Re: more ir now?

[Andrew Davidhazy <ANDPPH@xxxxxxxxxxxxxxxxxx>]

the thread goes roughly like this:

> > It occured to me that the color temperature
> > would now be generally lower than during the
> > other seasons. A check with a color meter
> > confirmed my  suspicion.

> > My question is : Has anyone noticed that there is a correlation between
> > the decrease in color temperature and an increase in the amount of IR
> > during the winter months. It it noticeable  as an increase in highlight
> > density in exposed film.

> I'm not surprised at all. You only have to look at the sun and notice,
> that during wintertime the sun doesn't climb high and looks therefore
> more orange. It is the same effect you see during summertime a few hours
> before sunset. The color temperature of a tungsten bulb is very low, but
> the amount of IR is very high, so high that you don't even need a filter!

The amount of infrared that the sun emits is constant ... at least I think it
is. Therefore, the only varialble is the amount of atmosphere the energy
emitted by the sun has to go through to get to the ground here on Earth.
As short wavelengths tend to be scattered more, the proportion of reds to blues
goes up and so the color of the light is more reddish ... and presumably
therefore also the _ratio_ of IR to visible also is greater. But the total
amount in my opinion can not be any greater than what the ratio naturally is as
it leaves the Sun.

Anyway, the _density_ that film produces is a function not only of the actual
amount of energy present or available but also the exposure time. This is the
basis for the "exposure" formula = Exposure being equal to the Illuminance
times the Time.

So, simply stating that the density of the film goes up or might go up becasue
the color temperature falls is problematic because the energy falling on the
film has not been defined.

As for the statement that one does not need a filter ... well, I am a bit
curious about this. The statement needs further clarification to the point of
making sure that what is meant is that before visible energy becomes apparent,
the tungsten filament is already emitting IR and one can make IR exposures by
such radiation.

Further, once the bulb has started to emit light, not only is the bulb
producing light energy but the actual amount of IR it produces is also higher
then what it was when the bulb was cooler and only emitting infrared. In any
case, once the bul is emitting light as well as infrared a filter is needed to
separate light from the IR wavelengths ... assuming one wishes to make
photographs by only IR.

... at least this is my take on this and subject to further correction.

Andy  o o  0 0 o . o  Andrew Davidhazy, Imaging and Photo Tech
       \/\/\/\/\/\/   http://www.rit.edu/~andpph  716-475-2592  
________|        |____________________________________________
 


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