Optical Properties of Pulp Brightness is the percentage of diffuse reflected light from a thick pad of paper at a wavelength of 457 nm. This is an arbitrarily defined standard. Reflectance in the blue portion of the spectrum was chosen because this is the area were yellowish/brownish pulps absorb light. A change at this wavelength was found to be very reliable in describing changes in appearance. Brightness is not the same as whiteness and is not a colorimetric measurement (can not describe colors).

Reflected light

Absorbed

Transmitted light

Light hitting a paper surface can be reflected, transmitted or absorbed.

Light, shining on a paper sheet is reflected, absorbed, or transmitted. At each air-fiber interface, light can be reflected or refracted. In the paper sheet there are layers of fibers and light is scattered through the air-fiber interfaces. If all the fiber surfaces were bonded in a sheet of paper (no air-fiber interfaces) the sheet would be transparent. On the other hand, if all surfaces were unbonded light scattering would be at a maximum, the sheet would be nearly opaque. A measurement of the ability of the sheet to scatter light is called light scattering coefficient (s).

The other factor affecting brightness is the ability of the fibers to absorb light . This is measured as characterized using the absorption coefficient (k). The bleaching of pulp can be considered a removal of light-absorbing substances from the pulp. The reflectance of an opaque pad is a function of the ratio of k/s and is described by the Kubelka-Munk equation:

2 K = ( 1-R ) S 2R or

2 R∞= 1 + k/s - √ 2 (k/s) + (k/s)

K= light – absorption coefficient S= light –scattering coefficient

R =reflectance with infinite sheets of same pulp backing == brightness if measured at 457 nm (note: R is in % and needs to be used accordingly, for example 81 % brightness is added as 0.81 into the equation)

This means brightness is dependent on scattering and absorption. A pulp with high scattering power appears lighter at the same absorption level (think about sawdust being lighter in color than the log it comes from). Mechanical pulps usually have high scattering and might have almost twice the level of chromphores than a chemical pulp with the same brightness.

K and s for mixtures are additive. For example, for mixtures of pulps and/or fillers the overall k and s can be determined based on the mass ratios:

S = c1 s1 + c2 s2 ….. c=mass fraction K = c1 k1 + c2 k2….

The Kubelka-Munk equations have been solved to allow calculations/predictions of several optical properties of paper. In addition there are graphic solutions (called Judd charts) that allow determination of opacity, reflectance scattering power.

Kubelka and Munk worked with the theory that during bleaching scattering coefficient (which is determined by fiber dimensions and degree of bonding) does not change significantly, while k (proportional to chromophore concentration) decreases. During bleaching chromophore concentration is reduced, reduction is not proportional to the change in brightness. At really high brightness small changes in chromophore concentrations change brightness considerably.

Brightness measurement There are different brightness measurement instruments, main difference is the optical geometry (angle at which light hits the paper)

TAPPI brightness (also called GE) 450 angle , MgO standard, handsheets sample dried at room temp

SCAN diffuse illumination, MgO standard, handsheets dried at 600C

ISO (introduced 1977) uses absolute reflecting diffuser as ELREPHO standard, diffuse illumination, is lower than SCAN

Opacity Opacity is the ability of paper to hide or mask color or objects in back of the sheet. Opacity is influenced by scattering coefficient and by absorption coefficient. High opacity is the result of light scattering in many directions when the path of the light is bent. Rough surfaces help scatter the light. Fiber-air interfaces scatter light. Fines have very high surface areas and irregular shapes that contribute to high opacity.

High absorption coefficients also result in high opacity; light is absorbed and does not go through the sheet either. Dark samples have higher opacity. Opacity is measured by determining how much light diffuses all the way through the paper and back. It is expresses as a ratio of the reflectance of a sheet of paper over a black body to the reflectance of a sheet of paper over a white backing. There are two kind of opacities used. The TAPPI opacity used a black body (as described in the sentence before ); the Print opacity used the ratio of the reflectance of a sheet of paper over a stack of the same paper to the reflectance of the paper over a white body.

Gloss Gloss is a measure of the sheen or polish of paper. It is measured by illuminating the paper at very low angel and reading the reflectance at a similar low angle.

Figure 6-4: Gloss measurement

Colors Visible light consists of electromagnetic radiation from about 380 nm to 700 nm. Colors we see have approximately the following wavelength

Violet 380-430 nm Indigo 430-450 nm Blue 450-500 nm Green 500-560 nm Yellow 560-590 nm Orange 590-630 nm Red 630-720 nm

If all wavelengths are reflected evenly we do not see any color, the paper appears to be white (if all wavelength are reflected strongly) or grey (if they are absorbed partially, but to the same degree) or black (if they all are absorbed). If one wavelength is absorbed preferentially we see the complementary color. For example, a sheet of paper absorbing light in the blue range appears yellow to us.

Tristimulus System Systems designed to define color using three variables. There are several of these systems, the first system used was the CIE system. Today the L*, a*, b*, systems are most common. For the L,a,b systems L has values of 0 ( = black) and 100 ( =perfectly white). Positive values of “a” correspond to “redness”, negative values of “a” correspond to greenness. Positive “b” correspond to yellowness and negative “b” corresponds to blue. When a and b are both “0” the sample is achromatic (grey, white or black).

“b” values ( corresponding to yellowness) are an important measure of papers having a yellow tint (for example high yield fiber containing paper).