P3D Re: projection

[jacob@xxxxxxxxxxxxxx (Gabriel Jacob)]

>>As to the reference about cameras using IR to produce an image that
                            ------------------------------------               
>>we can see, the camera is converting an invisible portion of the
>>spectrum into something we can see (i.e. light). 
>
>If we're going to be really correct, and make such a distinction between
>"radiating heat" and "radiating infrared energy", then please explain
>just how it is that a film camera 'produces light that we can see'. :-)

I never said cameras produce light. They produce an image. They convert
it into something we can see, an image, which we see using light.
See the underlined part from the line you quoted above. The original
implication was that we can see infrared with cameras, therefore IR is
some kind of light.

In any case I wasn't the one that initially made the distinction
between heat and infrared energy. What I did is clarify, if one
wants to be really correct and call "heat", infrared energy,
invisible light or whatever, THEN it's only fair to also make
sure the propagating mechanisms are correctly used. 

>>This doesn't make
>>the IR portion light. As to animals, it's true they can see some
>>IR and UV which is invisible to humans but again, this doesn't
>>redefine that portion of the spectrum to "light". 
>
>Never mind the animals - what about humans? Some people who have had
>cataract surgery can see much shorter wavelengths than the general
>population.

How "much"? I would venture it would be insignificant to revise the
photopic curves adopted by the CIE. 
  
> Does that redefine light for all humans, or just for them?

Maybe, maybe not. We could include them in the analysis but 
they would haven't much impact statistically. 

>What if the entire human population has such surgery - does that
>redefine light?

Yes.

>What if only 49 percent of the population has the surgery, so 
>"the majority" of the population still can't see the shorter wavelengths?

Yes, the CIE curves would have to be revised. :-)

>What if it's in the frequency range of visible light, but it's too dim for
>you to see it - is it still light? As your eyes get dark-adapted, does the
>definition of light change?

No the definition of light doesn't change in this case. It would still
be light but you would be considered temporarily functionally blind
till the rods take over. The photopic and scotopic responses are
defined by the wavelength and luminous efficiency curves. Of course
your eye response may be significantly different! A question for you!
Should we re-classify light as invisible because a blind person
(sorry, visually-impaired) can't happen to see it?

>So far, I like John Toeppen's definition better than all the others posted -
>he's the only one who sounds like he might have looked in an actual physics
>text to find the contemporary usage. I don't have access to such a text
>at present, but I suspect the division he described (microwave, light, X-ray)
>is based on *major* differences in the interaction of the EM energy with
>matter.

John, you'd better read my posts more carefully! :-) I said the same
thing as John Toeppen in the part you quote from me below! As for
contemporary usage, see my previous post regarding that subject.

Gabriel (me!) wrote:
>>As for equations applying to UV and IR as well as the visible 
>>spectrum this is correct, but since they have different physical
>>effects on matter (or 3-D slides!) the classifications (IR, light,
>>UV, etc.) are important to keep distinct.

>But do those different physical effects (relatively minor compared with
>X-ray/microwave differences) have wavelength boundaries that necessarily
>line right up with the threshold of sensitivity of the human eye? 

Yes these wavelength boundaries are significant enough, especially
the visible.   

>First you want to distinguish light from non-light by human perception,
>then you want to *also* apply criteria of physical interaction with matter.

Problem is, who is field of science is doing the slicing (of the spectrum).
Light can be treated as thermal energy, depending on your point of view
or interest. Another example is qualifying electromagnetic radiation,
radiometrically or photometrically. Depending on your field of interest
you will evaluate the radiation by photometric or radiometric units. 

>I'm much more concerned with the definition of "infrared" than with the
>definition of "light". The range covered by the term "infrared" includes
>wavelengths with very different thermal properties from one another, and
>that are predominantly produced under very different circumstances. 
>The confusion between near IR and far IR seems to cause much more trouble
>than confusion regarding the definition of "light".

Good points. See my previous post about "heat". 

>This whole business of defining light in terms of what most of us can
>see seems to have so many difficulties, I think it may be clearer to
>call what most of us can see "visible light", and the other kinds
>"infrared light" and "ultraviolet light", and for the generic term
>"light", stick with a physics definition.

There are no problems. These are clearly defined by the CIE curves
and tables. Don't get me wrong, I have no problem with describing
infrared and ultraviolet as light in certain contexts. The reason
I brought this all up is because of the rather strict definitions
of infrared and it not being heat, heat absorbing filters not 
absorbing heat, etc. I could go on and on. I didn't touch on the
subject of heat absorbing filters being a misnomer, which it isn't.
Lest we start a new mailing list on 3-D thermodynamics! ;-)

>"If a tree falls in the forest, and nobody was there to hear it, then
>where were they?"

Good question, let's start a new list to discuss this!

Gabriel





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