Color standards

Judy Jacob <judy_jacob [at] nps__gov> writes

>I am looking for a color standard, specifically a ceramic tile that
>is used for calibrating colorimeters.  I think they are available in
>4 inch squares, and I know they come in white--they might also come
>in different colors. Does anyone know what these are, and where I
>can get one?

Coloured ceramic tiles for use in calibrating colorimeters or
spectrophotometers can be purchased from the same company that made
your colorimeter or spectrophotometer, such as Minolta or BYK
Gardner. The calibration values which come with the tiles are
specific to the type and geometry of the measuring device. The type
you purchase must be appropriate for your measuring device, and some
will not be suitable for use on different machines. If you have an
old set of tiles, it may be possible to have them recalibrated by a
certified laboratory for use with your colour measuring device.

The calibration standards produced for sale with the Minolta CR-200
and CR-300 tristimulus chroma meters are about 2.5 x 2.5 inches (6 x
6 cm) square. The round white calibration standard for the Minolta
CM-2022 spectrophotometer is 1 inch (2.5 cm) in diameter and  2 x 2
inches (5 x 5 cm) coloured tiles can also be ordered.  We also have
a set of very old coloured enamelled "tiles" from Hunterlab that are
about 4 x 4 inches . Other manufacturers probably produce different
sizes according to their applications. Contact information for three
manufacturers are below:

    Minolta Corporation
    Ramsey, N.J.
    1-888-ISD-COLOR
    <URL:http://www.minoltausa.com>

    BYK Gardner-USA
    Columbia, MD
    1-800-343-7721
    <URL:http://www.bykgardner.com>

    Hunterlab
    Reston, VA
    1-703-471-6870
    <URL:http://www.hunterlab.com>

Although white and coloured ceramic colour standards are considered
stable, they should be periodically recalibrated by the manufacturer
against primary standards which are traceable to the colour
standards held by the international standards laboratories (it is
recommended that this be done every several years).  These standards
should be cleaned carefully, and generally you should not be used as
a backing material under translucent surfaces being measured. Older
white ceramic calibration tiles such as opal glass have very
specific and time-consuming cleaning protocols (which is why the
ceramic tiles are preferred for every-day use).

You will want your calibration standards to be "certified", that is,
to have a certificate that states when it was calibrated against the
primary standards, and what the calibration values are.  When
purchasing coloured calibration tiles there may be two
prices--without the calibration values (which means you can only use
them as colour standards, to measure instrumental stability) or with
the calibration values (which means you can use them for calibration
and standardization).

I generally carry out my chroma meter  and spectrophotometer
calibration against a white standard, and verify the stability in
colour space against a series of coloured tiles. In certain
instances, I will calibrate against a coloured tile if it is close
in colour value to the surface being measured.  The colour values
measured with the instrument calibrated against the white vs. the
coloured tile standard will be close, but not necessarily the same.
I do not use the "multi-calibration" option on the CR-200/CR-300
chroma meters where the colour values of  several or all colour
standards are stored in the measuring device as this option does not
allow me to know  which colour value is being used as the nearest
"calibration" when the sample measurements are being carried out. I
prefer to know exactly what the device is being calibrated with so I
can repeat the measurements at a later date if necessary.

Nancy E. Binnie
Conservation Scientist (Chemist)
Conservation Processes and Materials Research
Canadian Conservation Institute
Department of Canadian Heritage
1030 Innes Road
Ottawa, Ontario K1A 0M5
Canada
613-998-3721 ext: 158


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                  Conservation DistList Instance 15:76
                   Distributed: Friday, May 10, 2002
                       Message Id: cdl-15-76-002
                                  ***