Performance of video camera system (zoom lens, image tube, and pre-amp) is investigated by MTF measurement. 3 pair bar pattern which has 7 spatial frequences is used in the measurement of MTF for the total system ($\mid{\Psi_{total}}\mid$). The test pattern gives square wave input to the video camera system under test. As a sinusoidal wave input only gives correct MTF data, the measured MTF data obtained from the square wave input pattern is corrected.
The MTF of zoom lens ($\mid{\Psi_{optical} }\mid$) is measured by slit image scanning method, and finally the MTF of electrical system, image tube and pre-amp ($\mid{\Psi_{electrical}}\mid$) is calculated using the following equation;
$\mid{\Psi_{electrical}}\mid = \mid{\Psi_{total}}\mid / \mid{\Psi_{optical}}\mid$
The cutoff frequency of the total system is 12 lp/mm, and in the region of spatial frequency from 0 to 7 lp/mm, the system shows good performance, yet behind that frequency the performance falls off abruptly. The abrupt falling-off of the performance is found mainly due to the low level frequency characteristics of the electrical part of the system, which has good spatial frequency response only up to 7 lp/mm.
The MTF of the zoom lens, measured by knife-scanning of the line spread function, falls slowly to zero level at the spatial frequency of 24 lp/mm.
In conclusion, the electrical part of the video camera system over-rule the entire performance of the video camera system, and adequate MTF balancing between optical part and electrical part is important in the system design.
