Spectrum measurement

Spectral measurements of paper samples were the part of the experiment. Spectral measurements were carried out by one-pixel Minolta Spectrophotometer CM-2002.

Spectrophotometer represents the present-day device, which allows measurements in the laboratory conditions.

5.2.1 Measurement Principles

The spectrophotometer uses a double-beam feedback system for measurement. The specimen is illuminated diffusely and viewed at 8 angle to the normal to the specimen surface, the width of the viewing beam is 7.4 . The geometry satisfies the specification described in CIE publication 15.2, ISO 7724/1 and DIN 5033 teil ² 7 [25, p. 139]. ²

When user presses “Measurement” button, following operations in the viewing system are initiated [25, p. 139] (Fig 27):

1. Light from the pulsed xenon arc lamp is thoroughly diffused inside the integrating sphere and provides even illumination over the area of the specimen surface to be measured.

For the SCE (specular component excluded) measurements, the light trap is opened.

Then the light, which would have been reflected by the surface on the integrating sphere at the light trap position enters the light trap and is absorbed.

2. Light reflected from the specimen surface at an angel of 8 to the normal enters the optical fiber cable for taking measurements and is transmitted to the spectral sensor 2.

3. The light from each optical fiber cable is divided by wavelength (from 400 to 700 nm) at 10 nm pitch before striking the segments of the silicon photodiode array of the spectral sensor. The segments of the spectral sensor convert received light into electrical currents proportional to an intensity of light. These electrical currents are then passed to analog control circuits.

“Multispectral Analysis of Paper Production Phases ”, Irina Mironova, 2002

Two spectral camera sensors allow take into account both light, which illuminates the specimen surface, and light reflected by the specimen surface. Such double-beam feedback system eliminates effects of variations in spectral characteristics or intensity of the illumination [25, p.140].

Figure 27. Geometry of Spectrophotometer CM-2002 [25, p. 142].

5.2.2 SCE/SCI modes

The CM-2002’s geometry allows switching between two modes: SCI/SCE. The difference between two modes is that for SCI mode measuring, light includes both diffusely reflected light and specularly reflected light, while for the SCE mode, measuring light includes only light reflected diffusely from the specimen surface. User can easily switch from one mode to another by opening and closing a trap. When the trap is opened, specularly reflected light is released, while, when it is closed, specularly reflected light is ensnared (Fig. 27) [25, p.

142].

“Multispectral Analysis of Paper Production Phases ”, Irina Mironova, 2002 5.2.3 Color Data Software for CM-2002

Spectrophotometer is connected to the computer through the RS 232C interface, which allows the computer to control operations of CM-2002 and to visualize results of the measurement. The computer interacts with CM-2002 with the aid of Color Data Software (CM-1). CM-1 provides a variety of functions for controlling the display, processing of data, and controlling the CM-2002 [26, p. 1]. Such operations as calibrating and setting the target data are presented input to the CM-1. Data received by the CM-1 can be then stored on the disk and used without the CM-2002 connected to the computer. Moreover, data can be saved in the ASCII file, allowing it to be exported and imported from the other software, such as spreadsheet program [26, p. 1]. Color Data Software displays data both on the graph and in the data table.

5.2.4 Representation of Colorimetric and Color Difference Data

“Colorimetric and color-difference data can be represented by different notions including XYZ, Yxy, L*a*b*, L*C*H*, L*u*v*, Lab, Munsell, metamerism index (for up to 3 test illuminations), CMC (1:1, 2:1, or user specified ratio), FMC-2, whiteness index (ASTM E 313 or ASTM D 1925), colorimetric data can also be displayed on the L*a*b* or Lab graph” [25, p. 1]. Conversion of the measured spectral reflectance curve to the colorimetric and color-difference data is accomplished by the conversion to tristimulus values of XYZ space (Eq. 8-10, p. 33) followed by the conversion to the desired color space.

The experiment in the thesis includes measurements of spectral reflectance curves of paper samples and their conversion to the L*a*b* space to compare difference between values produced by the similarity metrics and difference between L*a*b* values. Mathematical formulas of conversion from the XYZ space to the L*a*b* space are as follows [25, 154]:

12

“Multispectral Analysis of Paper Production Phases ”, Irina Mironova, 2002

,where Xn, Yn and Zn are the tristimulus values of the reference white.

5.2.5 Measurements

The general flow of operations in using CM-1 to control the CM-2002 and take measurements is represented in Fig. 28. Before using CM-2002 spectrophotometer, several preparation steps should be done. They are inserting battery, inserting AC adapter, adjusting contrast, adding/ removing stabilizer, installing memory card. Memory card is used for storage measurement data (about 1000 measurement sets). Then, zero and white calibrations should be performed to achieve the best possible accuracy. It allows to eliminate variations in measurement values due to changes in ambient temperature or in the internal temperature of the CM-2002 due to the repeated measurements, vibration, shock, etc [25, p. 32]. Then, measurement and displaying conditions in the MENU mode should be set. After that, camera is ready for taking measurements. Measurements should be taken under the same conditions as for white calibration. Sample to be measured is placed on the CM-2002 flat and “Measurement” button is pressed. When all measurements are taken, measurements data are saved on the disk.

“Multispectral Analysis of Paper Production Phases ”, Irina Mironova, 2002 Figure 28. Operations performing on CM-2002 [25, p. 28].

Display of measurement data

Output of measurement data

Install memory card

Set power switch to On

Calibration

Set

measurements conditions

Set target color data

Save

measurement conditions

Take

measurement Preparations

Load memory conditions, target color data

If memory card is blank

If color difference measurements will be taken Zero calibration

White calibration

“Multispectral Analysis of Paper Production Phases ”, Irina Mironova, 2002