Spectral color research
Color is a subjective sensation in the brain and therefore it cannot be measured directly. However, the electromagnetic spectrum reaching the eye can be measured accurately. Since we see colors with three different types of photoreseptors, the eye can be modelled as 3-dimensional color coordinates such as RGB, CIE xyY or CIE Lab.
The most accurate color presentation of an object is obtained by measuring the spectrum of the electromagnetic radiation emitted from the object.
[University of Joensuu Color Research Group]
The color of the object is composed of three basic factors: illumination, reflection or transmission, and the observer. In a 3-component presentation, the color is the product of these three, and so changing one factor also changes the color. Furthermore, the same sensation can be achieved through varying the 3-component values. For accurate color representation the spectral information is needed.
Spectral approach and its benefits
A spectrum is the amount of radiation measured as a function of wavelength. With a spectral presentation we can manage, for instance, changes in illumination that are not possible in 3-dimensional presentation. The spectral image can also be adapted to be similar in different types of display units. A spectral approach is not restricted to standard color coordinate presentations. Moreover, these methods can be applied beyond the visible range (UV and IR).
Processing of spectral data
The data obtained from spectral measurements is processed computationally using methods such as the following:
- PCA (Principal Component Analysis)
- ICA (Independent Component Analysis)
- ALSM (Average Learning Subspace Method)
- SOM (Self-Organizing Map)
Example of software which computes the PCA
of a spectral image and a color spectrum.
Light that is visible to the human eye is located between the UV and IR wavelengths. In the Figure, the prism distributes white light into a spectrum.
Devices and materials
- Spectral camera
- Luminescence spectrometer
- UV meter
- UV lamps
- Standard light sources
- Color standards and samples
- Color vision test
- Color filters
Center, a picture taken with a digital camera. On the left, R, G and B components, and on the right, spectral image components at intervals of 50 nanometers at wavelengths of 400-700 nm.