This document describes a light scattering technique for measuring surface roughness and form. Some key points: - Light scattering from rough surfaces can be used to measure surface texture parameters like roughness (Ra, Rz) and waviness. - The optical sensor measures the angular distribution of scattered light up to 1000 times per second to calculate a statistical parameter (So) that correlates with standard roughness parameters. - The non-contact optical technique can measure moving parts and is tolerant of vibration, misalignment, and harsh environments where contact probes cannot be used. - Applications include 100% inline roughness inspection during grinding and profiling of roundness and other forms with precision comparable to contact techniques.

1. Bro
1/10/09
Roughness and Form Measurement
by means of
Light Scattering
OptoSurf GmbH I Jagdrain 12 I 76316 Malsch I Germany I info@optosurf.com

2. Bro
1/10/09
Light scattering basics
Rough surface
φ
φ
I
Smooth surface
φ
I
scatt. angle φ
Intensity
I
Incoming light
distribution

3. Bro
1/10/09
Measurement of form and waviness
D
D
Waviness Angle D
D D D
y
x
Slope measurement and integration of the angles
D1 D2 D3 D4
 
D
 dx
y i
x

4. Bro
1/10/09
Sensor principle
Angular resolved measurement of
scattered light

5. Bro
1/10/09
System components
Sensor
System controller
Software

6. Bro
1/10/09
Statistical profile analysis
The scattering value So is a statistical
roughness parameter which is closely related
to the DIN EN ISO profile parameter Rdq
Rdq represents the distribution of the profile
angles; δ and S0 describe the distribution of
the scattering angles φ. (φ = 2 · δ)
I.e. So correlates best with Rdq.
If the machining process does not change,
So or Rdq correlates also with the
vertical parameters Rz or Ra
φ
δ
Probability
P
Scatt. angle φ
Probability
P
Profile angle δ
Rdq
So
φ = 2 ∙ δ
So ~ Σ (φ – φ)
2
· P(φ)
R2
dq ~ Σ (δ – δ)
2
· P(φ)
φ
δ

7. Bro
1/10/09
Comparison with stylus parameters
Profilometer
Light
scattering
Stylus
profilometer
Part Rz
(µm)
1 1,2
2 1,5
3 1,8
4 1,4
5 3,5
6 2,9
7 1,0
8 1,7
9 2,5
Part So
1 10,5
2 11.8
3 14,0
4 12,0
5 25,3
6 15,7
7 10,8
8 12,9
9 19,8
Correlation measurements to
calculate the curve fitting
parameters a,b
So
Rz
Rz = a·So+b
1 2 3
20
15
10
Samples from production
Rdq
(%)
1,5
1,9
2,1
1,7
7,3
5,1
0,9
2,5
4,7

8. Bro
1/10/09
Application: 100% in-line measurement
Schleif 5
ALARM
456
1500
So: 24,6
I : 450
M: 0,4
Go part
No go part
Go/No go Trigger
Alarm signal
Teile Nr.
Rz
Teile Nr.
Sigma
Rz
Teile Nr.
Intensität
So: 17,6
I: 456
M: 0,5
I.O. Alarm n.I.O.
So-values
Computer based roughness measurement
1. Sensor trigger detects the part and sends a Start/Stop signal for measurement reading
2. Sensor measures 1000 values/s and transfers So +/- σ value to the PC
3. PC calculates ISO-value (Rdq, Rz, ..) and checks the I/M value (sensor alignment)
4. Exceeding the limit starts the „no go – trigger“ or sends an alarm to realign the sensor

9. Bro
1/10/09
Application: Profile pattern inspection
Wrong dressing can cause different
surface profile patterns although the
vertical roughness value Rz remains
constant (see picture bottom)
The profile angle parameter Rdq
differentiates this change (see picture
middle)
Also the optical light scattering value
So is able to detect this change within
the same order of magnitude (see
picture top)
In a grinding process the pattern of a
surface profile should not change,
because the functional behaviour of the
surface will be influenced (e.g. wear).
With light scattering measurement this
can be detected easily.
So=9 So=17
Rdq=1,2 Rdq=2,2
Rz=0,5 Rz=0,5

10. Bro
1/10/09
Benefits
1. Measurement speed Up to 1000 roughness measurements per s
2. Sensor alignment Due to the specially designed OS 500 optics the So value
is hardly influenced by small misalignments of the sensor
(tolerance: working distance: ~ 1 mm, Tilt: ~ 2°)
3. Workpiece speed The workpiece speed does not affect the So value
4. Working distance The measurement distance (sensor – workpiece surface) is
3mm. (with a special light protection adapter, 50 mm is
possible)
5. Vibration Small vibrations do not disturb the measurement (see 3:)
6. Environment Even the harsh environment of the shop floor is acceptable

11. Bro
1/10/09
Applications
Surface
roughness
Ra << 1µm
Polishing
Grinding
Honing
Rolling

12. Bro
1/10/09
Form measurement
Absolute measurement of roundness by
means of profile angle technique
Intensity
I
Profile angle δ
Roundness profile
y=∫δ(x)∙dx
OptoSurf
OS
500
x
y
δ

13. Bro
1/10/09
Roundness measurement
Hommel Tester FMS 4310
Light scattering OptoSurf OS 500
mechanical probe
optical probe
piston pin
Comparison with contact roundness measurement

14. Bro
1/10/09
Results
(P+V)mech= 0,44µm (P+V)opt= 0,40µm
BoDL1 - Z=06
Profilvergleich FMS (oben) - RM500-03 (unten)
-0,3
-0,2
-0,1
0,0
0,1
0,2
0,3
0 45 90 1
35 1
80 225 270 31
5 360

15. Bro
1/10/09
Correlation P+V – value of 6 pins
Average of 10 measurements of P+V Values (µm)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Mechanical (Hommel FMS 4310)
Optical
(RM
500)

16. Bro
1/10/09
Amplitude values averaged from 10 measurements (µm)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
mechanical (Hommel FMS 4310)
Optical
(RM
500)
Correlation of the Fourier components

17. Bro
1/10/09
Bo06 - Z=06 - RM500-03
Fourierspektrum 2. bis 50. Ordnung
0.074 0.080
0.141
1.000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
Ordnung [-]
Amplitude
[µm]
Example : chatter marks