I was reading about white noise in a textbook and it said white noise is different from white light, because white light has constant energy per wavelength. (White noise is constant energy per frequency.) I’d never heard this definition, or any definition of “white light” for that matter - who uses it, and for what?
I don’t know what the exact definition is, but white light sure as heck isn’t constant energy per frequency, nor is white noise constant per wavelength. Try that, and you run smack dab into the Ultraviolet Catastrophe; since there’s no limit on possible frequencies/wavelengths, the total energy would have to be infinite.
I don’t think that there’s any strict definition of white either (I know I’ve never heard “white light” used as a technical term); I think it’s just enough radiation to be significant at any frequency in the range you’re interested in. How much is significant, and what range you want, depend on the situation.
frequency and wavelength are directly related, one is the inverse of the other, so constant energy per frequency or per wavelength would be the same thing.
Chronos, of course it would not be in the infinite width of the spectrum. White noise would have constant energy in the entire width of the audible spectrum.
But the question is in error in the sense that NOISE is not SOUND as defined in communications. NOISE is extraneous signal that degrades the original transmitted signal. So any signal, whether it is carrying sound, image or whatever is subject to noise.
The question as posed is meaningless. Noise is what you see on your TV screen when it is not tuned to any station.
In the real world: Noise is the sound of the TV when it interferes with the sound of your mother calling (or maybe noise is the sound of your mother when she interferes with your hearing the TV)
Noise would be dust in the air, refraction and other phenomena that distort the original image. Or maybe your mother getting in front of the TV and blocking your view.
Noise can have a certain frequency and is more easily blocked out (for example, the phone ringing) or it can be scattered and be difficult to block out (for example, the sound of rushing water)
White light would not have equal energy in all colors because the human eye is more sensitive to some colors than to others. I believe the human eye is most sensitive to Blue-green so, if you receive equal energy in all colors, you will se it as blue green.
OTOH the human eye is very adaptable and would probably adapt quite well after a while. I know you can wear colored sunglasses and after a while the image does not seem tinted.
So, what are we talking about? White light as the eye sees it? White sound as the ear hears it? Or white noise as defined in information theory?
In my book it says the following about white noise:
p. 101 in Skoog, D. A., Holler, F. J., Nieman, T. A., Principles of Instrumental Analysis. Harcourt Brace & Company, 1998.
White noise is like white light only by analogy. They are actually two different things. Some of the confusion about may have come from the OP’s use of the word “energy”. It really should say constant intensity, I think. White noise has a constant intensity over the whole range of frequencies. Over the frequencies of the visual spectrum ,white light has a constant intensity.
Also, light is a quantum phenomenon, and the energy of a photon is proportional to its frequency. So if you had constant energy per frequency, you would have to have more red photons than blue photons and the resulting light would probably look kind of orange. I think a better definition of white light would be constant intensity (i.e. photon flux) per frequency.
I don’t think so. In any case “intensity” would have to be defined. I always studied that white noise has its energy equally distributed along its bandwith.
As I said, probably white light does not but, as you say, the name is just an analogy.
Yeah, now that I think about it, I believe I used “intensity” incorrectly in my previous post. I think intensity is normally defined (for light or sound) as energy per area, or maybe energy per steradian if you want to be really fancy.
Anyway, I still think “white light” is best defined as constant photon flux per bandwidth. But that’s IMHO, and I’m open to other arguments.
I may be getting in over my head with you science guys, but here goes anyway…
In the graphic arts field (and perhaps photography as well), light is defined in terms of its “temperature.” Quoting from a printing reference manual:
“For visual purposes, the color quality of a light source is evaluated in terms of the color of a perfect light radiator, or “black body,” heated to a certain temperature … expressed in degrees Kelvin. When the light source matches the black body in apparent color, it is said to have a color temperature equal to the actual temperature of the black body … The color of light is bluer with higher and yellower with lower color temperatures.”
Printers view proofs under special lights rated at 4,500 degrees Kelvin; that is what the printing trade, at least, considers “white light.”
temperature is just a handy way of talking about color balance. More blue = higher temperature, more yellow and red = lower temperature.
regarding what the eye would consider white, the eye can be very easily fooled and will correct color imbalance but NTSC uses Y = 0.299R + 0.587G + 0.114B which I believe is in line with the human eye’s sensitivity.
some related links:
Color in Image and Video http://www.physics.uq.oz.au/ph128/6d.html
At any rate, we are just speculating since the OP is unclear.
OK, here is the exact quote which prompted me to post the OP. From “Low-noise Electronic System Design” by C.D. Motchenbacher and J.A. Connely:
And sailor, those are not the same thing. With equal energy per wavelength, there would be more energy between 1 and 2 Hz (1/1 - 1/2 = 1/2) than between 1000 and 1001 Hz (1/1000 - 1/1001 << 1/2). And the word “noise” may refer to all unwanted signals, but the term white noise has a specific meaning, as given above, so I don’t see why the question is meaningless.
And yes, Chronos, I agree there would be infinite energy if the bandpass was infinite. But it’s the same case with thermal noise - since all real-world systems have finite bandwidth, there is always finite energy.
I believe that definition of white light must be specific for some field or purpose unrelated to the human eye. I also find it unusual they would use that term for that concept as it would be more correct to call it “white electromagnetic radiation”. With just that paragraph and no context i am quite lost and will not attempt to guess what the author might try to mean.
Regarding the question “what is the definition of white light?” (outside the context of that paragraph) I do not believe there is one universal definition but the general gist of all of them would be “the combination that the human eye perceives as white”. This definition is very removed from the paragraph you cite because the human eye has very differing sensitivity to different colors. So I have no idea what that book is talking about. For photography, printing and other uses where the last receiver is the human eye, white can be defined in terms of color balance, color temperature or any other way but at the end, the eye has to see it as white and if the energy of red, blue and green are equal, the eye will not see it as white. Note that the eye can be made to see white with an infinite number of combinations of inputs, as long as they balance out correctly.
Regarding the wavelength thing, I believe it is mistaken (or maybe I just do not understand it). Wavelentgh is not an intrinsic characteristic of an electromagnetic wave or of a sound wave. It is just a shorthand method of talking about the frequency which is an intrinsic characteristic. The wavelength depends on the speed of the wave through space which will vary depending on the medium.
So, to try to make sense of what the book says, we would have to restrict the whole thing to vacuum. I guess I could get myself into a long analysis comparing power vs frequency and power vs wavelegth but I’ll leave that for someone else while I get myself some coffee.
I thought I would chime in with the info that from a lighting perspective “white” light is defined in a way that mixes Sailor and Stuyguy’s definitions. The IES (Ilumination Engineering Society of North America) reports that lights typically defined by various manufacturers to be “white” (not warm or cool, but just white) have a corrolated color temperature of about 3450 deg. Kelvin.
Color temperature is based on the visible radiation emited by a lump of iron heated to that temperature. Said radiation is composed of different intensities of light at different frequencies that when combined are called “white”.
The color temperatures called white are all over the map, though, and there doesn’t appear to be any real well defined standard. Without a more detailed description of what the author quoted in the OP intended by white light the question is pretty much unanswerable.
I think the Velvet Underground describe it best. Sorry for the interuption.