The human eye contains two main types of photoreceptors: cones for daytime (photopic) vision and rods for nighttime (scotopic) vision.
Photopic vision peaks at wavelengths around 555 nm – a yellow‑green hue. NDE‑Ed notes that under typical daylight conditions, the eye’s photopic response is most sensitive near 550 nm, so fluorescent materials often emit at this wavelength to maximize brightness .
Scotopic vision shifts sensitivity to shorter wavelengths; the scotopic peak is roughly 507 nm (blue‑green) . As light fades, the rods dominate (the Purkinje effect), making blue‑green hues seem relatively brighter while red and yellow appear darker .
Evidence from the PLOS ONE study on fluorescent fabrics showed that fluorescent yellow provided the highest “brightness and conspicuousness sensitivity” across normal, protanopic and deuteranopic participants and was preferred for brightness across groups .
Because the human eye is most responsive to yellow‑green wavelengths during photopic vision and still sees luminous yellow well when rods begin to function, colors centered around fluorescent yellow‑green offer the best all‑round visibility.
Visibility in Daylight and Low‑Light Conditions
Daylight (Photopic) Conditions
Yellow‑green hues sit near the peak of photopic sensitivity (~555 nm), so they appear brighter and more easily detected. NDE‑Ed notes that materials used in fluorescent penetrant inspection fluoresce around 550 nm because the eye is most sensitive there .
Educational resources explain that emergency vehicles were historically painted red but many are now yellowish‑green or white because red is less visible at night; the human eye is more sensitive to yellow‑green hues .
The DayGlo article adds that green is the most visible color in full sunlight but as light levels drop, yellow becomes the most easily visible due to the shift toward rod vision .
Low‑Light (Scotopic/ Mesopic) Conditions
When illumination decreases, rods become more sensitive and the eye’s peak shifts toward 507 nm. This makes blue‑green hues brighter relative to red or yellow. However, rods do not perceive color; they are sensitive to luminance. Fluorescent materials increase luminance by converting short wavelengths into visible wavelengths, making the color appear brighter even under low‑light, fog, or haze .
Red appears darker in low light , which is why emergency vehicles and safety gear seldom rely on red alone.
Some research suggests that fluorescent orange‑red may stand out better at dusk because of the Purkinje effect – rods make the longest wavelengths appear darker, so contrast with orange‑red is more noticeable against darkening backgrounds. Nonetheless, fluorescent yellow‑green still performs well due to higher luminance.
High‑Visibility Colors in Safety Gear and Signage
ANSI/ISEA and ISO Standards
ANSI/ISEA 107‑2020: The standard for High‑Visibility Safety Apparel (HVSA) in the United States specifies three approved background colors: fluorescent yellow‑green, fluorescent orange‑red and fluorescent red . Clothing classes (1, 2, 3) dictate the amount of background material and reflective tape required.
The ANSI blog notes that although green is easiest to see, high‑visibility garments must provide contrast; bright yellow and orange show up better against darker backgrounds, and retroreflective strips enhance visibility in low light .
Safety+Health Magazine and Blain’s Farm & Fleet emphasize that the three fluorescent colors are the only acceptable high‑visibility backgrounds; other colors are allowed only when combined with reflective trim .
MUTCD (Manual on Uniform Traffic Control Devices) uses yellow for permanent caution signs because it is easily seen in daylight, orange for temporary construction warnings, and fluorescent yellow‑green for school and pedestrian zones where enhanced conspicuity at dawn, dusk or in fog is required .