evaluation of reflectivity of metal parts by a thermo-camera · measurement setup thermocamera:...

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Measurement setup Thermocamera: VarioThermo Head II PtSi-sensor MWIR (3400nm 5000nm) 256x256 resolution 25mm lens Radiation source: Hohlraumstrahler“ – quasi black body low reflection - high emission (ε=0.91) temperature monitored near the cavity by temperature probes 300°C mean temperature Evaluation of measurement data synchronization of data from camera and probes camera image processing dropping of out-of-range data segmentation of source in the images calculation of mean radiant exitance Abstract Assessment of specular reflectivity of metallic surfaces with different roughness in the MWIR (Medium Wave InfraRed) spectrum by a thermo camera, and comparison against a theoretical model. Reflection model from Bennett and Porteus σ s : specular component σ d : diffuse component σ 0 specular reflectance of a perfectly smooth surface (Figure 1) R q RMS-roughess (Root Mean Square) of the surface λ wavelenght of the light R Δq RMS-slope of the surface Δθ Instumental acceptance angle If λ is sufficiently long: (Rayleigh criterion) θ Incident angle Spectral reflectivity of perfectly smooth metal surfaces Evaluation of reflectivity of metal parts by a thermo-camera = 0 + 0 = 0 exp( (4 ) 2 2 )+ 0 2 5 4 2 4 ( ) 2 Zoltán Sárosi, Wolfgang Knapp, Andreas Kunz, Konrad Wegener contact: [email protected] = = 0 exp( (4 ) 2 2 ) < 8 cos Calibration evaluation of correspondence between camera- and probe-data scanning temperature range best fit of 4 th grade polynomial Test objects Sheet metal Machined surfaces Different roughnesses Surface roughness measurements Measurement of surface profile at 10 different positions Evaluation of R q , R a , R Δq , autocovariance, distribution of height and autocovariance Plane ground „Rugotest“ object Surface profile of a sample Reflectivity measurement measurements at small (5°) incident angle mean value from 60 measurements Surface roughness class N1 (R q =0.02μm) N2 (R q =0.07μm) N3 (R q =0.15μm) N4 (R q =0.27μm) N5 (R q =0.55μm) Reflected images of the radiation source on different surfaces Measured reflectivity from (a) plane ground parts, (b) aluminum sheets compared to the theoretical model of Bennett and Porteus b CAD drawing of the source and its thermo image Segmentation of the source and calculation of the mean value U Calibration curve: radiant exitance measured by the camera vs. temperature of the probe Evaluation of the measurement uncertainty (U): difference between the measured values and the calibration curve a

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Page 1: Evaluation of reflectivity of metal parts by a thermo-camera · Measurement setup Thermocamera: VarioThermo Head II •PtSi-sensor •MWIR (3400nm –5000nm) •256x256 resolution

Measurement setup

Thermocamera: VarioThermo Head II

• PtSi-sensor

• MWIR (3400nm – 5000nm)

• 256x256 resolution

• 25mm lens

Radiation source: „Hohlraumstrahler“ – quasi black body

• low reflection - high emission (ε=0.91)

• temperature monitored near the cavity by temperature probes

• 300°C mean temperature

Evaluation of measurement data

• synchronization of data from camera and probes

• camera image processing

• dropping of out-of-range data

• segmentation of source in the images

• calculation of mean radiant exitance

AbstractAssessment of specular reflectivity of metallic surfaces with different

roughness in the MWIR (Medium Wave InfraRed) spectrum by a

thermo camera, and comparison against a theoretical model.

Reflection model from Bennett and Porteus

σs: specular component σd: diffuse component

σ0 specular reflectance of a perfectly smooth surface (Figure 1)

Rq RMS-roughess (Root Mean Square) of the surface

λ wavelenght of the light

RΔq RMS-slope of the surface

Δθ Instumental acceptance angle

If λ is sufficiently long: (Rayleigh criterion)

θ Incident angle

Spectral reflectivity of perfectly smooth metal surfaces

Evaluation of reflectivity of metal parts by a thermo-camera

𝜎 = 𝜎0 + 𝜎0 = 𝜎0exp(−(4𝜋𝑅𝑞 )2

𝜆2 )+ 𝜎025𝜋4

𝑅𝛥𝑞2

𝑅𝑞

𝜆

4

(𝛥𝛩)2

Zoltán Sárosi, Wolfgang Knapp, Andreas Kunz, Konrad Wegener contact: [email protected]

𝜎 = 𝜎𝑠 = 𝜎0exp(−(4𝜋𝑅𝑞)2

𝜆2)

𝑅𝑞 <𝜆

8 cos𝜃

Calibration• evaluation of correspondence between camera- and probe-data

• scanning temperature range

• best fit of 4th grade polynomial

Test objects• Sheet metal

• Machined surfaces

• Different roughnesses

Surface roughness measurements

• Measurement of surface profile at 10 different positions

• Evaluation of Rq, Ra, RΔq, autocovariance, distribution of height

and autocovariance

Plane ground „Rugotest“ object

Surface profile of a sample

Reflectivity measurement

• measurements at small (5°) incident angle

• mean value from 60 measurements

Surface roughness class

N1 (Rq=0.02μm)

N2 (Rq=0.07μm)

N3 (Rq=0.15μm)

N4 (Rq=0.27μm)

N5 (Rq=0.55μm)

Reflected images of the radiation source on different surfaces

Measured reflectivity from (a) plane ground parts, (b) aluminum

sheets compared to the theoretical model of Bennett and Porteus

b

CAD drawing of the source and its thermo image

Segmentation of the source and

calculation of the mean value

U

Calibration curve: radiant exitance

measured by the camera vs.

temperature of the probe

Evaluation of the measurement

uncertainty (U): difference between the

measured values and the calibration

curve

a