1. An Improved CRT Resolution Measurement System Using MTF Method
Zhang Hong, Ge Changfeng, Ying Genyu, Wang Jianru
Department of Electronic Engineering,
Tsinghua Univ., Beijing, P.R.China

2. An Improved CRT Resolution Measurement System Using MTF Method
Outline:
Target
Principle
Test System
Improved point
Experiment
Conclusion

3. An Improved CRT Resolution Measurement System Using MTF Method
Target: test the Ultimate Resolution of a tube, such as CRT, CDT or PRT. It is respected by line-width.
Conditions: the electronic gun works at weak current (for example 150 uA), fully pre-worked, optimal focus state.
The line-width means:

4. An Improved CRT Resolution Measurement System Using MTF Method
The line-width WL is the width of electronic beam modulated by shadow mask, and assume it to be Gaussian distribution
Neither the pitch WP
Nor the width of electronic beam WB
And WP < WL < WB

5. An Improved CRT Resolution Measurement System Using MTF Method
Principle:
1. Let the Tested tube (ex, CRT) display a transverse line
2. Image the line on the grating through lens
3. Detect the modulated luminance with PMT behind the grating.
4. Analyze the luminance curve to get the Ultimate Resolution

6. An Improved CRT Resolution Measurement System Using MTF Method
dI0=dI×
6mm/8mm

7. An Improved CRT Resolution Measurement System Using MTF Method
M, a parameter describing the degree of modulation, is defined as:
In the above figure, we can easily conclude that the sum of Imax and Imin is a constant. So M is decided only by dI –- Imax - Imin.
Define M0 = dI0 / dI0 = 100%, then the other modulation degrees are:

8. An Improved CRT Resolution Measurement System Using MTF Method
Considering the grating is rectangular, the line-width is formulated as:
Where fs is the spatial frequency when modulation degree M is 60%.

9. An Improved CRT Resolution Measurement System Using MTF Method
Test System:
The whole test system is based on an imported Micro-spot Analyzer. It is shown as the right figure:

10. An Improved CRT Resolution Measurement System Using MTF Method
The original test system isn’t suitable to the new request, so it is improved to the following one:

11. An Improved CRT Resolution Measurement System Using MTF Method
Improved Point:
1) Before improvement:
lens: 5×, 10×
The image distance is fixed: 138mm
So test line-width: < 0.1mm

12. An Improved CRT Resolution Measurement System Using MTF Method
After improved:
magnification: 0.2× ~ 2.0×
Test line-width: 0.1 ~ 0.8mm
Can b test PRT, CDT and CRTs (14 ~ 34in)

13. An Improved CRT Resolution Measurement System Using MTF Method
2) Before improvement:
Micro-spot Analyzer work with X-Y recorder.
All data work should be do manually.
It would take about half an hour to finish one test.
Paper-based data are conserved inconveniently.

14. An Improved CRT Resolution Measurement System Using MTF Method
After improved:
2 minute once!
Database.
Automatic operation.

15. An Improved CRT Resolution Measurement System Using MTF Method
For only one line is scanned in one field period and the next line in next field, the sampling-hold circuit functions sampling the maximum luminance of a lighting line on tested tube and hold it to the next field period. That ensures the data whenever computer gets are true.

16. An Improved CRT Resolution Measurement System Using MTF Method
Experiment:
There are several important influence factors transforming the test result:
1) The image luminance variety: the image luminance on the grating is not uniform. That violates the base of calculating line-width.

17. An Improved CRT Resolution Measurement System Using MTF Method
1) resolution: normalizing the PMT output signal by the image luminance. The image luminance can be obtained by replacing the grating with a same-size window.

18. An Improved CRT Resolution Measurement System Using MTF Method
2) Optical magnification: the experiments show fs is affected by optical magnification obviously, and when only near some suitable value the magnification change affects little. Such magnification “K0” and real line-width has the following relationship:
It means when spot’s image size on the grating is about 0.17 mm the test result is credible.

19. An Improved CRT Resolution Measurement System Using MTF Method
2) The computer stimulation proves that such limit results from the method in which those modulation degrees would be fitted -- least squares method. It dose not precise enough at the ends of curve.

20. An Improved CRT Resolution Measurement System Using MTF Method
3) Scanning speed: The data-updating period of sampling-hold circuit equals to the CRT field period. So the time interval the computer acquires data from PMT should be less than it, otherwise some data will be lost. Commonly it is suitable to sample twice in a period.
Test results at different frequencies

21. An Improved CRT Resolution Measurement System Using MTF Method
Experiment result:
Note1: Made by Panasonic;
Note2: Toshiba gun.
Difference between measured result and reference values is within ±5%.

22. An Improved CRT Resolution Measurement System Using MTF Method
About the reference data:
1) empirical formula in this field: W/P*1.25*0.75=W/P*0.94
2) measured value: R=(1.10~1.17)*W/P
3)the measured fs using MTF Method can reflect the real ultimate resolution, but the result is valid only when the line is Gaussian distribution, when the current is very weak.

23. An Improved CRT Resolution Measurement System Using MTF Method
14 inches tube，magnification=0.45，work current is 150 uA, repeat 10 times
The repeatability is better than 98%.

24. An Improved CRT Resolution Measurement System Using MTF Method
Conclusion:
1) The consecutive magnification is necessary!
2) The improved system can be used in display devices from PRT to 34-inch CRT

25. An Improved CRT Resolution Measurement System Using MTF Method
3) after the tube fully pre-worked, and neither electrical condition nor optical condition is changed, It has high accuracy and satisfying repeatability in ultimate resolution tests.
The test results can reflect the real utmost resolution of the measured display device well !

26. Thank you!