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Re: Flash Colour Temperature (was Re: [OM] baby's coming - OM-1 flash re

Subject: Re: Flash Colour Temperature (was Re: [OM] baby's coming - OM-1 flash recommendations)
From: Frank Ernens <fgernens@xxxxxxxxxxxxxxx>
Date: Fri, 19 Feb 1999 22:19:01 +1100
Wayne Harridge wrote:

> When you bounce a flash off a "white" wall or ceiling, what
> is the effective colour temperature ?  What is "white" ?

Colour temperature is the temperature in Kelvin to which you would
need to heat up a nonreflecting incandescent source ("black body")
to get that colour. Of course, this can't represent all the colours in the
world, but most of the *sources* of light we have are indeed
incandescent, or look like incandescent sources to our imperfect
eyes.

The colour "white", or CIE Illuminant C, can be defined as
the spectrum of a black body at 6557 K. Because of the way
our eyes work, we also identify other spectra as white which
don't look at all white to film, e.g. daylight fluorescent tubes.

A reflective surface that looks "white" looks the same
as Illuminant C to our eyes, but not necessarily to film.
It's a surface which when illuminated by white light still
looks white. This includes any surface which reflects visible
light equally regardless of its wavelength. (But some others
don't reflect equally, but still look white because they
phosphoresce.) So when you bounce your flash off a nonphospohorescent
"white" ceiling, the spectrum, and therefore the colour temperature,
is unchanged.

Most white paint is white in this sense, I think including most
of the white ceiling paint used in Australia. A few paints
have phosphors, so don't have a black body spectrum or a colour
temperature. Those ones need CC filter correction, usually done
at the minilab if you use print film.



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