The probability of detecting an object by several observation points. Object detection using the otsu method

  • Specialty of the Higher Attestation Commission of the Russian Federation05.09.07
  • Number of pages 240

1. MODERN METHODS FOR CALCULATING PROBABILITY OF DETECTION

OBJECTS BY OBSERVER.

1.1. Functions of visual perception.

1. 2. Physiological models.

1.3. Information models.

1.4. Statistical models.

1.4.1. The simplest statistical models.

1.4.2. Models based on statistical decision theory.

2. DEVELOPMENT OF A STATISTICAL MODEL OF THE VISUAL ORGAN FOR

SOLUTIONS TO THE PROBLEM OF OBJECT DETECTION ON RANDOM BACKGROUNDS.

2.1. Determination of the threshold likelihood ratio in the model of the organ of vision.

2. 2. Determination of the probability of detecting objects against random backgrounds.

2.2.1. Calculation of the logarithm of the likelihood ratio in the field of spatial implementations.

2.2.2. Calculation of the logarithm of the likelihood ratio in the spatial frequency domain.

2.3. Taking into account the characteristics of optical elements of an optical-electronic image visualization system.

2.4. Analysis of the obtained expressions to calculate the probability of detection against random backgrounds.

2.5. Determining the probability of recognizing pairs of objects against random backgrounds.

3. DETERMINATION OF THE BASIC PARAMETERS OF THE VISION ORGAN MODEL

IN THE OPERATING BRIGHTNESS RANGE OF ADAPTATION.

3.1. Taking into account the real properties of the optics of the eye in the model of the organ of vision.

3.2. Features of the functioning of the organ of vision at high levels of background brightness.

3.3. Functional dependence of the sensitivity of the receptors of the visual organ on the brightness of adaptation and the position of the stimulus on the retina.

3.4. Determination of the main parameters of the model of the organ of vision using the APPRC0N program.

4. MATHEMATICAL MODEL OF THE VISUAL ORGAN IN THE FORM

APPLICATION PROGRAM PACKAGE.

4.1. Description of the DETECTOR program for calculating the probability of detecting objects against random backgrounds.

4.2. Modification of the DETECTOR program for the case of identifying pairs of objects on random backgrounds.

5. ANALYSIS OF CALCULATION ERRORS USING THE DETECTOR PROGRAM.

5.1. Error in cutting a blurred image of an object against a uniform background.

5.2. Errors in cutting correlation functions.

5.3. Sampling error of a blurred image of an object against a uniform background.

5.4. Discretization errors of correlation functions.

5.5. Taking into account the joint influence of errors

On the accuracy of calculations using the DETECTOR program.

6. EXPERIMENTAL RESEARCH METHOD AND INSTALLATION SCHEME FOR DETERMINING THE PROBABILITY OF DETECTION OF OBJECTS BY AN OBSERVER.

6.1. Development of a functional diagram of the installation.

6. 2. Development of software for the experimental setup.

6.3. Description of the adaptometer block.

6.4. Description of the method for calibrating the installation and conducting a visual experiment.

6.5. Methodology for assessing the error of experimental results.

6.6. Comparison of calculation results using the proposed model with experimental data.

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Introduction of the dissertation (part of the abstract) on the topic “Method for calculating the probability of an observer detecting objects against random backgrounds”

Currently, the main method for determining the output characteristics of optical-electronic image visualization systems (OEIS) is the method of direct experimental research (method of expert assessments). To obtain statistically reliable results, this method requires the involvement of a large number of trained observers. The consequence of this is significant time and economic costs for the creation of working samples of OESVI. When using the method of expert assessments to optimize system parameters, it becomes necessary to create several systems with different parameters or one system in which the parameter under study varies, which makes obtaining experimental results even more expensive.

Elimination of these shortcomings is possible by creating a mathematical model of the OESVI, which includes a mathematical model of the observer’s organ of vision (03). In this case, the volume of necessary experiments is significantly reduced, since they are needed only to find the unknown functions and coefficients included in the model, as well as to determine the limits of its applicability. Mathematical models make it possible not only to significantly reduce the time for researching existing types of OESVI, but also to analyze promising OES without creating prototypes, which significantly reduces time and economic costs.

A feature of most visual work, carried out both with the help of OESVI and with the naked eye, is the presence of a random, uneven distribution of brightness in the field of view. Thus, observation in OESVI can occur under conditions of additive superposition of noise from the electronic path ("additive noise") on a non-random object and background. During field observation, an object (in particular, one with a random coloring), as a rule, applicatively replaces a section of a random background such as a landscape, water surface, clouds, etc., as a result of which the image areas inside and outside the object’s contour cease to be correlated (“ applicative background"). In addition, cases of additive noise and applicative background are observed together.

As an analysis of the literature data has shown, a mathematical description of the process of detection by an observer of objects against real backgrounds that have both additive and applicative components is currently missing, which makes it impossible to develop a mathematical model of OESVI for this case. Therefore, the proposed topic of the dissertation can be considered relevant.

The dissertation consists of a table of contents, introduction, six chapters, conclusions on the work and a list of references.

The first chapter contains a literary analysis of more than 100 different models of human threshold vision, indicating their advantages and disadvantages. A classification of such models is proposed, according to which four approaches to their creation can be distinguished: empirical, physiological, informational and statistical. Based on the results obtained, we can conclude that the statistical approach is most promising for solving the formulated problem.

In the second chapter, a structural diagram of the statistical model 03 is proposed based on the likelihood ratio function, and calculated relations are obtained that allow one to find the probability of detecting objects and recognizing pairs of objects by an observer on random backgrounds, taking into account the joint influence of the applicative background, additive noise and masking coloring of the object. The obtained relationships also make it possible to take into account the influence of the optical elements of the OESVI and 03 observer on the calculation results using the model.

At the beginning of the third chapter, an analytical description of the scattering function of the eye optics point is obtained, based on the results of experiments of a number of authors and allowing to take into account the real properties of the eye optics in model 03. Next, an analysis of the operation of the visual analyzer at high brightness levels of adaptation is carried out and the process of obtaining the functional dependence of the distribution of retinal sensitivity is considered on the brightness of the background and the distance from the center of the fovea. The software created for this purpose is described. This dependence is an important component of the model, which allows us to move from the distribution of brightness in the space of objects to the distribution of receptor reactions across the retina 03.

The fourth chapter includes a description of a package of application programs that implement the developed model 03 and allow one to calculate the probability of detecting objects and identifying pairs of objects on random backgrounds.

The fifth chapter outlines the features of representing continuous functions in a computer and substantiates the need to take into account the errors that arise during calculations using the DETECTOR program, which implements the created model 03. A method for estimating these errors, caused by both coarse discretization and cutting at the edge of arrays of functions included in into the calculated expressions of the model. This technique allows you to select a sampling interval that minimizes the total error of calculations using the DETECTOR program.

The sixth chapter contains a description of the experimental research method and setup scheme for determining the probability of detecting objects and recognizing the orientation of objects by an observer against random backgrounds. The specified method and scheme are intended to test the performance of the proposed model 03 under various observation conditions 8. A description of the software of the experimental setup and the methodology for its calibration is given. Methods for conducting visual experiments and assessing the error of the results are also described. A comparison of the calculation results using the model with experimental data showed their agreement with a confidence level of 0.9 in a wide range of observation conditions.

The results of the dissertation work were published in six printed works, tested at the International Scientific and Technical Conference "0lighting"9b" (Varna, 1996), the Moscow Student Scientific and Technical Conference "Radio Electronics and Electrical Engineering in the National Economy" (MPEI, 1997), the III International Lighting Engineering conference (Novgorod, 1997) and scientific seminars of the Lighting Engineering Department of MPEI.

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Conclusion of the dissertation on the topic “Lighting Engineering”, Arkhipov, Boris Borisovich

CONCLUSIONS ON THE WORK

1. The most significant results in the development of 03 models for solving the problem of detecting objects on random backgrounds were obtained on the basis of TCP, however, in the literature there is no description of 03 models for solving the problem of detecting objects on real backgrounds that have both additive and applicative components.

2. A statistical model 03 has been developed, which allows one to calculate the probability of detecting an object taking into account the characteristics of the object, additive noise and applicative background. The performance of model 03 on random backgrounds, i.e., in conditions fundamentally different from the conditions of its normalization, indicates that the algorithm embedded in the model is quite close to that implemented by the organ of vision.

3. It is shown that to calculate the probability of detecting objects against real backgrounds, it is enough to determine only two characteristics of model 03: the threshold likelihood ratio and the functional dependence of retinal sensitivity on adaptation brightness and distance from the center of the fovea. A comparison of the calculation and experimental results indicates the constancy of the threshold likelihood ratio with variations in the average background brightness, the brightness distribution over the object, and the statistical characteristics of the observed images.

4. Multivariate calculations using model 03, implemented in the form of an application software package, showed a significant influence of the sampling interval of the functions included in the calculation expressions of the model on the calculation results. The developed method for estimating the calculation error makes it possible to select a sampling interval that gives the minimum value of this error, which for typical observation conditions does not exceed 20%.

5. It is shown that when observing on a fine-textured background, a rational choice of object coloring can always achieve its effective camouflage, whereas when observing on a strongly correlated background, it is impossible to camouflage an object by randomly coloring it. An increase in the dispersion of the applicative background leads first to a decrease in the probability of detecting objects, and then to its increase. Increasing the dispersion of additive noise can only worsen the conditions for detecting objects.

6. The developed experimental setup, combining electronic and optical methods of image formation, made it possible to obtain the dependence of the probability of detecting objects on observation conditions with an error of no more than 17%. It is shown that the calculation results using the developed model 03 coincide with the experimental results with a confidence probability of 0.9.

7. The results obtained show that the application of the developed model 03 to calculating the probability of detecting objects is legitimate when the background brightness is 10~2.102 cd/m2 and the angular sizes of objects are O.Yu0. Good agreement between the calculated and experimental dependences is observed when the statistical characteristics of the images vary in the following range:

Additive noise correlation interval 0.30";

Correlation interval of applicative background 0.80";

Object masking correlation interval 0.30";

Relative standard deviation of additive noise o.0.1;

Relative standard deviation of the applicative background o.0.14;

Relative standard deviation of object camouflage is about 0.05.

8. A computational and experimental study of the dependences of the probability of recognizing pairs of objects on observation conditions confirms the hypothesis that detecting an object and recognizing a pair of objects are equivalent tasks for the observer.

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Please note that the scientific texts presented above are posted for informational purposes only and were obtained through original dissertation text recognition (OCR). Therefore, they may contain errors associated with imperfect recognition algorithms. There are no such errors in the PDF files of dissertations and abstracts that we deliver.

Hello, dear habra readers and habra critics. I would like to devote this post to such a topical topic today as object detection in images.
As one of the algorithms for such detection, consider choosing a threshold that is fast and efficient Otsu's method.

Introduction

So, let's start in order. In general, the task of object detection is to determine the presence of an object in an image that has certain specific characteristics.

Such a characteristic could be, for example, brightness. One of the simplest and most natural ways to detect an object (or objects) is to select a brightness threshold, or threshold classification (thresholding). The purpose of such a threshold is to divide the image into a light object (foreground) and a dark background (background). Those. an object is a collection of those pixels whose brightness exceeds a threshold ( I > T), and the background is the collection of remaining pixels whose brightness is below the threshold ( I < T).

So the key parameter is the threshold T. How to choose it?

There are dozens of methods for choosing a threshold. A quick and effective method is the method invented by the Japanese scientist Nobuyuki Otsu in 1979. This is what we will talk about further.

Otsu method

Let there be an 8-bit image for which you need to calculate the threshold T. In the case of a 24-bit image, it can be easily converted to 8-bit using grayscale:
I = 0.2125 R + 0.7154 G + 0.0721 B

Otsu's Method uses an image histogram to calculate the threshold. Let me remind you that a histogram is a set of bins, each of which characterizes the number of sample elements that fall into it. In our case, the sample is pixels of different brightnesses, which can take integer values from 0 to 255.

Example image with an object:

Histogram for this image:

From the histogram, a person can easily see that there are two clearly separated classes. The essence of Otsu’s method is to set the threshold between classes in such a way that each of them is as “dense” as possible. In mathematical terms, this comes down to minimizing intraclass variance, which is defined as the weighted sum of the variances of two classes:

Here w 1 and w 2 - probabilities of the first and second classes, respectively.

In his work, Otsu shows that minimizing intraclass variance is equivalent to maximizing between class variance, which is equal to:

In this formula a 1 and a 2 - arithmetic averages for each class.

The peculiarity of this formula is that w 1 (t + 1), w 2 (t + 1), a 1 (t + 1), a 2 (t+ 1) are easily expressed through previous values w 1 (t), w 2 (t), a 1 (t), a 2 (t) (t- current threshold). This feature allowed us to develop a fast algorithm:

  1. We calculate the histogram (one pass through the pixel array). Next, you only need a histogram; passes over the entire image are no longer required.
  2. Starting from the threshold t= 1, we go through the entire histogram, recalculating the variance at each step σ b (t). If at any of the steps the variance is greater than the maximum, then we update the variance and T = t.
  3. The required threshold is T.
Naturally, this is only a general description of the algorithm. There are a lot of optimizations that can be made in an exact implementation. For example, a passage through the histogram can (and should) be done not from 1 to 254, but from minimum to maximum brightness minus one. At the end, an implementation in C++ will be given, taking into account some of these optimizations.

This is the result obtained by implementing the above algorithm:

Calculated threshold:

Real example

In addition to an artificially generated example, I would also like to show a real
using the method.

My current thesis requires localizing a barcode on an image:

Before using the Otsu method, you need to do preprocessing in order to somehow take into account the structural features of a one-dimensional barcode. If you don’t do this, then the method simply won’t do anything. The peculiarity of the barcode structure is that it consists of vertical stripes, and therefore has large horizontal derivatives and small vertical ones. Therefore, if we take the image as the difference between the horizontal and vertical derivatives, and then apply an averaging filter, we get this:

Not bad, right? The barcode image is clearly visible in the image and stands out with significantly higher brightness compared to surrounding objects. Now you can safely use the Otsu method:

As a result, we received a correctly localized barcode.

Implementation in C++

Well, as I promised, implementation of threshold calculation using Otsu’s method in C++ with comments:
  1. typedef unsigned char imageInt;
  2. // Determining the threshold using Otsu's method
  3. int otsuThreshold(imageInt *image, int size)
  4. // Checks for NULL and so on. let's lower it to concentrate
  5. // at work of the method
  6. // Calculate the minimum and maximum brightness of all pixels
  7. int min = image;
  8. int max = image;
  9. for (int i = 1; i< size; i++)
  10. int value = image[i];
  11. if(value< min)
  12. min = value ;
  13. if (value > max)
  14. max = value ;
  15. // The histogram will be limited below and above by the min and max values,
  16. // so there is no point in creating a histogram of 256 bins
  17. int histSize = max - min + 1;
  18. int * hist = new int ;
  19. // Fill the histogram with zeros
  20. for (int t = 0; t< histSize; t++)
  21. hist[t] = 0;
  22. // And calculate the height of the bins
  23. for (int i = 0; i< size; i++)
  24. hist - min]++;
  25. // Let's enter two auxiliary numbers:
  26. int m = 0; // m - the sum of the heights of all bins, multiplied by the position of their middle
  27. int n = 0; // n - the sum of the heights of all bins
  28. for (int t = 0; t<= max - min; t++)
  29. m += t * hist[t];
  30. n += hist[t];
  31. float maxSigma = -1; // Maximum value of interclass variance
  32. int threshold = 0; // Threshold corresponding to maxSigma
  33. int alpha1 = 0; // Sum of heights of all bins for class 1
  34. int beta1 = 0; // The sum of the heights of all bins for class 1, multiplied by the position of their middle
  35. // The alpha2 variable is not needed, because it is equal to m - alpha1
  36. // The beta2 variable is not needed, because it is equal to n - alpha1
  37. // t runs through all possible threshold values
  38. for (int t = 0; t< max - min; t++)
  39. alpha1 += t * hist[t];
  40. beta1 += hist[t];
  41. // Calculate the probability of class 1.
  42. float w1 = (float )beta1 / n;
  43. // It's not hard to guess that w2 is also not needed, because it is equal to 1 - w1
  44. // a = a1 - a2, where a1, a2 are arithmetic averages for classes 1 and 2
  45. float a = (float )alpha1 / beta1 - (float )(m - alpha1) / (n - beta1);
  46. // Finally, we calculate sigma
  47. float sigma = w1 * (1 - w1) * a * a;
  48. // If sigma is greater than the current maximum, then update maxSigma and the threshold
  49. if (sigma > maxSigma)
  50. maxSigma = sigma;
  51. threshold = t;
  52. // Let's not forget that the threshold was counted from min, not from zero
  53. threshold += min;
  54. // That's it, the threshold has been calculated, return it to the top :)
  55. return threshold;
* This source code was highlighted with Source Code Highlighter.

Conclusion

So, we looked at the use of Otsu's method for detecting objects in images. The advantages of this method are:
  1. Ease of implementation.
  2. The method adapts well to various types of images, choosing the most optimal threshold.
  3. Fast turnaround time. Required O(N) operations where N- the number of pixels in the image.
  4. The method has no parameters, just take it and apply it. In MatLab, this is the graythresh() function without arguments (Why did I give an example from MatLab? It’s just that this tool is the de facto standard for image processing).
Flaws:
  1. Threshold binarization itself is sensitive to uneven brightness of the image. A solution to this problem could be the introduction of local thresholds, instead of one global one.

Sources

  1. Otsu, N., "A Threshold Selection Method from Gray-Level Histograms," IEEE Transactions on Systems, Man, and Cybernetics, Vol. 9, No. 1, 1979, pp. 62-66.

Detection of an object using reconnaissance equipment is the process of making a decision about the presence or absence of an object in a given area of ​​space as a result of receiving and processing signals.

Reception The transmission of signals always occurs against the background of interference of one kind or another (the receiver’s own noise, radio noise from outer space, reflection from hydrometeors, the earth’s surface, etc.).

The presence of interference leads to distortion of transmitted signals and to errors in assessing the situation. When detected, four situations are possible.

Firstly, if there really is an object and the signals arrive in the presence of interference, then, based on reconnaissance data, the observer can make two decisions: first - there is an object and second - there is no object. In the first case (i.e., deciding what an object is when there is an object in reality) is called correct detection object. In the second case (making a decision about the absence of an object while the object is there) - skipping an object. The possibility of an indefinite answer - “it is not known whether the object exists or not” - is excluded.

Secondly, if there is no object, then in the presence of interference two solutions are also possible: there is an object and there is no object. In this case, making a decision about the presence of an object (when in reality it is not) is called false anxiety, and the decision about the absence of an object - correct non-detection.

Missing a target and false alarm are errors in object detection. Since, in the general case, signals and noise are random functions of time, the adoption of one or another decision is random in nature. Therefore, the possibility of the occurrence of the listed situations is usually characterized by the corresponding probabilities: the probability of correct detection of W by omission Wpr, false alarm Wlt and correct non-detection of W pn.

Correct detection and omission of an object (if the object is actually present) form a complete group of incompatible events, therefore

Similarly, a false alarm and a correct nondetection form a complete group of incompatible events in the absence of an object

W LT +W PH =l (2)

The four probabilities considered are conditional, because they characterize events that occur under the condition of the presence or absence of an object. In real operating conditions of a reconnaissance station, we cannot know in advance whether there are objects in the area of ​​space being viewed or whether they are not there.

Let us denote the probability of the presence of an object in the region of space of interest to us by W(t), and the probability of the absence of an object by W(o).

The four events mentioned also constitute a complete group of incompatible events, so

W(ts).Wpo+W(ts)Wpr+W(o)Wlt+W(o)W PN =l (3)

where: W(t).Wpo - unconditional probability of correct detection,

W(ts)Wpr - unconditional probability of missing the target,

W(o)Wlt is the unconditional probability of a false alarm,

W(o)W PN is the unconditional probability of correct non-detection. The optimal detection device will be a device with which the best (compared to others) value of the selected criterion can be achieved, all other things being equal. The most commonly used are three criteria: the ideal observer criterion, the Neyman-Pearson criterion, and the sequential analysis criterion.

In accordance with the ideal observer criterion, the optimal detection device must provide a minimum of the total unconditional probability of detection errors, i.e.

Wosh=W(ts).Wnp+W(o)Wlt -»min (4)

The ideal observer criterion is used for radio communication systems when the probabilities W(t) and W(o) are known a priori.

The relative frequency of errors is determined by the prior probabilities W(t) and W(o), respectively. Therefore, the average probability of the general (total) error is equal to

Wosh =W(ts) Wpr+ W(o)Wlt, (5)

and the probability of correct signal detection is equal to

In accordance with the Neyman-Pearson criterion, the optimal device is characterized by a maximum difference

Wpo*Wlt at Wlt<=(Wлт)доп (6)

Therefore, the optimal nature of the Neyman-Pearson test is that it maximizes the probability of a correct detection given a fixed probability of a false alarm.

In the receiving device, with the help of which signals are detected, the posterior probabilities of various messages are determined (for example, a message - there is an object or it is not) and an indication as a decision is that message, the probability of which is greater than the others. The main characteristics of the device used to detect signals are the operating characteristics of the receiver.

The operating characteristic of the receiver (detector) represents the dependence of Wpo on the signal-to-noise ratio at the detector input (q) for a given value of Wlt ■

In Fig. Figure 1 shows the corresponding dependencies for the detector described by the relation

and the case of a narrowband pulse signal. Thus, having calculated the signal-to-noise ratio at the reception point and knowing Wlt , W can be determined.

Figure 1 Dependence of Wpo on signal-to-noise ratio

Calculation of the probability of detecting an intruder

Having calculated the period of false alarms, it is necessary to calculate the probability of detecting a CO ASO.

Regardless of the information processing algorithm and the number of detection sites in each boundary, in general form (for SLOS m out of n), the formula for calculating the probability of detection of software will take the form:

where k is the ordinal number of the sum term, considering the first one to be zero, ;

Number of combinations of k+1 elements by k;

Probability of detecting the i-line;

The sum of all possible products of detection probabilities from n-terminals by n+k, factors, the number of members of the set is equal to the member of combinations of n elements n-m-k. In the case where the software includes 3 detection lines, when the intruder overcomes the software, different values ​​of detection probabilities are possible, depending on the selected SLOS.

Room1


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Option 2

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Option 3

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From the results obtained it is clear that only when using SLOS 2:3 and 3:4 in all selected TSO layout options for premises, the required value of the probability of detecting an intruder is achieved.

Determination of the optimal information processing algorithm

Having received an information system with a minimum cost, it is necessary to determine the probability of its detection of an intruder and the period of false alarms. The period of false alarms is a temporary characteristic of the CO. It is closely related to the information processing algorithm, which makes it possible to determine the optimal signal processing algorithm.

Currently, two SLOS algorithms are widely used, namely:

a) Algorithm A - consisting in the fact that after the first activation of one of the TSOs, another m-1 signals are received from the remaining n-1 TSOs over time, and upon receiving them, the detection subsystem generates an alarm signal. If the specified number of signals is not received within the time limit, the signal is reset and everything is repeated from the beginning.

b) Algorithm B - consisting in the fact that after the first operation, a signal is received from one of the remaining m-1 technical detection means for a period of time. When a second signal is received, again within a period of time a signal is received from one of the remaining m-2 TSOs, etc. Until m alarms are received and the CO alarm signal is generated. If during the process of typing information at least once during a period of time a signal from the TSO is not received, the information is reset and the process is repeated.

Note that the numerical values ​​and can be related by the relation:

Calculation of false alarm period

We will calculate the period of false alarms using the formula for algorithm B, since it is this algorithm that is most resistant to false alarms (preferred). This dependence has the following form:

where is the period of false alarms with algorithm B;

k - number of TSO sections in each boundary;

n is the number of milestones in the SO;

m-number of TCO signals to generate a software activation signal;

Logical signal processing time with algorithm B; (750 ms)

The sum of all possible products of TSO false alarm periods, various boundaries with n-m factors in each.

Option 1

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Option 2

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Option 3

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Room 2.

Based on the data obtained, I choose SLOS 2:4 since it provides maximum values ​​for the probability of detecting an intruder, and the false alarm period is higher than required.

When calculating the periods of LT of internal premises, it is necessary to take into account that the detectors installed in them, as a rule, do not work around the clock, but only part of them. This accounting can be done through the load factor, which represents the ratio of the detector operating time during the week to 168 hours (the number of hours per week). For rooms that are not opened regularly, for rooms with one-shift work, for rooms with two-shift work, and for rooms with three-shift work (round the clock) (due to two days off).

Option 1:

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Option 3

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Security Detection System Cost Calculation

The length of the detection zone (Loobn) significantly affects the cost of the software, and the degree of influence is determined by the relationship: