Frank,
Thanks for the detailed reply. What I was getting at was:
1. Is the ceiling "white" ?
Sounds like from your answer that it could well be when first painted.
2. Given that the ceiling was "white" when it was freshly painted, is it
still white ?
>From what I've seen after a while a ceiling ends up heading towards a brown
colour (rather than neutral grey), presumably giving a warmer tone to
photographs where the light is bounced off it.
...WAyne
>
> 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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