysis.? Modulation transfer function response of human visual characteristic is nonuniform in the above visual models. This can easily inhibit the small target and can’t search the small one.3 Vision enhancement systemSOLAS convention prescribes that ships must be equipped with GMDSS equipments, which have improved the search and rescue. However, for many non SOLAS convention ships, such as fishing boats and small crafts, the detection results are not very much unsatisfactory. With the plex sea environment, the searching of missing person bees a nailbiting task. Because of these physiological characteristics of human eyes and human visual systems, it is difficult for the rescuer to find small target in the adverse backlighting, night or dark condition, wave or clustered seas. Continuous long time observation also causes fatigue of human eyes, resulting poor sensitivity of detection. All those factors decay the results of searching operation. In order to improve the effect of maritime SAR operations during the dark hours or in adverse lighting or sea conditions, vision enhancement system could be used. The primary role of a vision enhance system is to be a secondary source of information to SAR operations. The vision enhancement system can, to some extent, overe the shortings and inadequacies human eyes. It can also improve the searching speed and accuracy, and is of significance in promoting rescue success rate and efficiency.Vision enhancement system, which is applied to maritime search and rescue, searches targets by visible light and infrared imaging equipments. The video images are proper processed to retrieve the position and dynamic information of the distress persons and life boats. The vision enhancement system can aid the rescuers to fulfill the task of search and rescue, especially for the small target, and could provide a good detection and identification result.4 Vision enhancement system for maritime search and rescueAccording to the requirements and special circumstances of maritime SAR operations, the vision enhancement system can be constructed to assist human vision, as shown in Fig. 4.Human eye observationVisible light imaging systemInfrared imaging systemTarget detectionTarget detectionImage fusionScene markingRescuer decisionSea environmentFig. 4. The structure diagram of vision enhancement system for maritime SARThe vision enhancement system, which is made up of visible light imaging system and infrared imaging system, can used to detect small and weak targets. High resolution video camera and high speed camera can be used to catch the visible light information. The infrared system is employed to enhance vision in weak light conditions. Special image processing techniques should be implemented according to the needs of SAR operation and the special circumstances and conditions. The image fusion technique can improve the observation sensitivity of single vision system, and simultaneously marks the possible rescued objects in the scene, which can help rescuers make decisions.5 ConclusionsDue to the limitations of human vision, it is necessary to employ advanced technology to promote the performance and standard of maritime SAR operations. Vision enhancement system, with proper design and implementation, can improve rescuers’ searching speed and accuracy at sea, and plays an important role in improving success rate and efficiency of the operation. It will play an important role in safeguarding people’s life and property and reducing the economical losses in marine accidents.