Analysis of MTF and Its Image Quality Parameters

Based on the principle of line spread function and the block diagram shown in Figure 7.11, the two methods of spatial domain filtering and spectral domain filtering are used to calculate the MTF of the continuous tone modulation image and the relevant parameters for evaluating the image quality. According to the optical Fourier transform, the spectral domain filtering method is used. Calculate the MTF of the dot image and the resolution R, and verify the data of both.

Figure 7.13 shows the MTF comparison of two different electronic platemaking systems. With the changes in the internal structure of the electronic platemaking system and the variation of matching conditions, the image quality has changed significantly. In addition, the level of process is different and the quality of the images obtained is also different.

The detection and calculation of MTF and related image quality parameters were performed on several different electronic platemaking systems and platemaking processes and the properties of the materials used. The results are shown in Table 1.

From frequency analysis, the image processed by the electronic platemaking system is always composed of a low-frequency part that determines the contrast of the image and a high-frequency part that determines the subtle level of the image. Finding the MTF can determine:

First, the relationship between the system image quality and the line diffusion function L(x)

The line spread function L(x) is the signal intensity distribution output by the electronic platemaking system through the input of the slit (function, d tends to 0), and the Fourier transform of the line spread function L(x) is the modulation transfer function MTF.

If the maximum value of L(x), then the equivalent width of L(x) is as shown in Figure 7.12, which is defined as:

The geometric meaning of the equivalent width is: The height is the density of the dot of a rectangle with an area equal to the area enclosed by L(x). Its physical meaning is the degree of concentration of the point energy distribution. The smaller the value, the more concentrated the energy, and the better the quality of the image delivery system by the electronic platemaking system.

Thus, it can be seen that the maximum value of the naturalization L(x), the maximum slope of the L(x) equivalent width EQW, and L(x) is effective as an objective criterion for image quality evaluation. The larger the EQW, the better the image transfer of the electronic platemaking system and vice versa.

Second, the relationship between system image quality and SUM (area of ​​MTF)

The area enclosed by the MTF and the horizontal axis represents the characteristics of the system from low frequencies. If the area SUM of the envelope is larger and the MTF value is larger, then the system has better transitivity. vice versa.

Third, the relationship between system image quality and effective bandwidth Beff

The effective bandwidth Beff is the integral of the square of the modulation transfer function on the spatial frequency:

It reflects the concentration of energy. If the energy is concentrated, the area of ​​the distribution curve is small, and the larger the Beff value, the better the system transitivity.

Fourth, the relationship between system image quality and solution force R

The image resolution reflects the fineness of the image, so the higher the resolution, the finer the image and the higher the image quality.

V. Systematic Image Quality and Characteristic Frequency Relationship

The image quality of the system can be assessed by selecting the MTF values ​​for several characteristic frequencies (f=10, 20, 30, 40, 50, 100...). The higher the MTF value of the eigenfrequency, the better the system's transitivity, and can more accurately reflect the overall resolution of the system with the characteristic frequency of the first minimum point of the MTF.

MTF is directly obtained from the image and can not be subject to any subjective factors, so it is a reasonable, effective and objective method of image quality assessment.