Brightness (3 Types) & Whiteness (2 Types): Full Disclosure

CRABLE ENGINEERING LLC

International Paper - Papermaking Process Solutions

“Marketing psychologists state that a lasting impression is made within ninety seconds and that c o l o r accounts for 60% of the acceptance or rejection of an object, person, place, or circumstance. Because color impressions are both quick and long lasting, decisions about color are critical factors in the success of any visual experience.” - About Color

The fields of shade (or color) and appearance are critical to the acceptance of paper and board products, yet these product attributes are often overlooked. Or, systems to support them are often an afterthought in the design of papermachine processes.

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Definition & Origin of Brightness

 It is the measure of light reflected from a sample at 457 nanometers +/- 44 nm.

 Brightness represents the diffuse blue reflectance of a sample, and the wavelengths indicated above were chosen on a somewhat arbitrary basis.



It was introduced as a visual aid for assessing and controlling the degree of pulp bleaching.

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Origin & Evolution of Brightness

 Originally, brightness was assessed by observing a pulp sample through a blue stained glass.

 The more the pulp is bleached, the more lignin is removed, and the higher the “blue reflectance” becomes.

 The reason for focusing on the blue-yellow axis is that the pulp starts out dark brown, changes to light brown, then to tan, and to diluted yellow as bleaching progresses.

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Pulp Bleaching Impact

International Paper - Papermaking Process Solutions



Origin & Evolution of Brightness

The need to simplify and standardize bleaching assessment of pulp led to the first instruments such as the GE-Photovolt.

 General Electric has long since sold this instrument business, but the term GE brightness remains. (Technidyne currently produces the instruments to measure it.)  In this instrument, samples are illuminated with an incandescent lamp at a 45 o angle and reflectance is measured perpendicularly or at a 0 o angle of observation.

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Evolution and Use of Brightness

 As there was no comparable instrument available for the papermaking process, the GE brightness meter (now referred to as Directional or TAPPI T452 Brightness) moved to the paper mill.

 Prior to the introduction of fluorescent whitening agents, and as a consequence of its purpose in describing the degree of bleaching, brightness over time became synonymous with how white the paper appeared (or simply whiteness).

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Types of Brightness

 There are two basic types of brightness measurement: Directional and Diffuse.

 Directional brightness (TAPPI brightness – TAPPI 452) employs the 45 0 /0 0 geometry of the original GE-Phovolt instrument. It has been the standard in the U.S. and Japan.

 Diffuse brightness employs a D/0 0 geometry where D indicates diffuse illumination from a sphere, making it insensitive to sample orientation. Diffuse brightness is the standard in much of the rest of the world.

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Illustration of Instruments

Photodetector Light source Sample Diffuse Directional Sample Light source Photodetector Informing our Customers: CRABLE ENGINEERING LLC

Types of Brightness

 Two types of Diffuse brightness are commonly discussed, and a standard exists for both of them.

 ISO C Brightness (ISO 2470-1, TAPPI 525) uses Illuminant C level of UV energy. It was selected as the best approximation of normal office lighting conditions.

 ISO D65 Brightness uses Illuminant D65 (daylight). Control of D65 level of UV energy is not good from instrument to instrument, but still there is now a standard for D65 Brightness (ISO 2470-2).

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Differences Between D65 and C

International Paper - Papermaking Process Solutions

Comparison of Brightness Types

 TAPPI (45 0 /0 0 ) , or Directional and Diffuse (D/0 0 ) Brightness (C or D65) cannot be correlated. This is due to the fact that multiple reflections which occur between the sphere and sample make unpredictable changes in Diffuse Brightness as compared to Directional (TAPPI) Brightness.

 The following two slides show brightness comparisons on a number of white papers.

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Brightness Comparisons

Sample E F G A B C D P Q R S T H I J K L M N O Brightness

Elrepho

D65 Brightness ISO Brightness 98.92

95.34

105.58

111.20

110.61

112.25

113.46

91.78

88.99

95.08

99.83

98.94

100.43

100.89

110.11

112.28

93.00

102.01

110.12

110.14

107.24

106.90

110.31

109.57

113.86

111.55

85.48

98.68

100.15

88.64

92.52

99.05

99.48

96.68

96.28

99.21

99.03

101.50

99.29

85.43

Technidyne

TAPPI 89.9

87.3

91.8

96.5

95.5

96.0

96.4

94.7

95.9

90.0

90.6

95.0

96.1

94.1

93.0

96.1

96.1

97.4

95.0

87.6

Fluorescence

Elrepho Technidyne

R457 C 6.93

6.14

10.58

11.48

11.90

12.58

13.45

11.73

12.97

5.44

9.56

12.08

10.97

10.66

10.61

11.51

10.90

13.25

12.80

0.02

S-4M 5.2

4.7

7.9

8.5

9.2

9.5

10.3

8.8

9.8

4.3

8.0

9.1

8.4

8.1

7.9

8.9

8.2

10.3

9.6

0.0

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Comparison: TAPPI vs ISO Brightness

Delta Brightness Comparisons - Real Data

5.00

4.50

4.00

3.50

3.00

2.50

2.00

1.50

1.00

0.50

0.00

86.0

-0.50

-1.00

-1.50

-2.00

-2.50

87.0

88.0

89.0

90.0

91.0

92.0

93.0

94.0

95.0

96.0

97.0

98.0

TAPPI Directional Brightness

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The Scattering Properties of Glossy and Matt Samples

Specular Reflection Diffuse Reflection GLOSSY SURFACE MATT SURFACE

The surface properties of a sample influence the quality and quantity of light that reaches our eye; influencing the way an object appears. In fact, we sometimes calender samples to get “the right look.” Informing our Customers: CRABLE ENGINEERING LLC

Problems and Limitations of Use

  Directional (TAPPI) brightness, first, has an issue in that it is highly sensitive to fiber angle. Indeed, one can impact directional brightness with changes in Rush/Drag ratio.

Per TAPPI method T 452, section 1.2: “This procedure is applicable to all naturally-colored pulps, and papers and board made therefrom. The measurement is not suitable for paper or paperboard containing coloring matter (such as yellow or green dyestuff) which appreciably absorbs light in that part of the spectrum extending from 400 to 500 nm. Colored papers must be measured spectrophotometrically or colorimetrically in order to obtain meaningful results.” Informing our Customers: CRABLE ENGINEERING LLC

Problems and Limitations of Use

  Also, brightness is not a linear function. A change from 84 to 90 brightness has much more visual impact than a change from 90 to 96 brightness.

 The biggest issue with use of brightness as a paper metric, though, is that it measures reflectance in a VERY narrow (blue) band of the reflectance spectrum. This does not correlate well with the perception of whiteness or visual appeal.

The higher the brightness of the paper (brought about through the use of FWA), the quicker the paper will fade.

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Ways to Increase Brightness

 Start with brighter pulp. This could involve additional bleaching or washing of the pulp or even purchase of High Brightness pulp.

 Cover the fiber with fillers of higher brightness. E.g. use very bright clay, PCC, TiO hide the yellower fiber.

2, or ATH to  Use Fluorescent Whitening Agents (FWA) to “boost” the blue reflectance measured as brightness.

 Keep the FWA on the surface of the sheet using good film formers such as PVOH or PEG.

 Use the cleanest, brightest coatings (starch or PVOH in uncoated paper) possible.

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Ways to Avoid Lower Brightness

 Use no post-consumer fiber (contains more fines and contaminants) which dull the paper.

 Avoid heavy metal ions (such as copper, manganese, and iron) in the process. Use a chelant to control them if necessary.

 Treat very hard water to prevent precipitation of the FWA.

 Do not overuse FWA or “green-over” may occur.

 Use minimum amounts of dye for shade control.

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CIE Whiteness

 Whiteness is a visual perception that involves the entire visible spectrum from 400 nm to 700 nm.

 It was developed, specifically, as a single metric to describe the appearance and appeal of white papers.

 Values of Whiteness increase as the paper becomes bluer and slightly redder.

 Whiteness can be increased both through use of FWA and tinting dyes.

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CIE Whiteness

 Most import papers are shade formulated to take advantage of the increase in Whiteness with blue and red (or violet) tinting of the paper.

 Whiteness can be less expensive to achieve than Brightness: a small addition of tinting dye (low cost) can significantly improve Whiteness. [Tinting dye use can generally only lower Brightness.]  Change in the shade of paper with light exposure is generally less severe than loss of Brightness with FWA fade.

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CIE Whiteness

 There are two types of CIE Whiteness.  First is outdoor CIE Whiteness (per ISO 11475). It yields the higher value due to the higher UV energy level of Illuminant D65. It came first and is the only one commonly used.

 Second is indoor CIE Whiteness (per ISO 11476). It yields lower values but is more representative of office lighting conditions, using the UV energy level of Illuminant C.

 CIE Whiteness should ONLY be used for ‘near white’ papers. (And this is becoming an issue.) Informing our Customers: CRABLE ENGINEERING LLC

Brightness, Whiteness, and L*

International Paper - Papermaking Process Solutions

Brightness Conclusions

 Either brightness or whiteness can be more expensive to achieve depending upon the shade of the paper and its ‘brilliance’.

 As one goes to very high brightness levels, production costs rise ever more quickly, and the visual impact becomes smaller.

 ISO Brightness values are generally higher (as much as 1.5 to 4.5 points) and less variable than TAPPI Brightness values.

 Whiteness often correlates better with visual appeal than does Brightness.

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Whiteness Conclusions

 The uppermost Whiteness values have now been pushed beyond their practical limits for true white papers.

 As one goes to very high whiteness levels, the positive impacts of FWA on the paper have been exhausted.

 The only way to continue to increase CIE Whiteness, then, is to make the paper bluer.

 Often, however, the product begins to look dull and grayish at the highest CIE Whiteness levels.  We need to refine the CIE Whiteness equations to prevent overuse of tinting dyestuff to artificially inflate the numerical value while actually making the paper look duller.

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

Whiteness Conclusions

Although much of the rest of the world makes and sells paper according to CIE Whiteness level (which is not printed on packaging) the attempt to introduce whiteness into the U.S. market was not well received.

 Much of the reason for this is that major U.S. customers are familiar with brightness (mostly TAPPI T452 brightness) and are unwilling to give up this metric.

 Brightness is still printed on the ream wrap and, oddly, is used to distinguish between quality levels.

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