Learn about light, reflection curves, optical brighteners, and more.
Illuminants
Electro magnetic radiation in the wavelength range from 380 nm to 730 nm is seen as light by our eyes. Low wavelengths show as blue light, then the spectrum continues from green to yellow, orange, and red. UV radiation is located in the range below 380 nm; the range above 730 nm is called infrared radiation. The visual impression of a colored body changes by the composition of the incoming light.
Reflectance
Each color has its typical spectral curve. The color pigments absorb specific wavelengths of the incoming light while other wavelengths are reflected. Perfect white reflects the entire incoming light (i.e.: no absorption) while perfect black absorbs all wavelengths at 100%. If you add saturated dyestuffs to a white base, specific sections of the spectral curve are lowered more and more. The spectral curve can never be raised by dyestuffs.
CIELAB System
The CIELAB color space is the most common used color space in the industry. The vertical L* axis reflects the lightness of a color. Here L*=0 represents absolute black and L*=100 represents perfect white. The positive a* axis represents the red parts of a color and the negative a* axis represents the green shares. The positive b* axis is for the color yellow and negative b* values mean blue. Thus, in this three-dimensional structure you can "address" all real existing colors at one light type, measurement geometry, and standard observer.
Another way to display the a* and b*-axes is the representation in polar coordinates. C* is named chroma and shows the difference from the neutral gray axis to the sample. H is called the Hue angle. It is always measured counterclockwise starting from the positive a* axis. This set of definitions is mainly used for saturated colors, since values in numbers are easier to understand. For instance, the color orange is more saturated than the sample, not more red and more yellow. The difference from a sample to the standard is stated in delta values (Δ or d):
- ΔL* = 0.5 sample is 0.5 units brighter
- Δa* = -1.5 sample is -1.5 units greener
- Δb* = -3.6 sample is -3.6 units bluer
- ΔC* = -3.9 sample is -3.9 units less colored
- ΔH = 0.7 sample is 0.7 depending on the location of the color shades
Tolerance
Similar to the axes of the CIELAB system, several directions must also be observed for the tolerances. The difference from standard value to tolerance limits need not always be the same. Especially in saturated colors, a tolerable color difference along the chroma axis (=ΔC) can be much higher than a color difference in the hue (=ΔH) or in the lightness (=ΔL).
Optical Brightening
Optical brightening agents (OBAs) absorb invisible UV radiation and emit in the visible range. This can create reflectance values of more than 100%, i.e.: at specific wavelengths more light is reflected by the sample than had come in at these wavelengths. This effect is utilized in white papers or textiles, where blue light is excited. The CIE whiteness correlates better to the visual assessment than ISO whiteness, since the entire visual range is taken into consideration.
