Figure 2: Colour palettes on Ag.
Coloured squares 25 mm2 in area produced using (a) nonburst and (b,c) burst irradiation. (a) The different colours were obtained by changing the line spacing, Ls, in steps of 0.5 μm (as marked above each panel) for a laser fluence of ϕ=1.29 J cm−2 and a marking speed of v=100 mm s−1. (b) Colour palette obtained using bursts containing 5 laser pulses by varying the machining speed in increments of 5 mm s−1 starting at 40 mm s−1 (as marked above each panel) with a fixed line spacing of Ls=7 μm and a laser fluence of ϕ=5.6 J cm−2. (c) Colour palette obtained by maintaining the line spacing fixed (row-wise, as marked) and varying the machining angle between each successive pass (that is, the angle between the light polarization and machining direction), where column A=2 pass at 270°, column B=1 pass at 270°+1 pass at 234°, column C=1 pass at 270°+1 pass at 191°, column D=1 pass at 270°+1 pass at 162° and column E=1 pass at 270°+1 pass at 126°. The burst parameters in c were identical to the parameters used in b and the marking speed was v=150 mm s−1. (d) Graph of Chroma versus Hue, using as markers photographs of the colours obtained by applying the different machining processes of (a–c) and using the same burst parameters and fluence as in b,c. The horizontal colour bar was constructed from a collection of photographs of the 30 highest Chroma colours in d, covering the entire Hue range (0 to 360). (e) Photographs of a blue-coloured coin taken at four tilt angles (0o, 30o, 45o and 60o) for two coin orientations.
