Re: Spectrophotometer's Illuminant and Angle Affect
Re: Spectrophotometer's Illuminant and Angle Affect
- Subject: Re: Spectrophotometer's Illuminant and Angle Affect
- From: Robin Myers <email@hidden>
- Date: Mon, 17 Jul 2006 20:22:41 -0700
Dear Mr. Whitfield,
On Jul 17, 2006, at 7:27 PM, Bill Whitfield wrote:
Greetings,
I have noticed that the illuminate and angle for my i1 and DTP41
are D50 and 2 degrees. I am guessing they are set the same so their
readings will be close.
The settings on the production floor's spectrophotometers
(DataColor) are D65, 2 degrees for our local plant and D50 10
degrees for our corporate plant.
Readings for the same PMs color (186) on the local production floor
spectrophotometer is different than the reading for the same color
on the corporate spectophotometer. I would imagine is because their
illuminate and angles are different(?). Does the illuminate and
angle make a difference, unless one is trying to standardize device
readings?
These are not the instrument's measurement geometry and illuminant,
but the measure of the field of view for the standard observer and
the reference illuminant by which the instrument's spectral data will
be processed. The DTP-41 and the Eye-One both use tungsten-halogen
illumination (roughly corresponding to CIE standard Illuminant A) and
45/0 measuring geometries for reflectance measurements. The "45" in
the term "45/0" refers to the angle of the illumination from the
normal to the measurement surface. The "0" in the "45/0" term is the
position of the instrument's optical pickup from the measurement
surface normal. Simpler put, the instrument is looking straight down
on the spot illuminated by the instrument's lamps set to a 45 degree
angle.
I do not know what type of illumination the DataColor is using, but
most DataColors use an integrated sphere for the measuring geometry.
This is a reflective sphere with a hole on one side for the sample,
another hole approximately 10 degrees down from the normal on the
opposite side and a hole down the side, and out of view, for the
illumination. The sample is lit from the reflected light from all
directions, simulating a more real-world view with light coming from
all directions to a surface. Because there is a specular reflection
visible to the optical pickup from it's 10 degree from normal
position, most integrating spheres have a black trap that can be
positioned at the opposing 10 degree position. This has the effect of
absorbing the reflection to give a "specular excluded" measurement,
or, by placing a reflective material at this position, producing a
"specular included" measurement. The "specular excluded" measurement
is roughly equivalent to a "45/0" geometry measurement.
Integrating spheres are the recommended method for measuring fabrics,
rough papers and other textured surfaces. "45/0" geometries are
recommended for smooth surfaces.
Once the sample is measured spectrally, the illuminant and sensor
components are subtracted from the measured spectrum to give the
reflectance of the sample itself. Once the spectral data from the
sample is obtained, the spectrum can be recalculated with any
illuminant and observer. There are two standard observers; 2 degree
and 10 degree. The 2 degree is used for small samples that occupy an
area up to 4 degrees of arc, the 10 degree observer is used for
samples greater than 4 degrees of arc in area.
The spectral data from the DTP-41, Eye-One and DataColor instruments
may be compared, but the colorimetric data from D50/2-degree cannot
be compared with D65/2-degree or D65/10-degree.
The data for your PMS patches should be different for instruments
with differing observer and illuminant calculations. Also since there
have been many generations of PMS color selectors, unless you are
measuring the same patch from the same sample book, you are almost
guaranteed to get different colorimetric values.
If you are interested in learning more about this subject, I can
recommend "Billmeyer and Saltzmann's Principles of Color Technology"
by Roy S. Berns as a good beginning to intermediate text.
Regards,
Robin Myers
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