Timo Jaeaeskelaeinen

Accurate spectral color measurements

Surface color measurement is of importance in a very wide range of industrial applications including paint, paper, printing, photography, textiles, plastics and so on. For a demanding color measurements spectral approach is often needed. One can measure a color spectrum with a spectrophotometer using calibrated standard samples as a reference. Because it is impossible to define absolute color values of a sample, we always work with approximations. The human eye can perceive color difference as small as 0.5 CIELAB units and thus distinguish millions of colors. This 0.5 unit difference should be a goal for the precise color measurements. This limit is not a problem if we only want to measure the color difference of two samples, but if we want to know in a same time exact color coordinate values accuracy problems arise. The values of two instruments can be astonishingly different. The accuracy of the instrument used in color measurement may depend on various errors such as photometric non-linearity, wavelength error, integrating sphere dark level error, integrating sphere error in both specular included and specular excluded modes. Thus the correction formulas should be used to get more accurate results. Another question is how many channels i.e. wavelengths we are using to measure a spectrum. It is obvious that the sampling interval should be short to get more precise results. Furthermore, the result we get is always compromise of measuring time, conditions and cost. Sometimes we have to use portable syste or the shape and the size of samples makes it impossible to use sensitive equipment. In this study a small set of calibrated color tiles measured with the Perkin Elmer Lamda 18 and the Minolta CM-2002 spectrophotometers are compared. In the paper we explain the typical error sources of spectral color measurements, and show which are the accuracy demands a good colorimeter should have.

Grabbing video sequences using protein-based artificial retina

Bacteriorhodopsin thin film matrix has been studied for real-time acquisition of video. The proton pumping property of bacteriorhodopsin is reversible, and the relaxation time back to the basic state is approximately 10 ms at ambient temperature. Photostimulation can be used to return bacteriorhodopsin to the basic state in 50 microsecond(s) . The measurements show that the photocycle becomes slower in polyvinylalcohol than in solution, thus the achievable acquisition frequency is limited by the composition of the thin film.

Representation of spectral images in data communication

We present techniques for representing spectral images in data communications. The spectral domain of the images is represented by a low-dimensional component image set, which is used to obtain an efficient compression of the high- dimensional spectral data. The component images are compressed using a similar technique as the JPEG- and MPEG- type compressions use to subsample the chrominance channels. The spectral compression is based on Principal Component Analysis (PCA) combined with a color image transmission coding technique of chromatic channel subsampling of the component images. The component images are subsampled using 4:2:2, 4:2:0, and 4:1:1-based compressions. In addition, we extended the test for larger block sizes and larger number of component images than in the original JPEG- and MPEG- standards.

Thermochromism in color measurement

Accurate color measurements have become more and more important during the past few decades. This is valid not only in physical research but also in industrial production, where the importance of accurate measurements is mainly due to increased quality requirements set by the customers of various goods. The development of technology enables more and more accurate measuring systems. While the accuracy has improved one has noticed, that many unexpected factors affect the color of an object. One of these factors is the temperature of the sample. It is known that for example the reflectance of the ceramic reference tiles used for calibration of colorimeters and spectrophotometers is temperature dependent. This phenomenon is called thermochromism, which is a reversible change of color of the sample as a function of temperature. It may be noticed already at room temperature if the temperature varies few centigrades. Red and orange samples are especially sensitive to temperature variation and may cause difficulties in precise color measurements. We show, how the phenomenon is based on physical processes and not only reflects the instability of red color pigments. We derive simple formulas, which are shown to explain the experimental data. We also discuss the meaning of thermochromism for color measurements, measure the magnitude of it and propose the experimental conditions to avoid this effect.

Model for spatial and chromatic vision

An artificial vision system with spatio-chromatic channels is proposed. A dynamic neural network is used for the spatial and chromatic information of a scene. The spatio-chromatic information is transmitted into two channels for processing. This segmentation allows accurate spatial and chromatic analysis of the visual input. For both channels, models based on the biology of the visual system are used. Spatial channel responses simulate, e.g., enhanced edges and subjective contours. Chromatic channel output is shown to correspond to the color characteristics found in the spectral color tests and in the literature of the physiology of color vision. The ultimate goal of the project is to find a biologically motivated model for an intelligent image sensor. In this report we describe potential candidates for both spatial and chromatic information.

Inverse grating diffraction problems

An important diffraction problem in modern optics is to design gratings that are able to produce given output power distributions from a given input. In this paper the coupled multiwave diffraction approach, together with an optimization method, is applied to design gratings, which diffract one incoming beam into N output beams with a given power distribution. Several gratings have been designed. We show that by choosing the material parameters properly relatively simple continuous grating profiles having better diffraction characteristics than binary gratings or kinoforms can be found.

Photocycle of the 4-keto bacteriorhodopsin

The photocycle of the 4-keto bacteriorhodopsin is investigated. We constructed a multilevel theoretical model for the transmittance properties of the material. Adjusting the relaxation parameters we are able to fit the theoretical intensity dependent transmittance curves into the experiments. Thus we determine the photocycle from simple optical measurements. This method provides a new approach for studying light-induced changes in photochromic proteins.

Model for a color perception system with learning capabilities

We present a model for color vision system with learning capabilities. The system adapts to statistical properties of its input. The adaptation is done by utilizing unsupervised learning techniques, as self-organizing feature maps and vectorial boundary adaptation maps. A color difference reflecting statistical properties of input to the system is defined. The model was tested by using color data with different statistics and two different sets of rhodopsin- based color sensors.

Spectral representation of paper opacity

Paper opacity is an important problem in the paper industry. In this paper we have studied methods to derive reflectance spectrum of an opaque pile of colored paper from the reflectance spectrum of the single sheet of the same paper type. Five opacity correction methods were tested. Four of them used algebraic equations for constructing corrected spectra and the fifth method was based on the multi-layer- perceptron (MLP) neural network. Two sample sets were used for testing opacity correction methods. The first set consisted of 9 colored newsprint samples and the second set was formed from 15 samples of colored photocopier paper. The measurements of both sample sets were done with Minolta CM-2002 spectrophotometer. The measured wavelength range was from 400 nm to 700 nm with 10 nm sampling interval. For both sample sets similar tests were made. The CIE L*a*b* color coordinates were calculated both for the spectra measured from opaque piles and for the opacity corrected spectra. For each sample the absolute color coordinate differences between the coordinates of the pile spectra and the corresponding corrected spectra were computed and the results of different methods were compared to each other. The method based on the transmittance of paper gave the best results with both test sets. The predefined error tolerance of 0.5 in the a* color coordinate was achieved. The MLP-network worked also fine but the results may have been affected by the small number of training samples. We have shown that paper opacity depends on wavelength and we have developed a method for deriving the color of an opaque pile of paper from the single sheet of paper of the same type.

Spectral color measurement

In this paper, we discuss a color measuring system, which is based on the measurement of the wavelength spectrum of an object of interest. The design of measuring head, the correction of errors in the measuring and color recognition and analysis methods for an intelligent colorimeter are studied and discussed. We have described a compact and shock resistant measuring head for color spectrum measurements. It is shown that the cut of a spectrum from the 640 nm can be corrected quite accurately by linear extrapolation function in the sense of error in CIELAB coordinate values. For construction of an adaptive color measurement system, the subspace network or the Multi-Layer-Perceptron network can be used as an intelligent method for spectral analysis and recognition.