Color assessment to ISO 3668

Color meters and optical instruments can only be one part if you need to compare colors and substrates. Since no technical device is as precise as the human eye, the visual color control is essentail for your quality control.

It is written in the ISO 3668:2009 standard how to assess colored substrates in manufacturing industries (automotive, plastics, furniture etc.). On the contrary, visual color machting in the graphic arts industry is described in the standard ISO 3664:2009.

ISO 3664:2009 ISO 3668, DIN 6173-2, DIN 5033
Graphic technology and photography – Viewing conditions Visual comparison of the colour of paints
Illuminant D50 (CIE) Illuminant D65 (CIE)
Graphic arts industry Manufacturing industry

Requirements to color viewing booths

Reproducible conditions are necessary for standardized and repeatable color matching processes. Natural dalight would be a good starting point for the light quality but the light spectrum varies depending on time of day and place. Therefore, for uniform viewing conditions color matching boxes with artifical CIE daylight D65 are used.

The requirements for a viewing booth and its lighting are defined in ISO 3668 and are fulfilled by all our Color Viewing Light cabinets:

  • JUST daylight 6500 proIndustry fluorescent tube (illuminant D65) fulfills quality class BC
  • Illuminance between 1000–4000 Lux
  • Internal wall for the shielding of extraneous light with matte, neutral gray color (Munsell N7)
  • Note: For very bright substrates, the inner walls may be brighter. For very dark substrates the inner walls should be matte black in order to reduce contrast between the environment and the colored sample. Corresponding interior walls are available on request.
  • Acrylic glass for diffuse, uniform light without reflection on the colored substrates
  • Operating hours meter to show when new lamps are required to fullfil the standard

Standardized color assessment

In order to standardize not only the viewing environment but also factor „human“, special requirement are placed on the observer. The observer must be perceive color normally and should have experience in color comparison as well as a good ability to distiguish different colors. Corresponding visual tests help to choose suitable observers. In addition, a neutral color such as gray must be worn during the color assessment.

Two-dimensional sample plates need to be viewed with illumination of 0° by a viewing angle of 45° (measuring geometry 0°/45° and 45°/0° respectively). Howeder, certain application fields require different viewing and illumination angles, that is why appropriate sample holders are available in the web shop.

Color differences are indicated to the ISO rating scale by hue, chroaticity and brightness. Finally, all details of the color matching procedure should be recorded in an assessment report. A corresponding function to create such reports is available with the PROFESSIONAL series of the Color Viewing Light booths.

Detecting metamerism

For the detection of metamerism, means two color samples that appear equal with one illuminant but looks different with another light, the substrates must also be assessed by CIE-illuminant A (incandescent lighting). In the illustrated example, both bodies have different spectrum, that may appear different depending on the illumination type and light source.

For standardized color control it is necessary to choose D50 lighting for prints and D65 for industrial products, e.g. coatings, plastics, varnish etc. If you need to detect metamerism of the substrates, additional illuminants are needed.

With our color viewing booths you can choose between different dimensions and numbers of illuminants but always with illuminant D65 und A for detection of metamerism.

Comprehensive recommendations for color assessment

  • Comply with measuring geometry 0°/45° or vice versa. If necessary, use sample holders
  • Put samples as close as possible. Replace position and repeat color viewing.
  • If possible, samples should have same dimensions
  • Ambient conditions shoulb be as neutral as possible. Try to avoid ambient light.
  • For very dark substrates a light box with matte-black interior lining can be used to reduce the contrast. Otherwise the viewing booths must be neutral gray (Munsell N7).
  • Obersers should wear neutral clothing to avoid reflections from their clothes.
  • Only the viewing samples can be used at the time of assessment in the viewing booth in order to avoid color irritations
  • The observer must be perceive color normally and should have a good ability to distiguish different colors.
  • Give your eyes time to adapt to the ambient and do not look at any strong colors directly before the assessment
  • If you own standards and guidlines are used, inform all parties who are involved in the color assessment procedure
  • Use several illuminants to detect metamerism
  • If the substrates are fluorescent, UV radiation must also be used for color assessment
  • Replace your fluorescent lamps regularly according to the manufacturer’s instructions. JUST Normlicht product contain an operating hours meter that shows you change of the lamps after 2,500 hours.

Source and further information:

DIN Deutsches Institut für Normung e.V. (2001): DIN EN ISO 3668:2001-12: Beschichtungsstoffe – Visueller Vergleich der Farbe von Beschichtungen (ISO 3668:1998); Deutsche Fassung EN ISO 3668:2001