Re: Subject: photogravures/metamerism
Re: Subject: photogravures/metamerism
- Subject: Re: Subject: photogravures/metamerism
- From: Robin Myers <email@hidden>
- Date: Wed, 24 Oct 2001 15:08:15 -0700
- Organization: Robin Myers Imaging
Dear Mr. Barstow,
>
Cyan dye absorbes red light, not green, and not infrared.
>
Where does red become infrared? The human eye falls off
>
gradually as red deepens into infrared. If the cyan dye opens
>
too soon in the near infrared, then the eye sees that infrared
>
as red. This makes the cyan dye appear weak under light that is
>
rich in infrared, such as an incandescent light, and a print
>
made with that cyan will have reddish grays under infrared rich
>
light. If you boost the amount of cyan to neutralize gray under
>
incandescent light, then under light that has no infrared, such
>
as flourescent, the cyan will be too strong and grays appear
>
cyanish green.
This statement is inaccurate. The human eye has NO infrared response. Accepting
the CIE standard observer as a reasonable measure of human visual
response, the
limits of human vision fall in the 380 to 780 nm range. Infrared light
is at
much longer wavelengths, sometimes reported as 1000 nm and greater, with the
range between 780 and 1000 described as the "near-infrared". Since the 2 degree
standard observer response at 700 nm is only 1% and falls almost by half again
for each 10 nm increase in wavelength, the practical human vision range is
generally described as 400 to 700 nm. So the human visual system has no infrared
response.
Cyan dyes do indeed absorb red and reflect blue and green wavelengths,
but they
are not perfect dyes. A perfect dye would reflect all green and blue wavelengths
and absorb all red wavelengths. However, cyan dyes continue to reflect
some red
wavelengths and they also absorb some blue and green wavelengths. It is the
balance between the red and blue-green absorbtion/reflections that determines
the strength of a cyan dye. As the lighting changes, so does this red/blue-green
balancing act.
<<Continued in next post>>
Robin Myers