Previously the blue wavelengths were accessible only through DPSS which are comparatively expensive and inefficient, but still widely used by scientists for applications including optogenetics, Raman spectroscopy, and particle image velocimetry, due to their superior beam quality. Lasers emitting in the blue region of the spectrum became widely available to the public in 2010 with the release of inexpensive high-powered 445–447 nm laser diode technology. On a colour wheel based on traditional colour theory ( RYB) where blue was considered a primary colour, its complementary colour is considered to be orange (based on the Munsell colour wheel). On the HSV colour wheel, the complement of blue is yellow that is, a colour corresponding to an equal mixture of red and green light. Red, green, and blue subpixels on an LCD display. This method could produce almost all the colours in the spectrum with reasonable accuracy. Later, printers discovered that more accurate colours could be created by using combinations of cyan, magenta, yellow, and black ink, put onto separate inked plates and then overlaid one at a time onto paper. The RYB model was used for colour printing by Jacob Christoph Le Blon as early as 1725. From the Renaissance onward, painters used this system to create their colours. Mixing all three primary colours together produces a dark grey. Red and blue mixed together form violet, blue and yellow together form green. In painting and traditional colour theory, blue is one of the three primary colours of pigments (red, yellow, blue), which can be mixed to form a wide gamut of colours. He included indigo, the hue between blue and violet, as one of the separate colours, though today it is usually considered a hue of blue. He chose seven colours because that was the number of notes in the musical scale, which he believed was related to the optical spectrum. Isaac Newton included blue as one of the seven colours in his first description the visible spectrum. Pure blue, in the middle, has a wavelength of 470 nanometres. Blues with a higher frequency and thus a shorter wavelength gradually look more violet, while those with a lower frequency and a longer wavelength gradually appear more green.
Human eyes perceive blue when observing light which has a dominant wavelength of roughly 450–495 nanometres. The same surveys also showed that blue was the colour most associated with the masculine, just ahead of black, and was also the colour most associated with intelligence, knowledge, calm, and concentration.
In US and European public opinion polls it is the most popular colour, chosen by almost half of both men and women as their favourite colour. Surveys in the US and Europe show that blue is the colour most commonly associated with harmony, faithfulness, confidence, distance, infinity, the imagination, cold, and occasionally with sadness. Distant objects appear more blue because of another optical effect called aerial perspective. An optical effect called Tyndall effect explains blue eyes. The clear daytime sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. Most blues contain a slight mixture of other colours azure contains some green, while ultramarine contains some violet. The eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres. It lies between violet and cyan on the spectrum of visible light. Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model.
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