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Terahertz: An Emerging Technology for Process Control of Printing, Coating and Various 

Other Aspects of Paper Manufacturing

Mark Kemper, Picometrix, LLC

Jeffrey White, Picometrix, LLC

Overview

• Introduction to Time‐Domain Terahertz Technology (THz)

• Measurements of Interest–Simultaneous Coat Weight–Substrate Basis Weight–Moisture Measurement

• Summary

What is Terahertz•Portion of EM spectrum between Microwaves and Far‐IR

•Used by Astronomers for yearsGas Phase spectroscopy of interstellar objects

• Typically used liquid He cooled Bolometer detectors and very long acquisition times (min)

What is Terahertz?•Terahertz is…

–An emerging technology for Non‐destructive Testing (NDT) combining imaging and chemical analysis

–Non‐contact, non‐destructive, non‐invasive –Safe, not harmful to operators or inspected objects

–Electromagnetic properties: 0.02 – 4 THz, 0.7 – 133 cm‐1,15 – 0.075 mm

•Terahertz can…–See through most materials

–Determine chemical composition

–Measure multiple properties simultaneously

•Moisture, density, weight, thickness…

Advantages• FAST (measurements can be done 100 times per second and soon to be 1000 times per second…eventually 4000 times per second)

•Non‐destructive, non‐contact, non‐invasive• Transparent to most materials

• Safe to user and material (µW energy at the sample, Class I laser)

•Physical and spectral properties can be measured

• Fiber optic flexibility and versatility•Allows on‐line and off‐line measurements that are difficult to achieve with any other technique

Advantages• FAST (measurements can be done 100 times per second and soon to be 1000 times per second…eventually 4000 times per second)

•Non‐destructive, non‐contact, non‐invasive• Transparent to most materials

• Safe to user and material (µW energy at the sample, Class I laser)

•Physical and spectral properties can be measured

• Fiber optic flexibility and versatility•Allows on‐line and off‐line measurements that are difficult to achieve with any other technique

General Areas of Application

•Imaging

•Spectroscopy•Time domain (physical parameters)

•Can be used for non‐destructive testing on‐ and off‐line

splitter

Typical TD‐THz System

GDC splittervariable delay

power meter

splitter

power meter

100fs Laser

Time Domain Terahertz (TD‐THz)

Time Domain Terahertz (TD‐THz)

Reason for Interest

•In Fall 2007, the Domestic Nuclear Detection Office (DNDO) of the Homeland Security (HLS) Department awarded a $1,000,000 grant to develop a functional economic replacement for nuclear gauges (Alpha, Beta, Gamma)

•The goal of the HLS office was to eventually remove all “orphaned” nuclear material from the private sector

Applications and Benefits for Paper

•Coating Thickness•Basis Weight

•Caliper Thickness•Moisture Measurement

•Ink Mileage

•Simultaneous Measurements (simplicity)

•Remote Measurements

•In Process Measurements

Sensor Installed on CLC

Samples

• S1C1W1• S1C1W2• S1C1W3• S1C1W4• S1C1W5• S2C1W1• S2C1W2• S2C1W3• S3C1W1• S3C1W2

S1 = RecycledS2 = Free SheetS3 = LWC

C1 = High SolidsC2 = Low Solids

• S1C2W1• S1C2W2• S2C2W1• S2C2W2• S2C2W3• A2C2W4

Results – B ‐ Scan

Coating Start

Coating Start

Velocity = 2500 ft/min

Velocity = 350 ft/min

Substrate Basis Weight

Caliper Thickness

Sample Caliper Thickness (mm ± 0.002) THz Thickness (mm) Deviation

HCC80 0.226 0.226 < 1 µm

HCC100 0.285 0.285 < 1 µm

K184 0.249 0.252 3 µm

K219 0.188 0.187 1 µm

K247 0.297 0.297 < 1 µm

K247 0.297 0.296 1 µm

Wet Coating Measurement

Cured Coating Measurement

Cured Coating Measurement

Coat Weight Measurement

Detection of Coating

-2 -1 0 1 2 3 4-0.2

-0.1

0.0

0.1

0.2

0.3

Light Weight Coating Up - Deconvolution / Fitting

Dec

onvo

lved

TD

-TH

z Si

gnal

(Arb

)

Waveform Time (ps)

Decon Sum2PkFit Sum3PkFit

-2 -1 0 1 2 3 4

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Light Weight Coating Up - CPT Fits

Tim

e-D

omai

n Te

rahe

rtz S

igna

l (V

olts

)

Waveform Time (ps)

Fit1 Fit2 Fit3

-2 -1 0 1 2 3 4

-0.2

-0.1

0.0

0.1

0.2

0.3Light Weight Coating Dn - Deconvolution / Fitting

Tim

e-D

omai

n Te

rahe

rtz S

igna

l (Vo

lts)

Waveform Time (ps)

Decon Sum2PkFit Sum3PkFit

-2 -1 0 1 2 3 4

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

Light Weight Coating Dn - CPT Fits

Tim

e-D

omai

n Te

rahe

rtz S

igna

l (V

olts

)

Waveform Time (ps)

Fit1 Fit2 Fit3

Basis Weight VariationFormation

Sample D Δt

5 10 15 20 25 30 35 40 45

5

10

15

20

25

30

35

40

45

4.8

4.9

5

5.1

5.2

5.3

5.4

5.5

Basis Weight Measurement

4 6 8 10 12 14 16 180.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

6 Paper/Card Stock Types - 4 Paper Weights, 4 Card Stock Weights

R^2=0.999

Paper Basis Weigth Measurement

Mea

sure

d 4

Pass

TH

z Ti

me

Del

ay (p

s)

Paper Weight (g)

Measured Values Linear Fit

Summary

•THz has potential to make measurements of coating thickness, basis weight, caliper thickness and moisture simultaneously. Ink Mileage may also be possible.

•Freely positionable (single sided measurement, fiber optic connected) sensor easy to operate for laboratory measurements or for use in process.

•THz measurements can provide accurate process control to allow improved quality and consistency.