DE102005055393A1 - Light signal`s range recognizing and reading method for rail traffic, involves obtaining threshold value by multiplying luminance or intensity of individual light with power of number of light spots and corrected factor - Google Patents

Abstract

The method involves evaluating recognition and readability range of light signals based on measured luminance or intensity of light. The recognition range is evaluated by using a threshold value for the total luminance or intensity of the light. The value is obtained by multiplying the luminance or intensity of an individual light with the power of the number of light spots (101) and corrected factor.

Description

Territory of invention

The The invention relates to optical measurement and test methods for traffic signals, especially with light signals, as used in rail transport Find. In particular, the invention is concerned with methods for Determining the recognizability and readability of a light signal at a predominant background or ambient luminance.

background the invention

in the Rail traffic is used to regulate traffic signals. These are usually on or over arranged the tracks. The signals serve the train drivers of the Trains as Information source about the traffic situation and allow a smooth and safe operation of rail transport.

The Signals can be designed as so-called shape signals. For shape signals the signal concept is indicated by a change in the shape of the signal. The shape signals, however, are outdated and are becoming increasingly prevalent replaced so-called light signals. In the light signals of the Signal concept by a specific light output of the light signal displayed. Such light signals are in principle with the the traffic comparable to known traffic lights. A further introduction to the Theme leaves light signals the article "Fundamentals about light signals" by Dr. K. Grosskurth in light engineering 8. No. 8 (1956).

A Basic requirement for The use of a light signal in rail transport is that this off enough Distance, especially in daylight, recognizable by the driver or must be readable. In this case, a light signal is recognizable as soon as it from the driver can be perceived.

at Complex light signals representing symbols come to the premise the recognizability of readability added. In fact it is enough with these light signals is not that the driver recognize this, i.e. can perceive. Once the driver perceived the signal has, he must in addition also be able to read the symbol represented by the signal. The readability of a light signal is given if that of this symbol symbolically identifiable as a particular symbol is. requirement for The readability of a signal is accordingly first Recognizability.

Therefore must be ensured with light signals be that from different directions at least up to one certain minimum width are recognizable or legible. In other words, light signals must be one certain minimum distance and possibly also a certain Have minimum readability. To determine if this requirement for a given light signal fulfilled Therefore, its recognizability and readability must be determined.

The Detection distance of a light signal is direction-dependent. Depending on which of Direction seen from the light signal, there is another Erkennweite. For example, e.g. the detection distance along the direction at biggest, up which the light signal is aligned. This direction of registration the light signal is at the same time the optical axis of the light signal. On the other hand, e.g. the detection distance of the light signal obvious zero if you look at it from the back. There the detection distance of a signal depends on the viewing direction for each direction a different detection distance, so that total a recognition distribution is present.

The Detection distance is for defines a given direction as the distance along that direction, to which the light signal at a prevailing ambient or background luminance can still be perceived. The detection distance of a light signal can also be as a consequence of the light signal be designated.

The readability range of the symbol of a light signal is defined for a given direction as the distance along that direction, to which the symbol in a prevailing environment or direction background luminance can still be identified.

Out the article "Scope of light signals "from Dr. K. Grosskurth in Lichttechnik 9. Nr. 11 (1957) is a procedure known, with which determines the detection distances of a light signal can be which consists of a single point of light. A major disadvantage This known method is that it is not the determination the Erkennweiten of light signals allows, which consists of several There are points of light. In particular, can not with this method determines the detection distances of self-illuminating additional indicators which are increasingly being used in rail transport. In the Indeed, these additional indicators have several points of light, with which different icons can be displayed.

Incidentally, is Also no method is known with which the readability a light signal can be determined.

Task of invention

in view of It is the one of the problems of the prior art just described Object of the present invention, method for determining the Recognizability and readability in the case of light signals, in particular light signals with multiple light points propose.

description the invention

According to the invention, this object is achieved by a method for determining the detection distance, which is present in a given direction, of a light signal comprising a plurality of light spots at a prevailing ambient luminance with the following steps:
Measurement of the luminance or light intensity of each individual light spot in the given direction, and calculation of the detection distance on the basis of the measured luminances or light intensities with the aid of the threshold value calculated from these measured values for the total luminous intensity or the total luminous intensity of the light signal, which is obtained by multiplying the luminance or luminous intensity Gives the luminous intensity of a single point of light with the number of points of light and with a correction factor. This correction factor depends on the number of light spots, the light intensity, the diameter and the position of each of the light spots and on the distances between the light spots. In particular, the correction factor is always less than or equal to 1.

• – Bei Tage werden die Lichtpunkte als Beugungsscheibchen auf der Netzhaut des Auges aufgrund der Beugung des in das Auge fallenden Lichtes an der Pupille des Auges abgebildet, so dass der Korrekturwert 1 ist.
• – Bei Dämmerung und Nacht ergibt sich eine unscharfe Abbildung der Lichtpunkte als Scheiben auf der Netzhaut des Auges, so dass der Korrekturfaktor kleiner als 1 ist. Es tritt eine Überstrahlung der einzelnen Lichtpunkte auf.
• – Bei Tage und insbesondere bei Nacht ergibt sich zusätzlich eine Streuung an Schwebeteilchen in der Flüssigkeit des Auges innerhalb des Augapfels, so dass der Korrekturfaktor kleiner als 1 ist. Das Auftreten von Schwebeteilchen im Auge ist insbesondere eine Alterserscheinung des Auges oder krankhaft.

Cause of the varying correction factor are imaging properties of the eye:

• By day, the points of light are projected as diffraction slices on the retina of the eye due to the diffraction of the light falling into the eye at the pupil of the eye, so that the correction value is 1.
• - At dusk and night, there is a blurred image of the points of light as slices on the retina of the eye, so that the correction factor is less than 1. There is an over-radiation of the individual points of light.
• In addition, there is a scattering of suspended particles in the fluid of the eye within the eyeball during the day and in particular at night, so that the correction factor is less than 1. The appearance of suspended particles in the eye is especially an aging of the eye or pathological.

The Magnification of the practical readability removal or increase the legibility is by consideration the first two causes of origin possible. However, this is a healthy eyesight the viewer, so possible the almost nonexistent / effective third Cause of origin, required.

This Correction factor is advantageously determined numerically. This is a virtual additional indicator with user-definable light point arrangement given and by means of an eye model, in particular the Gullstrand eye model and Light ray calculation at given distance between eye and additional indicator the correction factor is calculated. The virtual additional indicator becomes formed by a virtual grid with freely selectable Positions occupied with points of light.

The over-radiation increases with the distance to the signal consisting of several points of light, since the angle between each two adjacent points of light decreases with the distance from the signal. To improve the readability or to realize a larger practical readability range up to The amount of theoretical readability is the distance between each two adjacent points of light to increase, but this can reduce the scope of the signal.

The inventive method is also applicable if the total luminance or the total luminous intensity is not inversely proportional to the number of points of light, e.g. proportional to a power of the number of points of light.

The selected above Wording "distance to each other make it clear that this is the distance of a light spot to any other light spot of the light signal. in the Contrary to that gives a "distance from each other "the Distance of a point of light to its nearest neighboring point of light at.

The Light points of the light signal can also as essentially point-like Light sources are called. Preferably, the light signal is through the entirety of the points of light formed. So the points of light can total in their geometric arrangement a certain symbol, such as a speed indication, map. If the points of light then shine together, resulting in a luminous symbol, which as a source of information for a driver can serve.

The Measurement of the luminance or the intensity of each light spot over a commercial one Meter. The measurement must differ depending on the measuring device used be performed. For example, if a device is used that directly single average luminance or light intensities measures, so does the device from a certain direction to the corresponding point of light aligned with the light signal and the average luminance detected by the device. or intensity value will be recorded.

If For example, a measuring camera for measuring luminance or Used light intensity distributions the camera will be at a known distance from the camera aligned overall light signal and the luminance or light intensity distribution the entire signal is recorded. Based on the measured distribution can then the values for the individual points of light are determined.

at the calculation of the detection distance using the total luminance threshold or the total light intensity it is assumed that the threshold for total luminance or the total light intensity the light signal is the product of the luminance or luminous intensity of a Light point with the number of light points and the correction factor is.

The The assumption just described applies in particular to long distances (i.e., more than 10 m) from the corresponding light signal or until to a certain maximum distance of the light spots to each other.

It is advantageous when measuring the luminance of a light spot the luminance both in the on state and in the off Condition is measured. It is in the switched-on state Measured luminance is the sum of the pure caused by the point of light Luminance and the prevailing ambient luminance during the measurement. On the other hand, the luminance measured in the switched-off state exists only from the ambient luminosity. Thus, by forming the Difference of the two measured values of the ambient luminance the measured values are removed, so that only the pure Luminance of the light spot retains, i. E. the luminance of the turned on light spot in a completely dark environment.

Preferably becomes the luminance difference obtained by subtraction converted to the optical axis of the light signal. This is necessary if the measurement of luminance is not along the optical axis the light signal has occurred and the detection distance in the direction of optical axis is to be determined.

From the luminance difference along the optical axis, which corresponds to the pure luminance of the light spot along the optical axis, a normalized luminance can then be calculated by adding a defined ambient luminance or background luminance. The defined background luminance is preferably a defined value which prevails under the most difficult light conditions, ie in conditions in which light signals are particularly difficult to detect. This is the case, for example, in bright sunshine in a snowy landscape. The defined background luminance is preferably at a value of 10,000 cd / m ² .

The object mentioned at the beginning is also solved by a method for determining the readability width of a symbol of a traffic signal present in a given direction at a given surrounding luminance with the following steps:
Determination of the characteristic length of the symbol, determination of the required minimum viewing angle of the eye for the given surrounding luminance and calculation of the readability width using the determined characteristic length and the determined required minimum viewing angle.

at the symbol of the traffic signal may be any geometric Figur act, which is suitable for the instruction of a train driver is. In particular, the symbol is a letter or a number.

Of the Minimum viewing angle is the smallest angle that the eye observes dissolve can. Objects at a smaller angle than the minimum viewing angle can be seen be perceived by the eye no differentiated. The minimum viewing angle is also called "Riccoscher Limit angle " and hang from the surrounding luminance.

Prefers is the characteristic length the symbol the length, their resolution through the eye requirement for the reading of the symbol is.

Prefers becomes the readability distance in the direction of the optical axis of the traffic signal certainly.

If the readability is not determined in the direction of the optical axis will have, may need in determining the characteristic length of the angle between the optical axis and the relevant direction.

Preferably the symbol consists of several elements.

• 1. bei einer dunklen Strecke durch die Entfernung zwischen den die Dunkelstrecke begrenzenden Lichtpunkten;
• 2. bei einer leuchtenden Strecke durch die Entfernung zwischen den zur leuchtenden Strecke gehörenden äußeren Lichtpunkten.

If the symbol consists of several elements and the readability distance is determined in the direction of the optical axis of the traffic signal, then the characteristic length of the symbol is defined as follows:

• 1. at a dark distance by the distance between the light points delimiting the dark distance;
• 2. in the case of a luminous route, by the distance between the external points of light belonging to the luminous route.

The Elements in their entirety form the symbol. At the elements these may in particular be individual points of light.

The Traffic signal is preferably a light signal, in particular a light additional signal.

The described inventive method enable an accurate and reliable Determination of the recognizability and readability of traffic signals, especially with light signals that consist of several points of light. Thereby can traffic signals or prototypes in use on their recog- and readability. For example, you can so old traffic signals that are no longer sufficient from a distance recognizable or readable are easily recognized and exchanged. This increases the safety of the transport system.

• – Verbesserung der Lesbarkeit bzw. Realisierung einer größeren praktischen Lesbarkeitsweite (vom Betrage der theoretischen Lesbarkeitsweite) oder Verringerung des Energiebedarfs für die Lichtpunkte zur Erzielung der geforderten Lesbarkeits- und Tragweite: der auf den freien Strecken im Bereich der Deutschen Bahn AG vorhandenen Geschwindigkeits- und Richtungsanzeiger der Bauart Siemens durch die Verwendung anderer Rastersteckplatten-Positionen für die Lichtleiter-Lichtaustrittsflächen;
• – Erreichung einer möglichst guten Lesbarkeit bzw. Realisierung einer möglichst großen praktischen Lesbarkeitsweite (im Idealfall vom Betrage der theoretischen Lesbarkeitsweite) bei gleichzeitiger Aufwendung/Einsatz eines möglichst niedrigen Energiebedarfes für die Lichtpunkte zur Erzielung der geforderten Lesbarkeits- und Tragweite: neuer, noch einzusetzender LED-Zusatzanzeiger durch Vorgabe der Anzahl der Lichtpunkte, des Durchmessers und der Lichtstärke eines einzelnen LED-Lichpunktes sowie der Position jedes der Lichtpunkte;
• – Wegfall von Probandenversuchen zur Ermittlung der Lesbarkeits- und Tragweiten von Zusatzanzeigern durch die Bestimmung dieser Größen aus den in der Lichtmessstrecke gemessenen Lichtverteilungen der Zusatzanzeiger analog dem Vorgehen bei der/zur Ermittlung der Tragweiten von Einzelsignalen/Lichtpunkten durch die Bestimmung dieser Größen aus den in der Lichtmessstrecke gemessenen Lichtverteilungen der Einzelsignalen/Lichtpunkte.

Particular advantages of the invention are:

• - Improving the readability or realization of a greater practical readability range (the amount of the theoretical readability range) or reducing the energy requirement for the points of light to achieve the required readability and scope: the existing on the free routes in the area of Deutsche Bahn AG speed and direction indicator type Siemens by the use of other Rastersteckplatten positions for the light guide light exit surfaces;
• Achieving the best possible readability or realization of the greatest possible practical readability (ideally the amount of theoretical readability) while simultaneously applying / using the lowest possible energy requirement for the light points to achieve the required readability and scope: new, still usable LED Additional indicator by specifying the number of points of light, the diameter and the intensity of a single LED spot as well as the position of each of the points of light;
• - Elimination of test subjects to determine the readability and carrying ranges of additional indicators by determining these quantities from the light distributions measured in the light measuring path, the additional indicator analogously to the procedure for determining the carrying ranges of individual signals / light points by determining these quantities from the light distributions of the individual signals / light points measured in the light measuring path.

The Invention will be described below with reference to two embodiments and a drawing closer with a figure explained. The figure shows in

1 schematically an additional indicator whose 16 pixels represent a letter "E".

description preferred embodiments

in the The following will each be a preferred embodiment of the method according to the invention for determining the recognizability and readability of a light signal described.

For a self-luminous additional indicator, which has been used on a railway line for 15 years, the recognition and the readability range should be determined. The symbol 100 , which indicates this additional indicator is in 1 shown. It is the letter "E." The symbol 100 consists of sixteen points of light 101 , When they are turned on together, the result is a glowing "E".

detection the detection distance

The detection distance t _{n of} the illuminated symbol 100 and thus to determine the switched-on additional indicator for a particular direction, first from this direction, the average luminance of each light spot 101 measured. For this purpose, a corresponding luminance meter is aligned to each of the individual points of light and read the resulting measured value. The measurement is carried out in such a way that an angle α exists between the optical axis of the additional indicator and the line between the measuring device and the penetration point of the optical axis on the front side of the additional indicator.

For each light spot, the luminance L _on in the switched-on state and L _off in the switched-off state are measured. Thus, for each of the sixteen points of light, two measured values L _an and L _result . The luminance difference .DELTA.L = _α L is then _at each of these two measured values - _from L formed.

Subsequently, ΔL _{α is converted} into a luminance difference along the optical axis ΔL _{0 °} . This is done with the following formula: .DELTA.L 0 ° = ΔL α · K · cos (α) (1)

there k is the ratio between the maximum light intensity and the light intensity at the angle α against the optical axis of the additional indicator. k is measured from the additional Light distribution of the additional indicator determined.

Subsequently, the average normalized luminances L _{n of} the light spots for distances over 100 m are determined by the following equation: L n = ΔL 0 ° + L H (2)

L _{H is} the background luminance. For these, the value 10000 cd / m ^{2 is} taken, which corresponds to the most difficult lighting environmental conditions. With this transformation, sixteen luminances L _n normalized to a single common background luminance are obtained.

Furthermore, a common mean value L _{i is} calculated from the sixteen normalized luminances L _n . This common average represents the average luminance of any of the sixteen points of light in the direction of the optical axis. This value can then be finally used in the following formula for the detection distance t _{n of} the additional indicator:

der Korrekturfaktor,. A die Querschnittsfläche eines Lichtpunktes und n die Anzahl der vorhandenen Lichtpunkte, also in diesem Beispiel sechzehn.It is

the correction factor ,. A is the cross-sectional area of a light spot and n is the number of available light spots, in this example sixteen.

= 1 und n = 1, d.h. für Lichtsignale mit nur einem Lichtpunkt, bekannt. Bisher unbekannt ist jedoch die Einfügung der Faktoren n und vor allen Dingen

, welche die genaue Berechnung der Erkennweite auch für Lichtsignale mit mehreren, beliebig angeordneten Lichtpunkten ermöglicht. Die Einfügung des Faktor n basiert auf der Erkenntnis, dass die für eine bestimmte Gesamtlichtstärke (Lichtstärke ist Leuchtdichte mal Fläche) des Lichtsignals von jedem einzelnen Lichtpunkt benötigte Lichtstärke umgekehrt proportional zur Anzahl n (n = 16 im vorliegenden Beispiel) der Lichtpunkte ist.This central formula (3) is for

= 1 and n = 1, ie for light signals with only one light point known. So far unknown is the insertion of the factors n and above all

, which allows the accurate calculation of the detection distance for light signals with multiple, arbitrarily arranged points of light. The insertion of the factor n is based on the knowledge that the luminous intensity required for a given total luminous intensity (luminous intensity is luminance times area) of each individual point of light is inversely proportional to the number n (n = 16 in the present example) of the points of light.

Of the Correction factor φ reflects the dependence the threshold for the total luminance of the signal from the number of points of light, from the light intensity, the diameter and position of each of the points of light and of the mutually present distances between the points of light, while the factor 2 influences of the signal screen present in most light signals describes the signal during the day. To determine the detection distances a light signal at night or in the tunnel is in place of the factor 2 use factor 1000 in the above formula.

mit dem Schwellenwert E_min für die Beleuchtungsstärke. Anhand dieser äquivalenten Formel (4) erkennt man sofort die umgekehrte Proportionalität zwischen der Lichtstärke I_n und n.Formula (3) can also be expressed as follows using the light intensity I _{n of} a light spot (I _n = A · L _i ):

with the threshold E _min for illuminance. On the basis of this equivalent formula (4) one immediately recognizes the inverse proportionality between the light intensity I _n and n.

It has been found that the novel formula (3) for t _n and the equivalent formula (4) for I _n are applicable for any distances of the light spots from each other.

The detection distance t _n determined via formula (3) can then be compared with the minimum value for the detection distance that is permissible in the specific case. So it can then be determined whether the measured additional indicator is still sufficiently recognizable or needs to be replaced.

detection readability

If the symbol 100 of the additional indicator can be detected in good time by the driver, this does not mean, however, that he can read this in good time. So that the perceived symbol 100 can also be read, it must be additionally resolved by the eye. As long as the symbol 100 is only seen as a blurred light spot and not as "E", the additional indicator is indeed recognized, but can not be read yet.An additional indicator so not only a certain Erkennweite but also a certain legibility be guaranteed the readability distance from the direction from which the symbol to be read is viewed.

The following describes how to use the icon for the additional indicator 100 the readability I is determined along its optical axis. The readability along the optical axis is also the maximum readability.

The readability width I of the symbol 100 along the optical axis of the additional indicator can be determined by the following formula: I = (ζ / 2) / (tan (γ / 2)) (5)

Here, γ is the minimum value for the visual angle at which an observer can read or resolve a symbol at a given surrounding luminance. γ is also called "Ricco's critical angle" and can be looked up for a known ambient luminance. With average daylight, the value of γ is about 1 '.

To every blur disc corresponds to a background luminance in the immediate vicinity of the respective Light spot.

The theoretical readability is a function of the environmental luminance, in this case, the average weighted with the respective area shares from the background luminance and the luminance in the immediate Environment of the points of light is.

ζ is the characteristic length of the symbol and can be in the case of the symbol 100 take the figure.

Basically for a given symbol, that length is the characteristic length, its resolution by the eye requirement for the reading of the symbol is.

• 1. bei einer dunklen Strecke durch die Entfernung zwischen den die Dunkelstrecke begrenzenden Lichtpunkten;
• 2. bei einer leuchtenden Strecke durch die Entfernung zwischen den zur leuchtenden Strecke gehörenden äußeren Lichtpunkten.

For any symbol consisting of multiple points of light, the characteristic length of the symbol is defined as follows:

• 1. at a dark distance by the distance between the light points delimiting the dark distance;
• 2. in the case of a luminous route, by the distance between the external points of light belonging to the luminous route.

If the readability width along a direction other than that the optical axis should be determined, so must in the specified Formula in addition still the angle between the relevant direction and the optical Axis considered become.

In summary can be determined by the characteristic length of the symbol, which is directly from the geometric design of the symbol, and based on the suggested minimum viewing angle with the mentioned formula for one known ambient luminance the maximum readability of a Be detected light signal.

As soon as the readability of a signal with the described method is determined, can be compared immediately with permissible minimum values to judge whether the signal has sufficient readability.

α

Winkel

L_an

Leuchtdichte bei angeschaltetem Zustand

L_aus

Leuchtdichte bei ausgeschaltetem Zustand

ΔL_α

Leuchtdichtedifferenz

ΔL_0°

optische Achse

k

Verhältnis zwischen der maximalen Lichtstärke und der Lichtstärke bei

dem Winkel α gegen die optische Achse des Zusatzanzeigers

L_n

mittlere normierte Leuchtdichte

L_H

Hintergrundleuchtdichte

L_i

gemeinsamer Mittelwert aller mittleren normierten Leuchtdichten L_n

t_n

Erkennweite

φ

Korrekturfaktor

A

Querschnittsfläche eines Lichtpunktes

n

Anzahl der vorhandenen Lichtpunkte

I_n

Lichtstärke eines Lichtpunktes

E_min

Schwellenwert für die Beleuchtungsstärke

I

Lesbarkeitsweite

ζ

Charakteristische Länge

γ

Mindestwert für den Sehwinkel, ab dem ein Beobachter ein Symbol bei

einer gegebenen Umfeldleuchtdichte lesen bzw. auflösen kann

Directory of used formula characters:

α

angle

L _on

Luminance when switched on

L _off

Luminance when switched off

ΔL _α

Luminance difference

ΔL _{0 °}

optical axis

k

Ratio between the maximum light intensity and the light intensity at

the angle α against the optical axis of the additional indicator

L _n

average normalized luminance

L _H

Background luminance

L _i

common average of all average normalized luminances L _n

t _n

Erkennweite

φ

correction factor

A

Cross-sectional area of a light spot

n

Number of existing points of light

I _n

Light intensity of a light spot

E _min

Illuminance threshold

I

readability distance

ζ

Characteristic length

γ

Minimum value for the visual angle, from which an observer an icon at

can read or resolve a given surrounding luminance

Claims (11)

Method for determining the detection distance, in a given direction, of a light signal comprising a plurality of light spots at a defined ambient luminance with the following steps: measuring the luminance or luminous intensity of each individual light spot in the given direction and calculating the detectable and readable width on the basis of the measured luminance Luminances or light intensities, characterized in that the calculation of the detection distance takes place with the aid of the threshold for the total luminous intensity or the total luminous intensity of the light signal, which is obtained by multiplying the measured luminance or luminous intensity of a single light spot with a power of the number of light spots and with a correction factor results. Method according to claim 1, characterized in that that the correction factor of the number of points of light, from the Light intensity, the diameter and position of each of the points of light and of the mutually present distances between the points of light depends. Method according to claim 2, characterized in that that the correction factor is determined numerically. Method according to claim 3, characterized that a virtual additional indicator with user-definable light point arrangement given and by means of an eye model and light beam calculation for a given distance between eye and additional indicator, the correction factor is calculated. Method according to claim 4, characterized in that that the virtual supplementary indicator is formed by a virtual grid with freely selectable Positions occupied with points of light. Method according to at least one of claims 1 to 5 with the further steps: - Measurement of a first luminance for each individual light spot in the switched-on state of the light signal and - Measurement a second luminance for every single point of light when the light signal is switched off. Method according to Claim 6, with the further step: - Calculation the luminance difference of each light spot along the optical Axis of the light signal based on the measured first and second Luminance. Method according to claim 7 with the further step: - Calculation the normalized luminance of each light spot by addition each calculated luminance difference with the specified Surrounding luminance. Method according to one of the preceding claims, characterized in that when measuring the luminance or the light intensity of a average luminance or a mean light intensity is measured. Method according to one of claims 1 to 9, characterized that when measuring the luminance or the light intensity of a Luminance distribution or a light intensity distribution is measured. Method for determining the in a given Direction present readability of a symbol of a traffic signal at a given luminance level, with the following steps: - Determination the characteristic length the symbol,