Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!sri-unix!rutgers!mcnc!rti!xyzzy!throopw From: throopw@xyzzy.UUCP (Wayne A. Throop) Newsgroups: sci.misc Subject: Re: Color Message-ID: <426@xyzzy.UUCP> Date: Fri, 4-Dec-87 13:59:06 EST Article-I.D.: xyzzy.426 Posted: Fri Dec 4 13:59:06 1987 Date-Received: Wed, 9-Dec-87 07:09:32 EST References: <162300002@uiucdcsb> Organization: Data General, RTP NC. Lines: 71 > kadie@uiucdcsb.cs.uiuc.edu (Carl Kadie) > 1) On TV's and computers screens, why is it RGB (red, green, blue) > instead of RYB (red, yellow, blue) the primary colors? RYB are the so-called "subtractive" primary colors. Red, green, and blue are the so-called "additive" primary colors. When a color sensation is produced by removing selected components of white light, as in light reflected from pigments for example, the process is subtractive. When a color sensation is produced by adding selected components to "no light" (or, "blackness"), as in television for example, the process is called additive. To oversimplify, the human eye has sensors for red, green, and blue. Thus, starting with nothing, and adding these three components, any color perception can be reproduced. When subtracting from white light, the "primary subtractive colors" are those gotten by removing one element of white light at a time. For example, removing red from white light gives a bluish color called "cyan". In fact, the subtractive primary colors are often called red, yellow, and blue, but to photographers and others who need to be a little more precise, they are called magenta, yellow, and cyan. To sum up, there are two "kinds" of primary colors. In each case, there are three of them, to a large extent because there are three types of receptors in the human eye. In an additive mixing process, these colors are red, green and blue. This corresponds to the human eye's receptors. In a subtractive mixing process, these colors are magenta, yellow and cyan. The two processes can be related by noting that the additive primaries can be produced from the subtractive primaries like so: red = magenta - yellow green = yellow - cyan blue = magenta - cyan And the subtractive primaries can be produced from the addative primaries like so: magenta = red + blue yellow = red + green cyan = green + blue Again note, this is an oversimplification. (The "green = yellow - cyan" is what corresponds to the over-cute "ye-low an' bloo make green" of the ziplock comercials) > 2) Some light wave length produces the color green. A mixture of > the wave lengths of blue and yellow also produces green. > Even though these two greens are indistinguishable to our eyes, are there > (could there be) instruments that distinguish them? Yes and no. Subtracting cyan and yellow gives green, which is not the same as "mixing the wave lengths of blue and yellow". In fact, if you add blue and yellow *light* (as opposed to adding blue and yellow *pigment* which subtracts light), you'll get white, not green. With that slight nit out of the way, and translating the question into additive terms from subtractive, the rest is correct. One can add red to green to get a yellow that the eye couldn't tell from a pure monochromatic yellow. But the difference would be obvious to a spectrograph. In short, the one could be decomposed by a prism, and the other could not. -- Another interesting facet of ninja was the use of magic. They had a reputation as sorcerors and wizards who could fly and hypnotize, and walk through walls, and get away with huge deductions on their taxes. --- Bruce Israel martial-arts-request@brillig -- Wayne Throop!mcnc!rti!xyzzy!throopw