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quote:Originally posted by mdigweeddo we see colour consistently? in other words is the blue i see the same as the blue you see?Doh!
quote:Originally posted by Dr BHow do we see black?
quote:Originally posted by tonycsmIf I remember correctly from my early studies, when a light source in our visible spectrum such as daylight strikes an object, the result is that energy is released from the object's atomic structure in the form of photons. These released photons usually have a specific wavelength, depending upon the release energy, which our brain then interprets as a specific colour, therefore I would presume, if we all have similar abilities to detect and interpret these wavelengths correctly ( excluding those with defects such as colour blindness etc) then we should all see the same colour.However, what we actually call these colours we see is abitrary!Objects which are black, have the ability to absorb the light source, hence very low reflectance.Objects left in light sources such as strong sunlight will eventually suffer some atomic degeneration such as we see in colour fading!
quote:Cones come in three kinds, with peak sensitivities in different parts of the spectrum. These are conveniently called the red, green and blue cones. However, the responses are quite broad, with considerable overlap. Green cones have the strongest response, red is close but slightly weaker, while blue is 20 times lower, i.e., the eye’s response to blue is much lower than red and green. The blue peaks at about 440nm, green about 545nm, and red about 580nm. Interestingly, the red and green peaks are within the yellow part of the spectrum. Furthermore, red cones have a secondary response in the blue part of the spectrum. The eye’s limits are about 400nm for violet and 700nm for red.This use of three receptors to explain low-level colour is called the trichromatic theory or the tri-stimulus model. Basically this states that all perceivable colours can be represented as RGB stimuli to the eye. (This is almost true but not quite, as we will see shortly.) Thus, when we see, say, yellow light, what is really happening is that the red and green cones are being excited to certain relative levels. If we were to take red light and green light of the right intensities and mix them, the relative excitement of the cones would be the same, and we would perceive yellow. This, of course, is the principle of TVs. (The technical term for two colours which are visually the same but generated by different mixtures of wavelengths is ‘metamers’).Overall, the eye’s response to light of constant luminance has a peak at about 550nm, roughly the colour of sunlight.Even though we have a completely non-linear response in the eye, we will start with a simpletri-stimulus system. This means we shall represent a colour by a mixture of standard red, greenand blue sources, mixed in some proportion. These are our primary colours.We get secondary colours by mixing the primary ones in equal measure:yellow = red + green;cyan = blue + green;magenta = red + blue.We get white by adding all three primaries in equal proportion:white = red + green + blue.It is important to understand that the basis of the tri-stimulus model is the human eye, not physics. The spectrum is a continous, linear concept. If you were to invent something to approximate it, you might choose one light at each end of the visible part - blue and red say - and hope you could linearly interpolate the colours in-between. The reason this does not work fully is exactly because of the need to stimulate eye’s cones. Equally, the existence of metamers is a consequence of the overlap of the cones’ responses: there is no unique physical stimulus for any given perceived colour. One immediate consequence is that our computer screens depend on the viewing eyes having three types of cone: an alien - or even a bee - will not see what wesee.As an interesting digression, you might like to know that a bee has three receptors too, but one is in the ultraviolet and bees are less sensitive to red than we are. Many birds can also see in the ultraviolet but in their case they have four receptors and so have a wider viewable range than we do. So do some fish and turtles. Sometimes male-female pairs of birds look similar to us but are very different in the UV, so they can find the opposite sex more easily: the blue in a blue tit’s head is different in the UV, according to the sex, but is very similar to us. Some white feathers reflect strongly in the UV, others do not, so they will look different to other birds. Blue tits, zebra finches and even starlings use UV in mate selection. Maybe that UV“black light”, once popular at discos, had its uses after all.
quote:Originally posted by Dr BHow do we see black?Dr BIstanbul