On the Sharpness and Definition of Television Images of Moving Objects

K. M. Shulzhenko

(Presented by the scientific seminar of the Radio Engineering Faculty)

Introduction

The main purpose of television, unlike telephotography, is the transmission of images of moving objects. However, the definition of television images is still evaluated using stationary test charts. In doing so, it is assumed that moving objects are transmitted with the same definition. In reality, however, the definition of images of moving objects when transmitted by tubes with energy storage is always lower than the definition of images of stationary objects.

When moving objects are transmitted, various distortions arise.

The principal ones are:

1) geometric distortions, leading to deformations of the image;

2) distortions caused by optical defocusing, leading to a loss of sharpness (definition) of the image;

3) distortions caused by velocity smearing, also leading to a loss of image sharpness.

The first type of distortion was investigated in the works of O. B. Lurie and F. Schröter as early as the 1930s. These studies concerned transmitting devices without energy storage. Similar distortions also occur when images are transmitted by tubes with energy storage.

Distortions of the second kind are observed in all cases of transmitting moving objects, owing to the inevitability of optical defocusing with existing methods of focusing. The influence of defocusing of the optical section of a television system on image definition was investigated by O. A. Aleksandrov [1].

Distortions of the third kind are characteristic only of transmitting tubes with energy storage. They also occur in photography and cinematography of moving objects. To reduce distortions due to velocity smearing in photography and cinematography, the exposure time is set in accordance with the speed of motion.

of the objects being photographed. In the practice of television transmission this is not yet done. All television transmissions, regardless of the speed of motion of the objects being transmitted, are carried out with a constant exposure equal to the transmission time of one frame, i.e., \(1/25\) sec. As a result, rapidly moving objects are transmitted with a considerable loss of image sharpness.

In the simplest case, a modern television system consists of a camera lens, a transmitting tube, an electrical communication channel, and a receiving tube. In the case of projecting the image onto a large screen, one more link is added—a projection lens; and in film transmission, a film-projection lens. When stationary objects are transmitted, the definition of the image will be determined by the resolving power of these links; whereas when moving objects are transmitted, the definition of the image will also depend on velocity blur and optical defocusing. A study of the influence of the motion of the transmitted object on the sharpness and definition of the television image [2] shows that the component of the object’s velocity that causes velocity blur affects the decrease in definition more strongly than the component of the object’s velocity that causes optical defocusing. However, it is necessary to take into account the circumstance that velocity blur occurs only during the motion of the object, whereas optical defocusing may remain after the object’s motion has ceased, if the operator does not manage to make the corresponding adjustment.

In all cases of studio and outside-broadcast transmission, the optical image of the transmitted object, moving relative to the mosaic, causes blurring of the boundaries (velocity blur), depending on the following principal factors:

1) the speed of motion of the transmitted object relative to the transmitting camera;
2) the focal length of the lens;
3) the optical distance of transmission, i.e., the distance between the camera and the object;
4) the exposure time;
5) the degree to which the storage effect in the transmitting tube is used;
6) the resolving power of all the links of the television system.

In addition, the afterglow of the screen of the receiving tube may have an effect, if it exceeds the frame-transmission time.

In transmitting tubes with electron image transfer, displacement of the optical image relative to the photocathode causes displacement of the electron image over the mosaic (target), i.e., it likewise leads to velocity blur.

It is convenient to judge the sharpness of television images from the curve of brightness variation along a line on the screen of the receiving tube when transmitting an ideally sharp boundary between a dark and a light field, arranged perpendicular to the direction of line scanning. This curve is the transition characteristic of the television system.

In the case of transmitting a stationary object, the transition characteristic of the system can be calculated if the energy distributions over the cross sections of the scanning elements of the transmitting and receiving tubes, the energy distribution over the cross section of the object’s circle of confusion, and the transition characteristic of the electrical channel are known.

When transmitting a moving object, the boundary is additionally blurred as a result of velocity blur and optical defocusing.

To judge how much the motion of the object being transmitted can affect image sharpness, it is sufficient to find the change in the steepness or the rise time of the front of the transient characteristic of a television system as a function of the speed of motion of the transmitted object. However, calculating the transient characteristic is associated with certain difficulties. First, the exact energy distribution over the cross sections of the scanning elements in the transmitting and receiving tubes, and also in the lens, is unknown, especially under defocusing. Second, it is difficult to take into account the nonlinear distortions that occur both in transmitting tubes with energy storage [3] and in the electrical channel, especially when transmitting a single sideband. Finally, the sharpness (definition) of television images can be unambiguously determined only along the lines.

Consideration of this question [2] shows that the experimental results¹) are close to the theoretical data obtained under the following assumptions:

1. The energy distribution over the cross sections of the circle of confusion of the lens and of the scanning elements of the receiving and transmitting tubes can be approximated by one of the known distribution laws: rectangular, sinusoidal, probabilistic, etc.

2. The television system is linear, remains adjusted during transmission, and introduces a certain blurring of the image.

To formulate practical recommendations concerning the choice of the optical transmission distance or exposure time, it is necessary to know the permissible blurring of the boundaries of television images of moving objects. If one proceeds from the assumptions adopted in photography and cinema, then for a television image scanned in 625 lines the permissible magnitude of blurring is \(1 \div 2\) picture elements. But blurring of this order already occurs when transmitting stationary objects. Therefore the permissible blurring in television transmission of moving objects should be established on the basis of the relative loss of sharpness (definition); for example, one may allow a 20–30% drop in sharpness relative to the sharpness of a stationary image.

The question of the blurring of image boundaries of moving objects in photography and motion-picture filming has been considered by many authors. On the basis of the permissible blurring of boundaries, nomograms and tables have been compiled that make it possible to determine the maximum exposure time as a function of the speed of motion of the photographed object, the distance to it, and the focal length of the lens used. In doing so, only the component of the object’s velocity that causes displacement of the optical image relative to the photographic film was taken into account, and defocusing and the blurring caused during development, fixing, printing, etc., were completely disregarded. However, while such neglect is permissible in photography, since the processes of photographic and cinematographic production give a considerably smaller blurring of boundaries than motion smear, in the case of television transmission one must take into account the distortions introduced by all parts of the television system, which as yet limit image definition to a greater extent than the processes of photographic and cinematographic production.

¹) The experimental data were obtained under laboratory conditions, when the electrical channel was represented only by the low-frequency path. The loss of image definition as a function of object motion was determined only along the lines.

Transient characteristic of a television system in the transmission of moving objects

In determining the transient characteristic of a television system, the influence of the motion of the object being transmitted can be taken into account by the dependence of the lens’s circle of confusion on the velocity of the object’s motion and by introducing an additional link—the displacement zone, whose transient characteristic is determined by the distribution of potential on the mosaic of the transmitting tube when an ideal optical image—a maximally sharp boundary of a dark-and-light field—is displaced over it. Thus, the transient characteristics of two links of the system will depend on the velocity of motion of the object, while the transient characteristics of the remaining links of the given system remain unchanged in the transmission of moving objects.

In the case of different configurations of the scanning elements of the individual links of the system, computation of the resultant transient characteristic is very laborious, and the expression for the transient characteristic of the system becomes cumbersome and poorly suited for analysis [2]. If, however, probabilistic distributions of energy over the sections of the scanning elements are adopted for all links of the system, i.e., impulse characteristics of bell-shaped form, then the expression for the resultant transient characteristic is substantially simplified. Moreover, the scanning element of any known configuration having a monotonic transient characteristic can always, with sufficient approximation, be replaced by an “equivalent” scanning element of probabilistic configuration [4]. The equivalence of scanning elements of different configurations can be evaluated by the resolving power determined from their frequency characteristics, and also by the maximum steepness or the rise time of the transient characteristics. As calculations show [2], when different configurations of several links of a system are replaced by probabilistic ones equivalent in resolving power, the discrepancy of the resultant transient characteristics of the system will be insignificant.

If the system contains a link having a transient characteristic with overshoot, it may be taken into account together with other links selected in such a way that the transient characteristic of this group of links would be close to monotonic, i.e., would have an insignificant overshoot [5].

The transient characteristic of the “displacement zone” for a transmitting tube operating in the ideal-storage mode has the form of an inclined segment and is determined by the expression:

\[ \begin{aligned} P_a(x)&=0 &&\text{for } x<0,\\ P_a(x)&=\frac{x}{a} &&\text{for } 0<x<a,\\ P_a(x)&=1 &&\text{for } x>a. \end{aligned} \tag{1} \]

Here \(a\) is the displacement zone, i.e., the distance over the mosaic by which the image of the moving object is displaced during the exposure time \(T_{\mathrm{э}}\).

For objects located along the optical axis of the camera lens,

\[ a=\frac{V_x T_{\mathrm{e}}}{\dfrac{A}{f}}, \]

where \(V_x\) is the component of the object velocity perpendicular to the optical axis of the camera lens,

\(f\) is the focal length of the lens,

\(A\) is the optical range of transmission (the distance from the camera lens to the object being transmitted).

The relative distribution of energy over the cross sections \(x'\) of the equivalent scanning-element smear zone of a probability configuration (or the impulse response of the smear zone for \(x'=-x\)) is determined by the formula:

\[ R_{ea}(x')=e^{-\left(\frac{x' + \dfrac{a}{2}}{r_{ea}}\right)^2}. \tag{2} \]

In this expression, \(r_{ea}=\dfrac{a C_e}{2 C_{\square}}\) is the conventional radius of the equivalent scanning element of probability configuration; here \(C_{\square}\) and \(C_e\) are coefficients determining the relative extent of the scanning elements of rectangular and probability configuration:

\[ C_e=\frac{2r_e}{\delta}\quad \text{and}\quad C_{\square}=\frac{a}{\delta}, \]

where \(\delta\) is the scanning pitch, and \(r_e\) is the radius of the probability-configuration scanning element, reckoned at the energy-distribution level

\[ \left. R_e(x') \right|_{x'=r_e}=\frac{1}{e}. \]

The transient characteristic of the smear zone corresponding to the probability configuration of the equivalent scanning element (2) will be determined by the formula:

\[ P_{ea}(x)= \frac{\displaystyle \int_{-x}^{\infty} R_{ea}(x')\,dx'} {\displaystyle \int_{-\infty}^{\infty} R_{ea}(x')\,dx'} = \frac{1}{2} \left[ 1+\Phi\left( \frac{x-\dfrac{a}{2}}{r_{ea}} \right) \right], \tag{3} \]

where \(\Phi\left(\dfrac{x-\dfrac{a}{2}}{r_{ea}}\right)\) is the probability integral.

The distribution of the mosaic potential in the displacement zone, corresponding to the transition characteristic determined by formula (3), instead of formula (1), takes into account to some extent the redistribution and leakage of charge over the mosaic that occur in real transmitting tubes.

Assuming for the lens also a probability distribution of energy over the sections of the lens circle of confusion

\(R_{eo}(x') = e^{-\left(\frac{x'}{r_{eo}}\right)^2}\),

we obtain the transition characteristic of the lens in the form:

\[ P_{eo}(x)=\frac{1}{2}\left[1+\Phi\left(\frac{x}{r_{eo}}\right)\right], \tag{4} \]

where \(r_{eo}\) is the conventional radius of the lens circle of confusion, measured at the level of the energy distribution

\[ R_{eo}(x')=\frac{1}{e}. \]

\[ x' = r_{eo} \]

When moving objects are transmitted, owing to unavoidable defocusing, the radius of the lens circle of confusion \(r_{eo}\) will depend on the velocity of motion of the object, the parameters of the lens, and the optical distance of transmission.

The transition characteristic of the remaining links of the system, under the assumptions adopted, will not depend on the velocity of motion of the transmitted object and, in the case of monotonic transition characteristics of the individual links, may likewise be represented by a probability integral:

\[ P_{e}(x)=\frac{1}{2}\left[1+\Phi\left(\frac{x}{r_{e}}\right)\right], \tag{5} \]

where

\[ r_{e}=\sqrt{r_{e1}^{2}+r_{e2}^{2}+r_{e3}^{2}+\cdots+r_{en}^{2}}. \]

Here \(r_{e1},\ r_{e2},\ r_{e3},\ldots,r_{en}\) are the relative extents of the scanning elements (or impulse characteristics) of the individual links of the system.

Applying the convolution theorem to formulas (3), (4), and (5), we obtain the resulting transition characteristic of the entire system

\[ P_{ec}(x)=\frac{1}{2}\left[1+\Phi\left(\frac{x-\frac{a}{2}}{r_{ec}}\right)\right], \tag{6} \]

where

\[ r_{ec}=\sqrt{r_{eo}^{2}+r_{ea}^{2}+r_{e1}^{2}+r_{e2}^{2}+\cdots+r_{en}^{2}}. \]

The transition characteristics of the television system, calculated by formula (6) for different values of the velocity \(v_x\) of image displacement over the mosaic, are given in Fig. 1.

In calculating the transition characteristics, the conventional extents of the scanning elements or the impulse characteristics of probabilistic (bell-shaped) form are determined on the basis of equality of the longitud-

longitudinal and transverse resolving power of the system \([4, 2]\) for the number of links \(n = 3\) (links whose transient characteristics do not depend on the velocity of motion of the transmitted object).

Fig. 1. Transient characteristics of a television system at different velocities \(v_x\) of image displacement over the mosaic, measured in lines (picture elements) per second

Dependence of Image Sharpness on the Velocity of Motion of the Transmitted Object

We shall agree to evaluate image sharpness by the maximum steepness of the transient characteristic. From formula (6) we obtain

\[ S_{\max} = \frac{1}{r_{ec}\sqrt{\pi}} . \tag{7} \]

In the case of transmission of stationary objects,

\[ S_{0\max} = \frac{1}{r'_{ec}\sqrt{\pi}}, \tag{8} \]

where

\[ r'_{ec} = \sqrt{r_{e0}^{\prime 2} + r_{e1}^{2} + r_{e2}^{2} + r_{e3}^{2}} . \]

Here \(r'_{e0}\) is the conventional radius of the lens spot under exact focusing.

Dividing (7) by (8), we obtain the dependence of the relative change in image sharpness on the velocity of motion of the transmitted object:

\[ \frac{S_{\max}}{S_{0\max}} = \frac{r'_{ec}}{r_{ec}} = \frac{ \sqrt{r_{e0}^{\prime 2} + r_{e1}^{2} + r_{e2}^{2} + r_{e3}^{2}} }{ \sqrt{r_{e0}^{2} + r_{e1}^{2} + r_{e2}^{2} + r_{e3}^{2} + r_{e}^{2}} }. \tag{9} \]

Here \(r_{eo}\) and \(r_{ea}\) are functions of the velocity of motion of the transmitted object.

The relative decrease in sharpness, calculated by formula (9) for motion of the object perpendicular to the optical axis of the lens, is shown in Fig. 2. Here

\[ \frac{a}{\delta}=\frac{V_x T_e f Z_a}{A h}, \]

where \(h\) is the height of the image on the mosaic, and \(Z_a\) is the active number of scanning lines.

Fig. 2. Relative change in the sharpness and definition of the image as a function of image displacement over the mosaic

Sometimes it is more convenient to estimate the sharpness of an image by a quantity reciprocal to the duration of the rise of the front of the system’s transient characteristic.

From (6) one can determine the duration of the rise of the front of the transient characteristic (i.e., the blur zone), measured at levels \(0.1 \div 0.9\) of the established value:

\[ x_{z.r.}=2 r_{ec}\arg \Phi(0.8)=1.81\, r_{ec}. \tag{10} \]

Here \(\arg \Phi(0.8)\) is the argument of the probability-integral function \(\Phi(x)\) when the value of the function is equal to 0.8.

We determine the blur zone in the transmission of stationary objects from (10), substituting the values \(r_{ea}=0\) and \(r_{eo}=r'_{eo}\) into the expression for \(r_{ec}\):

\[ x_{0z.r.}=1.81\, r'_{ec}. \tag{11} \]

Dividing (10) by (11), we see that the quantity reciprocal to the relative increase in the blur zone is equal to the relative decrease in image sharpness:

\[ -\frac{x_{\text{ob.r.}}}{x_{\text{s.r.}}}=\frac{S_{\max}}{S_{0\max}}=\frac{r'_{e_c}}{r_{e_c}}, \tag{12} \]

Consequently, estimates of the relative change in image sharpness by the maximum steepness and by the rise time of the front of the transition characteristic coincide.

Definition of a television image of a moving object

The definition of television images is usually evaluated by the sum of the dark and light lines \(m'_h\) reproduced on the receiver screen over a segment equal to the height of the image [4]:

\[ m'_h=\xi_{\text{nom}} Z_a=\xi_{\text{nom}} Z(1-t_y), \]

where \(Z\) is the number of lines,
\(t_y\) is the loss in frame retrace,

\[ \xi_{\text{nom}}=\frac{m'_h}{Z_a} \]

is the relative frequency, determined from the point of intersection of the system frequency characteristic with a visually sufficient amplitude level.

We obtain the complex frequency characteristic of the television system by calculating the spectrum of the impulse (time) characteristic [6].

From the expression for the transition characteristic (6) we find the impulse characteristic of the system:

\[ R_{e_c}(x)=\frac{d}{dx}P_{e_c}(x)= \frac{1}{r_{e_c}\sqrt{\pi}}\, e^{-\left(\frac{x-\frac{a}{2}}{r_{e_c}}\right)^2}. \tag{13} \]

The complex frequency characteristic of the system corresponding to (13) is

\[ F(\omega)=\int_{-\infty}^{\infty} R_{e_c}(x)e^{-j\omega x}\,dx = e^{-\left(\frac{r_{e_c}}{2}\omega\right)^2} e^{-j\omega \frac{a}{2}}. \tag{14} \]

The modulus of (14) determines the frequency characteristic of the system

\[ \vartheta(\omega)= e^{-\left(\frac{r_{e_c}}{2}\omega\right)^2}. \tag{15} \]

In this case the phase characteristic of the system

\[ \varphi(\omega)=\frac{a}{2}\omega \]

will be linear.

Introducing into (15) the relative frequency \(\xi=\dfrac{\delta}{\lambda}\), we obtain

\[ \vartheta(\xi)=e^{-\left(\frac{\pi r_{e_c}\xi}{\delta}\right)^2}. \tag{16} \]

Taking the visually sufficient amplitude level \(\Delta'_0=0.122\) [4], from (16), for \(\xi=\xi_{\mathrm{nom}}\),

\[ \vartheta(\xi_{\mathrm{nom}})=e^{-\left(\frac{\pi r_{e_c}\xi_{\mathrm{nom}}}{\delta}\right)^2}=\Delta'_0 \]

we find the relative frequency

\[ \xi_{\mathrm{nom}}=\frac{\delta\sqrt{-\ln \Delta'_0}}{\pi r_{e_c}}=\frac{m'_h}{Z_a}, \]

which determines the definition of the television system,

\[ m=m'_h=\frac{\delta\sqrt{-\ln \Delta'_0}}{\pi r_{e_c}}\,Z(1-t_y). \tag{17} \]

When transmitting images of stationary objects, the definition is determined from (17) with \(r_{e_c}=r'_{e_c}\):

\[ m_0=\frac{\delta\sqrt{-\ln \Delta'_0}}{\pi r'_{e_c}}\,Z(1-t_y). \tag{18} \]

From (17) and (18) we obtain the relative change in the definition of the image as a function of the speed of motion of the transmitted object,

\[ \frac{m}{m_0}=\frac{r'_{e_c}}{r_{e_c}}. \tag{19} \]

Comparison of (12) and (19) shows that the relative decreases in the definition and sharpness of images of moving objects coincide (Fig. 2):

\[ \frac{S_{\max}}{S_{0\max}}=\frac{m}{m_0}=\frac{x_{\mathrm{oz.r}}}{x_{\mathrm{z.r.}}}=\frac{r'_{e_c}}{r_{e_c}} =\frac{\sqrt{r_{e_0}^{\prime 2}+r_{e_1}^2+r_{e_2}^2+r_{e_3}^2}} {\sqrt{r_{e_0}^2+r_{e_a}^2+r_{e_1}^2+r_{e_2}^2+r_{e_3}^{\prime 2}}}. \tag{20} \]

The relationship between definition and sharpness of the image in the present case can be obtained from (7), (10), and (17):

\[ S_{\max}=\frac{\sqrt{\pi}\,m}{\delta Z_a\sqrt{-\ln \Delta'_0}} =\frac{1.81}{x_{\mathrm{z.r.}}\sqrt{\pi}} \simeq \frac{1}{x_{\mathrm{z.r.}}}. \]

Figure 3 shows the relative changes in sharpness of the image of a moving object, calculated by formula (20), for different values of the product of the exposure time \(T_{\mathrm{e}}\) and the active number of scanning lines \(Z_a\). The rate of displacement of the image over the mosaic is referred to the height of the image on the mosaic. On the right-hand side of Fig. 3

Fig. 3. Relative decrease in the sharpness of the image of a moving object for different values of the product \(T_{\mathrm{e}} Z_a\)

there is given a table, in the left column of which, for each curve, are given the values of the active number of scanning lines corresponding to the constant exposure time \(T_{\mathrm{e}} = 1/25\) sec, and in the right column—the values of the exposure time for a constant number of scanning lines \(Z_a = 600\). From the curves of Fig. 3 we see that the relative decrease in image sharpness proves to be the greater, the greater the exposure time and the number of scanning lines. The curves of Fig. 3 are calculated for systems ensuring equality of the longitudinal and transverse sharpness of images of stationary objects.

Experimental verification of the dependence of the resolving power and sharpness of television images on the velocity of motion of the transmitted object confirms the correctness of the initial assumptions and the principal conclusions of the theoretical analysis.

The experiments were carried out by the author on a laboratory setup using an iconoscope transmitting tube with image transfer.

In order to observe on an oscilloscope screen the transient characteristic of the television system at different velocities of motion of the object, a displacement of the boundary of a dark-light field was produced, repeating at the frame frequency. By selecting on the oscilloscope screen the signal of a line containing the image of the moving boundary, a curve was obtained representing the transient characteristic of the system in transmitting a moving object.

Measurements of the dependence of the sharpness of the television image on the velocity of motion of the transmitted object were made using moving test charts.

Experiments were also carried out with the use of an obturator and pulsed illumination. The pulsed illumination was produced by lamps.

daylight, when they were supplied from a half-wave rectifier. The pulse duration in this case was 0.4 of the period of the supply-line frequency. The exposure time corresponding to this pulse duration was \(T_{\mathrm{e}}=\dfrac{1}{125}\) sec.

Theoretical and experimental investigations of the dependence of the sharpness and definition of a television image on the speed of motion of the objects being transmitted indicate the necessity of combating losses of sharpness (definition) in moving images.

Optical defocusing can be reduced to an admissible minimum by an appropriate choice of the relative aperture of the objective. Then the principal cause producing a decrease in image sharpness and definition as a function of the speed of motion of the object being transmitted remains the displacement of the image over the mosaic. The nomogram calculated for this case from formula (20) is given in Fig. 4. Starting from the permissible loss of sharpness (definition)

\[ \frac{S_{\max}}{S_{0\max}}=\frac{m}{m_0}, \]

the focal length of the camera objective \(f\), the height of the mosaic (photocathode) \(h\), and the velocity of motion of the object \(V_x\), which causes displacement of the image along the lines, one can determine the optical transmission distance \(A\) for a given exposure time, or the exposure time for a given optical transmission distance. For example, if a 20% loss of sharpness is allowed

\[ \left(\frac{S_{\max}}{S_{0\max}}=\frac{m}{m_0}=0.8\right), \]

then, for

\[ T_{\mathrm{e}}=\frac{1}{25}\ \text{sec}, \quad V_x=5.5\ \text{m/sec}, \quad f=20\ \text{mm}, \quad \text{and} \quad h_{\mathrm{f}}=18\ \text{mm}, \]

television transmission can be carried out from a distance \(A \geq 100\ \text{m}\). In order to carry out transmission from shorter distances with the specified permissible loss of sharpness (definition), it is necessary to reduce the exposure time. Thus, from Fig. 4 it is seen that, when the exposure time is reduced to

\[ T_{\mathrm{e}}=\frac{1}{100}, \]

the same loss of definition is obtained at an optical distance \(A=25\ \text{m}\).

A more convenient nomogram for determining the exposure time or the transmission distance can be constructed for a particular television system.

Conclusion

A direct reduction of the exposure time leads to incomplete utilization of the charge-storage effect in the pickup tube. Consequently, increasing the definition of moving images by reducing the exposure time is not consistent with the desire for maximum utilization of the charge-storage effect. The resolution of these contradictory tendencies may be the use of pulsed illumination.

In those cases where the illumination of the objects being transmitted exceeds that required for normal operation of the pickup tube, it is advisable to reduce the exposure time in order to increase the definition

images of moving objects. Adjustment of the exposure time can be accomplished, for example, by using a rotating shutter similar to that used in motion-picture apparatus. In television, it is apparently also possible to adjust the exposure time electrically. For example, when using transmitting tubes with electronic image transfer, the exposure time can be adjusted by applying voltage pulses of specified duration to the photocathode of the tube. This possibility, however, must be verified experimentally.

Fig. 4. Nomogram for determining the exposure time \(T_{\mathrm{e}}\) or the optical transmission distance \(A\), for a specified relative decrease in image sharpness (definition)

\[ \frac{S_{\max c}}{S_{0\max c}}=\frac{m}{m_0} \]

and given transmission conditions. \(f\)—focal length of the objective; \(h\)—height of the photocathode or mosaic; \(V_x\)—velocity of motion of the object causing displacement of the image over the mosaic.

When transmitting by means of tubes with electronic image transfer, in certain special cases of television use, when the direction of motion of the transmitted object is definite, and the speed of motion is known in advance and changes slowly, the blurring of the image can be reduced by using compensation scanning of the electronic image. The charge-accumulation effect will then be used fully, while a certain compensation or stopping of the displacement of the image over the target during frame transmission will, from the standpoint of increasing image sharpness, produce the same effect as the use of a shutter.

References

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2. Shulzhenko, K. M. On the influence of the motion of transmitted objects on the sharpness of a television image. Candidate’s dissertation, LETI, 1953.

3. Ryftin, Ya. A. Transient processes in transmitting television tubes with energy storage. ZhTF, vol. XXIII, no. 9, 1953.

4. Ryftin, Ya. A. On the evaluation of the resolving power of transmitting television tubes. In: Collection dedicated to the 70th anniversary of Academician A. F. Ioffe, 1950.

5. Khalfin, A. M. Fundamentals of Television Engineering. “Soviet Radio” Publishing House, Moscow, 1955.

6. Kharkevich, A. A. Spectra and Analysis. GITTL, 1952.