Maths & me @CIE Whiteness

What is Newsprint Paper?

Newsprint paper is a cost-effective thin paper used by publishers for years to produce newspapers, magazines and other similar marketing publications. An uncoated paper solution, newsprint is produced using recycled, mechanical, chemical and deinked wood fibres. This paper is typically very thin, meaning manufacturers can produce it more economically. However, the material is still thick enough to allow for printing on both sides. Newsprint is most often available in off-white or grey but can be dyed in any colour to suit customer requirements. Possessing both great runnability and high opacity, newsprint paper is the ideal printing solution for a variety of commercial and marketing communications.  

Different Types of Newsprint Paper

When it comes to printing, three main types of newsprint are typically used.

Standard Newsprint: The most common form of newsprint, most often used by large newspaper companies for printing. Standard newsprint is also the thinnest form on the market, making it the most cost-effective.

Improved Newsprint: A higher quality form of newsprint paper due to its higher thickness and brightness, which is achieved by additional bleaching throughout the manufacturing process. Improved newsprint often has an attractive look and finish, making it an ideal solution for high-end newspaper printing, inserts and advertising materials.

Specialty Newsprint: This variant of newsprint is bleached during the manufacturing stage, with standard colours available in green, pink, blue, yellow, orange and salmon. As this form of newsprint is also used for high-quality printing materials, this paper is thicker than standard newsprint and possesses great absorbent capabilities required for printing

From above paragraph we learned about Newsprint paper, now a days even SNP (Standard Newsprint) paper need more whiteness and brightness to print advertisement color images to meet the competition. So it is important to study the difference between Whiteness and Brightness of paper, which allow color pictures to reproduce in good quality.  

CIE Whiteness, Brightness and Shade - Overview 

WHAT IS BRIGHTNESS?

Brightness represents a narrow measurement of reflectance, measuring only a specific wave-length (457nm) of blue light. The industry uses blue light to measure a paper’s brightness because the human eye perceives a slight bluish tint as “whiter” than the neutral white of the color spectrum.

HOW IS BRIGHTNESS MEASURED?

The scale used in the United States to measure brightness is the TAPPI (GE) scale, which is from 0 to 100. A 96 bright paper reflects more light of a particular wavelength than a 92 bright paper. Brightness does not measure other wavelengths of light. This means that two papers of identical brightness can be different colors (shade), which can affect how color graphics or imagery appears.

For the most part, good newsprint, such as "Hi-Brite," will range from approximately 65 to 75 brightness. and all other newsprint used commercially today move below 60 in brightness test.

BRIGHTER THAN 100?

Some papers have a brightness of greater than 100 (by adding OBAs). This is possible because there are two basic types of brightness measurement scales: The TAPPI (GE) scale used in the United States, which uses a directional light source, and the ISO and D65 scales, which use a diffuse source to measure brightness. There is no correlation between directional and diffuse brightness due to the differences in measurement techniques too there will be increase in brightness test.

Brightness does not indicate the color or relative shade of the paper since a single number reflective value measured at 457 nm ignores all other wavelengths of light reflected across the visible spectrum. A brighter paper, therefore, reflects a greater amount of blue light than does the surface of a less bright paper. ISO brightness tester special instrument, which use blue light of 457 nm with diffuse geometry with scale range 0 -100+>   

Three different brightness measurement methodologies are used worldwide. 

Each of these methods will produce a different brightness number when used to measure the same paper sample. It is, therefore, important to understand basic concepts of each of the three testing methods, including photometric principles, characteristics of the illuminants, and illumination/observer—reflected light measurement geometries.

Due to differences in illuminants and illumination/reflected light measurement geometries, there are no direct correlations between brightness measurements produced by the three methods.

Standard Illumination Viewing Geometry Illuminant

TAPPI T 452 / GE Brightness Directional (45°/0°) CIE illuminant C (filtered response)

ISO Brightness (ISO 2470-1) Diffuse (d/0°) CIE illuminant C

D65 Brightness (ISO 2470-2) Diffuse (d/0°) CIE standard illuminant D65

VIEWING  GEOMETRY

There are tow types of  optical geometries  for brightness testers most commonly used in the Pulp & Paper Industry at present. One employing directional geometry adopted by the American Pulp and Paper Industry and the other employing diffuse geometry adopted by the European and Canadian Pulp and Paper Industries. 

WHAT IS WHITENESS?

Whiteness describes the degree to which a paper reflects light of all wavelengths (colors), and, therefore, more closely corresponds to our perception of a paper’s appearance.  Paper whiteness quantifies a paper’s ability to equally reflect a balance of all wavelengths of light across the visible spectrum. Paper brightness already said measures the amount of light reflected from the surface of paper at one specific wavelength (457 nm) in the blue region of the spectrum, whereas whiteness measures the relative balance of light reflected across the full visible spectral range (approximately 380nm – 720nm).

An object (e.g., paper) appears to be white if it reflects light equally at all wavelengths across the visible spectrum. If some wavelengths are absorbed while others are reflected, the object exhibits the color of the reflected light—color that can be measured. Pure white, therefore, is an achromatic reflector of perfectly balanced light.

HOW IS WHITENESS MEASURED?

Unlike paper brightness which only quantifies a specific wavelength of reflected blue light, paper whiteness formulas are designed to closely parallel the experience of normal human vision. Multiple standards and measurement methodologies exist for determining the whiteness of paper.

The most common whiteness measurement standard used worldwide is ISO 11475:2004, also known as CIE whiteness. ISO 11475:2004 specifies a whiteness index and measurement methodology which utilizes diffuse illumination with a light source configured to CIE standard illuminant D65.

In papermaking, to create a “white” sheet, dyes are mixed to produce a color balance that appears white. A slightly bluish tint helps whites appear their brightest; a neutral white helps colors pop. Since printing inks are translucent, the shade of paper you use can change how your colors perform.

Whiteness index calculation:

Regarding the CIE 15 standard, the whiteness index WI is based on illuminant CIE D65 and observer 1931 calculated from the following equations:

WI = Y + WI,x * (xn  -x) + WI,y * (yn-y)

where 

Y,x,y: Luminance factor and chromaticity coordinates of the sample
xn  yn: Chromaticity coordinates for CIE standard illuminant and the source used
(WI,x) (WI,y): Numerical coefficients

WI = Y + 800 (xn - x) + 1700 (yn - y) 

This equation is used for 2 degree standard observer and 10 degree standard observer. 

WI increases the more white an object appears to be. It takes the value 100 for a perfect reflecting diffuser (PRD). The WI value also depends on tint as well as lightness, e.g. if WI increases with increasing blueness of the object. For details, please refer to the tinting index calculation.

Now, this equation (WI) should be used only to show that they satisfy that: 

40 < WI < (5Y - 280) y 

The application of the equations is restricted to samples commercially called White, similar in color and fluorescence and measured with the same instruments and at the same time.

The higher the WI value, the higher the whiteness.  For a perfect reflective diffuser: WI will be 100 without OBAs, 


Tint index calculation:

Regarding the CIE 15 standard, the tint index TI is based on illuminant CIE C, D50 and observer 1931 calculated from the following equations:

TI = TI,x * (xn  -x) - TI,y * (yn-y)

where 

x,y: Luminance factor and chromaticity coordinates of the sample
xn  yn: Chromaticity coordinates for CIE standard illuminant and the source used
(TI,x) (TI,y): Numerical coefficients

TI = 1000 (xn - x) - 650 (yn- y)      -> This equation is used for 2 degree standard observer 

TI = 900 (xn - x) - 650 (yn- y)      ->  For 10 degree standard observer. 

Now, this equation (TI) should be used only to show that they satisfy that:  -3 < TI < +3

The higher T, the greener the sample. Negative T indicates reddish sample.
For a perfect reflective diffuser:  T will be 0.

ADDITIVES TO IMPROVE BRIGHTNESS AND WHITENESS

Where you are when you compare papers can make a big difference in how they appear. Since outdoor light includes ultraviolet, papers treated with OBAs (optical brightening agents) can appear brighter under outside light than they do indoors. Similarly, a white sheet not treated with OBAs may appear whiter indoors, but seem dull outside when compared with a sheet with OBAs.

In recent years, print buyers have begun to prefer papers in brighter and whiter shades. Paper makers utilize optical brightening agents (OBAs) to make substrates brighter and whiter to satisfy these demands while also lowering their costs, as OBAs are less expensive than traditional brighteners such as titanium dioxide.

OBAs are fluorescent material that absorbs ultraviolet radiation of illuminant wavelengths below the 400 nanometer (nm) region and then re-emits energy in the visible short wavelength region. This effect makes the paper substrate to appear whiter and brighter.

Disadvantage of using OBAs is the paper get color change in exposing to light for more than 40 days. 

WHAT IS SHADE?

The term shade describes the subtle differences in color of different papers. There are three groups of shade for whites: cream white, true white, and bright white.

Our eyes are more sensitive to color than to brightness. If you take two sheets with the same shade (color balance) but two points apart in brightness values, the human eye would have a difficult time detecting the difference. As we will see, each shade of white is useful in the right situation.

Yellowness index calculation:


As far as the yellowing index is concerned, the calculation formula used has been derived from the Hunter equation of 1942.

This equation was later modified to improve rounding. Finally, the formula recommended in the procedure E313, to minimize the residual error in the values ​​of the white points, is the following:

YI = 100 * (Cx * X - Cz * Z) / Y

with Cx and Cz - co-efficient

Observer 2 Degree 10 Degree

Illuminant C; 1931 D65; 1931 C; 1964 D65; 1964

Cx 1.2769 1.2985 1.2871 1.3013

Cz 1.0592 1.1335 1.0781 1.1498

were XYZ are  tristimulus values.

Following table is measured with help of XRite 528 Paper indices mode with Status E, CIE D50/2° - for comparison between different substrate. Even though Brightness tester device give accurate result, we can get approximate readings form our spectrophotometer too.

were Br% - Brightness percentage , Ct % - Color tint percentage

Here 1 - Densitometer reference plate higher white with gloss surface reflect more blue light, so Br% move to 88,
4 & 5 -  Newsprint paper with no/less OBAs or whitener agent  than magazine coated paper reflect medium blue light and result Br%. below 50.
9 & 11 - Absorbed all short wavelength (blue) so get Br% close to zero.

From the above table all pulp made paper color tends to lie between Y -> M, we can also take in account that Y -> C paper may use additive to improve brightness/whiteness were short wavelength get reflected.

By calculating CIELUV values we can clearly place / spot  points of measured paper white in graph and analyse our sample Newsprint shade with target, it is also easy to calculate distance between reference and sample. White paper shade is more important that decide the gamut size and shape, so this CIE76 chart give clear picture and help for Newsprint selection process.