Glossary
The technical terms this site uses, in plain language: Jerlov type, Secchi depth, CDOM, beam attenuation, Weber contrast, λmax, A1 and A2.
Jerlov type
A published classification of natural waters by the shape of their light transmission — five oceanic types (I to III) and nine coastal ones (1C to 9C). It is shorthand: "Jerlov type II" is a complete description of a water body’s optics that another person can look up.
Secchi depth
The depth at which a white disc lowered into the water disappears. It is the only optical measurement most people will ever make, it needs a dinner plate and a piece of string, and it is how this site asks "which of these is your water?".
CDOM
Coloured dissolved organic matter — the tannin stain from decaying vegetation that makes lake water the colour of tea. It absorbs almost exponentially with wavelength and hardest at the blue end, which is why stained water passes red and kills blue.
Diffuse attenuation (K_d) vs beam attenuation (c)
Two different numbers that are easy to confuse and expensive to mix up. K_d describes how fast the ambient light fades as you go down. c describes how fast an image fades as you look sideways through the water. In turbid coastal water c can be 8–12 times K_d, so using one where the other belongs overstates sighting distances by an order of magnitude.
Veiling light
Light scattered back toward the viewer by particles between the eye and the object — underwater haze, lit from within. It is what makes murky water bright as well as opaque, and it is the term that forces contrast to zero with distance.
Weber contrast
(target brightness − background brightness) / background brightness. Zero means the object is indistinguishable from what is behind it. It is bounded below at −1 (a perfectly black object) but unbounded above, which is why bright lures beat dark ones looking sideways while dark lures win in silhouette.
λmax
The wavelength a visual pigment absorbs best. A fish’s set of λmax values is the closest thing to a specification of what colours it can tell apart.
A1 and A2 (rhodopsin and porphyropsin)
The two chromophores a visual pigment can be built on. A2 shifts sensitivity toward the red — a few nanometres in the blue, but 40 to 60 in the red — and it is very common in freshwater fish. Many species switch the ratio seasonally, so a bass in October is measurably more red-sensitive than the same bass in May.
Tapetum lucidum
A reflective layer behind the retina that gives light a second pass at the photoreceptors, roughly doubling photon capture. Walleye and zander have one, which is exactly why they feed so well at dawn, at dusk and in stained water.
Cone monochromat
An animal with only one cone type, which therefore has no colour vision at all — only brightness. Every shark species examined by Hart et al. (2011) was one. For a shark, lure colour is close to irrelevant and lure brightness is everything.