Honey Yellow, Raspberry Red, Sapphire Blue or Moss Green are very appealing and descriptive color names. But are you sure that every person means the same color by it? Usually not. How do you clearly describe a color and guarantee that same color over time?
Our color perception is dependent on our individual “taste,” which is influenced by our mood, gender, age, but also the light source used, the viewing environment being light or dark, neutral or colorful as well as our deficiency to exactly remember and communicate one specific color.
Standardized viewing conditions
For controlled visual and instrumental evaluation the light source, the surrounding and the observer are to be defined. Colors may match under one light source (daylight), but not under another (fluorescent light). This effect is known as metamerism and is a crucial quality requirement for multicomponent products. Therefore, the match needs to be verified with the kind of light likely to be found where the product is sold or used. The CIE (Commission International de l’.clairage) standardized commonly used light sources.
ISO and ASTM standards define the surroundings as portion of the visual field immediately surrounding the specimens as well as the ambient visual field, when the observes glances away from the specimen, such as the interior surfaces of the light booth. It shall have the color with Munsell notation N5-N7 and a 60° gloss not greater than 15 GU.
The observer for visual appraisal should have normal color vision and be trained in observing and classifying colors. Visual tests are recommended to check an observer’s color vision periodically as it can change over time (see Guide ASTM E1499). The observer for instrumental color control was standardized with two different viewing fields: 2° standard observer and 10° standard observer. Today mainly the 10° observer functions are used as the eye integrates over a larger area.
Standardized measurement parameters
For instrumental color measurement the optical properties of the product need to be measured. A spectrophotometer measures the amount of light that is reflected by the object at different wavelengths in the visible range (400 – 700 nm). The reflectance curve shows the spectral data and acts as a “finger print” for the object color.
Internationally standardized color systems, like the widely used CIELab system, combine data of standardized illuminant, standardized observer and spectral reflection data in three color components describing the lightness, hue and chromaticity of a color.
Over the years new color systems and equations (ΔECMC – ΔE94 – ΔE99 – ΔE2000) were developed based on visual comparison studies for solid colors to improve the visual correlation, which shows elliptical tolerance behavior.
Standardized instrument geometries
International standards define the geometric conditions of spectrophotometers:
45/0 – Control color as you see it
For final QC of solid colors a 45° circumferential illumination is defined to achieve repeatable results on unstructured and structured surfaces.
d/8 – Control the hue of a color
If the color without influence of surface gloss or texture is to be controlled, diffused illumination is required.
Consistent quality control needs an OBJECTIVE EYE!
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byko-spectra light booth
Standardized visual color appraisal
- Color neutral surround and ambient field
- Several light sources: D65 – A – CWF/TL84 – UV
- UV light to detect fluorescence
- Timer to track daylight lamp usage
- Compact design