Color Measuring Technique
Functioning of spectrophotometers and measuring geometries
A spectrophotometer uses the light source to illuminate the sample to be measured. The light diffused diffusely from the sample is incident on a monochromat (e.g.holographic grid) where it is diffracted into its spectrum.The spectrum is projected onto a diode array that captures the relative amount of light at each wavelength. A downstream processor gives the reflection curve. These data are multiplied by the values of the selected CIE illumination type and the selected standard observer function to obtain the X, Y, Z values.
The spectral distribution of the reflected light is recorded at fixed wavelength intervals over the entire visible spectrum, usually 400 to 700 nm (although there are also devices operating in the UV or IR range). The type of light is added mathematically in this technique, the measured values can be applied to different types of standard light, e.g. to detect metamerism. The standard observer (2 ° or 10 °) can be selected and the measured data can be displayed in different color spaces.
Spectrophotometers are offered as laboratory or handheld devices.
Spectrophotometer with 45/0 ° or 0/45 ° measuring geometry
In the 45/0 geometry, the sample is circularly illuminated at an angle of 45 ° and measured perpendicular to the surface below 0 °. With the 0/45 ° geometry it is the other way around. The circular illumination is important for repeatable measurement results on structured surfaces. The measured values depend on the sample surface (gloss, texture) and largely correspond to the human perception of a color.
Areas of application: Comparison of different batches in production, color consistency control of products composed of several parts, all applications in which the measurement result should match the visual impression.
Spectrophotometer - d/8° sphere geometry
In the d/8° geometry, the sample is diffusely illuminated by means of a white coated sphere. Shadowers inside the ball prevent the light from falling directly onto the sample surface. The measurements are made at an angle of 8°, the light source can be mounted anywhere on the ball. The color is measured independently of the surface properties (gloss, structure).
Areas of application: Color formulation and all applications in which the color is to be measured independently of the surface properties of the sample. A bullet gauge can be used under two different measurement conditions: gloss included (spin), gloss excluded (spex).
The d/0° geometry was mainly used in the paper industry, but is not used today because d / 0 ° measures only spex.
d/8° spherical geometry and the gloss trap - spin and spex
The issue of the gloss trap seems so complicated only because too many terms are used that describe the same facts and those that describe the opposite sound very similar: so the best conditions for misunderstandings!
Common terms are:
- di:8°, SCI, spin = specular included, closed gloss trap, gloss included, no gloss trap
- de:8°, SCE, spex = specular excluded, opened gloss trap, gloss excluded, with gloss trap
Tristimulus meter - Tristimulus Colorimeter – Colorimeter
A tristimulus meter uses a filtered light source to illuminate the sample to be measured with a standard illuminant. The diffuse reflected light from the object then penetrates a red, green and blue filter (usually glass filter). This simulates the standard observer functions. A photodetector behind each of these filters detects the amount of penetrating light. These signals are represented as standard color values X, Y and Z. The tristimulus measurement is no longer sufficient for many industrial applications because only one light / observer combination is available. As a result, the measured values cannot be converted into other types of light and metamerism cannot be determined. In addition, the color filters change over time, but the aging of the filters is not defined and cannot be controlled. Three-range measuring instruments are still used today in the food and pharmaceutical industry as well as in various areas of chemistry.