Code 4. “Discontinuierliche true oil colour” for thicker layers of oil with a thickness of 50 to 200 μm. For oil films with a thickness of 50 μm or more, the light is reflected by the oil and not by the underlying sea surface, which makes it possible to visualize the true oil color of the exported oil. Real oil painting only gradually becomes the dominant color, brown oil and black oil appears black. However, code 4 is a transition code, as thinner layers alternate with thicker layers due to wave action. This code is also often described as a “real oil painting on a metallic background”. The minimal and maximum volumes are deduced, on the one hand, from the minimum thickness of the layer in which a coloration is visible and from the maximum thickness of the layer in which the coloring moves to a “higher” code. The volume of oil is then calculated with a simple mathematical formula, in which the code-polluted surface is multiplied by the percentage of that code present in a slick and by the minimum and maximum thicknesses of the layer. The estimation of the minimum volume is generally used in criminal matters (to indicate the minimum quantities), while the estimation of the maximum volume is used in cases of accidental pollution of the sea (typical assessment of the most pessimistic cases in crisis management). The observation of colour differences within an oil spill and the approximate estimation of the severity of a discharge or pollution at sea by distinguishing between thicker layers of oil and thinner layers (in principle, areas where pollution can be combated and those where it cannot be combated). The other codes for thinner oil layers, which often cover the largest areas of an oil spill, contain too little oil to be a reasonable prospect, even when dispersants are used. Code 3.
“Metallic” for oil layers with a thickness of 5 to 50 μm: this appearance is typical of metallic matte shine. The color of this metallic brilliance is largely determined by the color of the underlying water, as the oil layer is not thick enough to block the light reflected from the surface of the water, and also in part by the “color” of the sky (e.g..B blue sky or gray clouds) produced by the reflection of light. In the event of an emergency at sea, a ship is often the first to arrive at the site.