38 Proceedings Spring 2025 (nm) to 760 nm, though the actual visible range may vary slightly among individuals.7 The ability to sense light and color is dependent on photosensitive rods and cones in the eyes, which become active under specific lighting conditions.8 Rods are highly sensitive to light and rod activity dominates in low light, or scotopic conditions. Scotopic vision is char- acterized by achromaticity or, for example, a lack of color, or grayness in color, and low visual acuity, which is why an object’s color and physical details may be difficult to identify in low lighting. In brighter light, or photopic conditions, rods deactivate and cone activity dominates. Photopic vision is characterized by full color vision and high visual acuity. The light range where some propor- tion of both rods and cones are active is called mesopic vision.9 Since color is most discernable during photopic and mesopic conditions, RDC efforts are focused on color detectability during the full range of daylight hours, including dawn and dusk. The perception of color can vary by individual, ambi- ent lighting conditions, and a host of environmental factors. For this reason, high-precision color spectropho- tometers are used to eliminate variables by quantifying colors mathematically within a color space. A color space maps colors onto a three-dimensional coordinate system, allowing for accurate and repeatable color representa- tion. The International Commission on Illumination (CIE) developed a perceptual color space called CIELAB that closely models human color vision10 and is preferred in many color perception and vision research applications. The coordinate system axes in the CIELAB color space are labeled L*, a*, and b*. The term L* denotes lumi- nance, a measurable quantity most closely associated to the human psychophysical perception of brightness11 and often referred to as perceptual lightness.12 The terms a* and b* define chromaticity, the color of an object inde- pendent of its luminance.13 The numerical values of L*, a*, and b* are called color coordinates and plot a discrete point within the CIELAB color space. Detectability and Physical Salience The detectability of a target is greatly influenced by its physical characteristics and the surrounding environ- ment. The physical characteristics or properties that increase the likelihood that a target will draw the atten- tion of an observer is called conspicuity.14 Conspicuity is often increased by maximizing the chromaticity and/ or luminance of an object.15 High chromaticity colors appear intense or pure, while low chromaticity colors contain larger amounts of grey and may look muted. High luminance colors contain larger amounts of white and appear brighter while low luminance colors seem darker. Accordingly, high chromaticity, high luminance colors are typically perceived as intense and bright. The Coast Guard’s search and rescue (SAR) envi- ronment is maritime and highly dynamic. A SAR crew may encounter any range of lighting, cloud cover, sea state, visibility, and precipitation which may change the apparent color of the water and increase the number of distractors, or irrelevant targets, in the search environ- ment—whitecaps and debris, for instance. Apparent water color also changes with the observer’s viewing angle, or the height of their eye,16 proximity to shore, and the waterbody in which the target is located, such as the Pacific Ocean versus the Gulf of Mexico. The degree to which the conspicuity of a target draws attention in its environment is called physical salience. Physical salience is how much the basic, low-level visual features of a stimulus—the target’s properties—differ from its environment.17 For this project, the RDC corre- lates high detection probability to high physical salience of a target. The likelihood of target detection increases as the physical salience of that target increases. Specifically, the RDC is attempting to quantify the extent that various colors affect target salience, while holding all other target features constant, under assorted maritime environmen- tal conditions. Psychophysical Factors Another significant aspect of target detectability is the psychophysics of visual search. Psychophysics studies the relationship between the physical characteristics of a stimulus and the human perception of that stimulus.18 A key challenge in human information processing is the vast amount of sensory information constantly taken in by our sensory organs, like photoreceptors and hair cells, among others. Humans cannot fully process all sensory data received and require selective attention, or guidance, to focus the processing of relevant information while suppressing irrelevant information.19 While several forms of guidance exist in visual search, two primary modes are bottom-up atten- tional guidance and top-down attentional guidance. Bottom-up attentional guidance is driven by external, or stimulus-driven, factors that capture the observer’s attention and is typically associated with the salience of an object in the search environment.20 The salience of a search target is impacted by several factors such as the contrast between the target and distractors, amongst distractors,21 as well as the target against ambient back- ground color.22 Additionally, when a single feature is unique compared to its surroundings—a red X among black Xs, for example—it is a feature singleton, which can pop out to an observer.23 Examples of bottom-up search guidance include a single bright orange lifeboat against a dark blue ocean or the flash of a strobe light in contrast with the dark of night. Although these attributes can greatly assist in target detection, search distractors
