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Colour spectrum

In the printing industry, the term colour spectrum usually refers to an achievable colour space.

The ‘colour spectrum’ describes the totality of colours that a printing process or a specific printer can reproduce. The colour spectrum is determined by the printing inks, inks and printing techniques used. ‘The new CANON printer has a super colour spectrum’ would be a classic colloquial formulation.

The larger the colour space of a printing process, the more different colours can be reproduced accurately. Printing technologies such as offset printing, flexographic printing and digital printing have different colour spectrums that depend on the printing ink, substrate and other technical factors such as process colours or spot colours.

Colour gamut control is an important aspect of the printing industry, especially when it comes to reproducing accurate and consistent colours in printed materials. Colour management systems are used to ensure that the colours on the printed product match the intended colour values, digital proofs are used to check on the machine and in advance of the design process to ensure colour accuracy in the final print.

However, a colour spectrum can also be a single spectrum of a single colour.

If a colour is measured with a spectral measuring device, a file is created in which, for example, an emission colour spectrum is written to a file when measuring monitors or a reflectance colour spectrum is written to a file when measuring prints. Typical files such as CXF (Colour Exchange Format) or CGATS contain a colour spectrum or several colour spectra that are spectrally represented in the file. A value is stored for each wavelength measuring point of the measuring device, from which a colour value can then be calculated.

An example of such a CGATS file can be found in our paper white tables for printing papers, where we have measured the colour spectrum of the paper and published it as a file. Such a CGATS file looks like this:

CGATS.17
ORIGINATOR "Proof GmbH, https://www.proof.de | BabelColor CT&A, version 4.0.0 b298"
LGOROWLENGTH 1
CREATED "2019-02-28" # Time: 16:51:26
INSTRUMENTATION "i1Pro 2 (XRGA)"
# Instrument Serial Number: "1095898"
#
# Sample-1 (S1) name: "Arktika-GC1-M2"
#
MEASUREMENT_SOURCE ""S1: Reflectance (M2) Illuminant:D50 Observer:2_deg Units=Reflectance(0-1) WhiteBase=Automatic Filter=no""
#
KEYWORD "SAMPLE_ID"
KEYWORD "SAMPLE_NAME"
NUMBER_OF_FIELDS 44
BEGIN_DATA_FORMAT
SAMPLE_ID SAMPLE_NAME L*a*b*_L L*a*b*_a L*a*b*_b L*C*h*(ab)_L L*C*h*(ab)_C L*C*h*(ab)_h nm380 nm390 nm400 nm410 nm420 nm430 nm440 nm450 nm460 nm470 nm480 nm490 nm500 nm510 nm520 nm530 nm540 nm550 nm560 nm570 nm580 nm590 nm600 nm610 nm620 nm630 nm640 nm650 nm660 nm670 nm680 nm690 nm700 nm710 nm720 nm730
END_DATA_FORMAT
NUMBER_OF_SETS 1
BEGIN_DATA
1 Arktika-GC1-M2 95,45 0,6834 3,022 95,45 3,098 77,26 0,4987 0,5423 0,5984 0,7088 0,7831 0,8209 0,8362 0,8462 0,8558 0,8625 0,8682 0,8732 0,8764 0,8783 0,8805 0,8803 0,8811 0,8822 0,8806 0,8840 0,8856 0,8896 0,8943 0,8997 0,9078 0,9182 0,9316 0,9424 0,9501 0,9546 0,9563 0,9571 0,9612 0,9646 0,9671 0,9711
END_DATA

In the ‘Sample_ID’ line, the measured spectra are listed in columns from 380-730 nanometres after the LAB and LCH values; the respective measured values from which the LAB and LCH colour values were calculated are then listed in the line under ‘BEGIN_DATA’.

The term colour spectrum would therefore refer here to the actual individual colour spectrum of a measured colour, and not to the colour space of a printer or printing process.

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