P3D Re: projection (part 1 of 2)

[roberts@xxxxxxxxxxxxxxxxx (John W Roberts)]

>Date: Fri, 26 Feb 1999 20:32:07 -0700
>From: jacob@xxxxxxxxxxxxxx (Gabriel Jacob)
>Subject: P3D Re: projection

>>>As to the reference about cameras using IR to produce an image that
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>>>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. 

We can see the image in a *camera obscura*, but in a film camera, we generally
do not see the image produced by the camera. The function of a film camera
is to control the exposure of the film, which results in chemical reactions.
When using regular camera film, we don't see anything then, but have to
wait until the film has been processed.

All of which I think we all knew already, but if we're going to make an
issue of everything being correct, might as well describe it properly.
(No doubt I accidentally left a few imprecisions in my description! :-)

>The original
>implication was that we can see infrared with cameras, therefore IR is
>some kind of light.

I agree that this does not show that IR is visible light. However, I believe
you also mentioned (and I accidentally deleted) that the definition is most
usefully based on the context in which it is used. When using a film camera,
I think it would be much more convenient and useful *during the taking of
the photograph* to refer to anything that's going to expose the film as
"light", though that may refer to IR, UV, or X-rays. If these are present,
to fail to think of them as light is to risk accidentally leaving them out
of the calculations in making a correct exposure. When *viewing the resulting
photographs", visible light is the item of main interest as far as the
immediate appearance of the photographs is concerned.

John R


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