Focal Spot of Microfocus Tubes_a

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Size of Micro Focus Spot Ewert et al.June 2008

Focal Spot Size of MicrofocusTubes

U. Ewert, U. Zscherpel, J. Goebbels, Y. Onel, M. Jechow,

K. Bavendiek

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Focal Spot Size of µ-Focus Tubes

EN 12543-5:1999: • Revision in 2008 possible! Harmonization with ASTM recommended

Old:• Test object: sphere or wire cross with diameter between

0,9 bis 1,1mm (± 0,01mm)• Focus range 5 bis 300 µm

New:• New test object: duplex wire of EN 462-5 or ASTM 2002.• Extended standard for µ and nano focus tubes below 300 µm spot• Usage of smaller wire diameters than 1 mm.• Detectors: Film, CR, DDA, intensifier with computer, • Image processing with profile function with line integration,

perpendicular to the wire or sphere direction

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Principle: edge unsharpness measurement (ASTM-Draft)

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Edge Method:Edge Method:

Edge method for :• Micro focus tubes EN 12543-5• “normal” tubes EN 12543-4

Round wire simple adjustment!

Result of modeling:Edge transmission increases the unsharpness in the low intensity range

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EN12543-5: Focus size = 2x (90%-50%) = EF +GHEN12543-5: Focus size = 2x (90%-50%) = EF +GH

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Background

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Gauss edgeGauss lineFWHH10-90 %

If the focal spot (line distribution) has a shape of a Gaussian distribution,

the 10-90 % edge width corresponds to the full width at half height of the profile.

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Gauss Iedge functionGauss IISumm od Gauss I+II

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Two lines, separated by the full width at half height, show a dip of about 7.3 %.

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Lorentz lineLorentz edgeFWHH10-90 %

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The Lorentzian distribution is typical for focal spots (line profile) with long range wings of about 50 times the half width at half height.

The 10-90 % edge width corresponds to 10 – 30 % width of the spot line distribution depending on the zero line determination.

Typical focal spot line distributions are between Gaussian and Lorentzian.

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J. Goebbels, Y. OnelAG-CT

2008-05-09Measurements for focal spot size measurementUnit IOffset – Measurement: average over 100 exposures à 1 secGain – Measurement: average over 100 exposures à 1 sec

Duplex wire exposures: average over 60 exposures à 1 sec 225 kV Micro focus tubePE Detector 2048 x 2048 Pixel à (0,2 mm)2

SDD: 1147,5 mmSOD: ca. 10-15 mm , Distance of sample as near as possible in front of the tube window!Magnification about 80.

55 kV 100 µA, no filter3227.his Complete Duplex wire

3228.his Element 1D 3229.his Element 1D vertikal Position

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Self Calibration to Known Wire Diameter

Software ISee was used inclusive the calibration function.

The calibration measure is the 800 µm width of the wire 1 of E 2002 IQI

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Measured Edge Profile is Asymmetrical

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Spot width:edge profile as taken corresponding the previous slide presentation.

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MeasuredLorentzGauss50%

Fit with Lorentzian and Gaussian unsatisfying

67 µm10 % full width

30 µm50 % full width

30 µm2*90-50% edge

50 µm10-90% edge

The first derivative of the edge function is the “measured” line spread function of the focal spot. This corresponds to a slit camera exposure.

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Imax/2192,5

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MeasuredLorentzGauss50%

Allocation of values between edge profile and line profile not useful.

The asymmetry of the measured profile disturbs the allocation considerably.

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Inaccuracy: Distance, Left vs. Right Wire

The distance between the duplex wires is 10% less than its diameter.

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Second measurement with DDA at different day

2*90-50% 33 µm

Edge method

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2*half width 30 µm

Second measurement with DDA at different day

Measured at first derivative: line spread function

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Measurement with HD-CR (DÜRR) immediately after DDA-Measurement (HD-IP)

2*90-50% 33 µm

Edge profile Edge profile

20 µm pixel size

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2*90-50% 32 µm

Measurement with FujiFilm-CR immediately after DDA-Measurement (IP: UR1)

50µm pixel size

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Asymmetrical spot shape in length direction

(vertical)

2*90-50% = 47 µm

Higher distance due to long range distribution

DDA of PE

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2*half width = 32 µm

Asymmetrical spot shape in length direction(vertical)

Less sensitive to asymmetry

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2*90-50% 18 µm

Edge transmission when measured with thin wiresMeasurement possible

Measurement after new calibration!

Edge transmission

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Wire diameter

2*90-50 %

400 µm

6 µmUnit II100 kV Micro focus tube (Transmission target)CCD Camera + Fiber optic ( 2:1) + GdOS SzintillatorImage integration over 20 sec, no correctionsSDD: 245,0 mmSOD: ca 3 mmM about 66. 85 kV 90 µA, no filters en462.his Element 4D (0,4 mm)

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Conclusion:• Duplex wire EN 462-5 / E 2002 is suitable for the measurement of

the focal spot size of micro focus tubes.• The limitation to 1 mm wire is not necessary. • The standardized method “2*(90%-50%)” provides stable values

also if edge transmission is observed or focal spots are asymmetrical.

• The measurement of asymmetric spots with wide wings is considered.

• The full width at half height of line profiles (calculated as derivative of edge profiles) are almost equal to the edge distribution if calculated by “2*(90%-50%)” and close to Gaussian distributions. This applies in many cases for the central part of the spot distribution.

• Since the method is now applicable with the thin wires of EN 462-5 or E 2002 IQI the lower limitation to 5 µm is not required anymore.

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Focal Spot of Microfocus Tubes_a

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