WO2011001316A1

WO2011001316A1 - Method and system for measuring photometric quantities

Info

Publication number: WO2011001316A1
Application number: PCT/IB2010/052661
Authority: WO
Inventors: Umberto Grasselli; Stefano Bugoni
Original assignee: Università Degli Studi Di Roma "La Sapienza"
Priority date: 2009-07-02
Filing date: 2010-06-15
Publication date: 2011-01-06
Also published as: IT1394610B1; ITRM20090340A1

Abstract

The present invention refers to a method and system for measuring photometric quantities, in particular related to Aeronautical Ground Lights (AGL), such as approach lighting systems or the like, consisting of sets of Aviation Light Signals (ALS), allows to assess the efficiency conditions of a high number of ALS, further allowing to examine ALS photometry, in terms of luminance and chrominance, up to a distance of several hundreds of meters and even in daytime.

Description

METHOD AND SYSTEM FOR MEASURING PHOTOMETRIC QUANTITIES

DESCRIPTION

The present invention refers to a method and a system for measuring photometric quantities, in particular related to Aeronautical Ground Lights (AGL), such as approach (path) lighting systems or the like, consisting of sets of Aviation Light Signals (hereinafter referred to as ALS).

To date, the measuring of photometric quantities related to the individual ALS on site in order to check the effectiveness thereof is typically performed through individual detecting, lamp by lamp, by means of movable measuring instruments.

However, it is known that the typical configuration of lamps of a precision approach lighting system does not allow an efficient measuring of the photometric features of the same through any movable detecting instrument currently available on the market, as in most cases the lamps are installed on an elevated trestle or on loose ground.

Moreover, photometric checking as currently implemented impose runway engagement and, when envisaged, taxiway engagement, with evident inconvenience for airport traffic; they are performed under dark conditions such as not to significantly alter them by background luminosity.

Again, it should be considered that in the presence of a high number of lamps (e.g., Category (CAT) Il and III airports), the use of measuring instruments with a stationary positioning in front of the light source at issue may require a high overall time for completing the measuring campaign for a runway, even in excess of 20 hours. This usually mandates the performing of different measuring sessions, over runway closure times, thereby setting non-negligible problems of economic and logistic nature, unavoidably affecting measuring campaigns frequency.

Concerning the issue of provisions, ICAO (International Civil Aviation Organization) recommends a periodic checking of the photometric characteristics of all ALS comprised in the precision approach lighting system of CAT Il and III runways, and suggests the use of movable measuring devices.

However, such a provision is actually not applicable since, among other things, there is no movable device for on-field measuring of photometric parameters of aeronautical ground lights meeting the technical-operational standard APS-02 of ENAC {Ente Nazionale Aviazione Civile) Standards, describing minimum requirements for use of movable measuring apparatuses which may be efficiently used on lamps of the precision approach lighting system.

Currently, the sole theoretical option for performing photometric detecting on lamps of the precision approach lighting system is the use of a stationary device (ICAO grid- like) manufactured, e.g., by locally assembling, under the operator's responsibility, a matrix of illumination sensors. In fact, movable devices can currently be used only on the runway, or on taxiways. Therefore, also in view of the fact that the first 300 meters of the path lighting system are the most critical to the ends of the conduct of flights for CAT Il and III runways, ICAO, practically recognizing the difficulties highlighted above, suggests to perform in-flight "qualitative" inspections of the approach lighting system.

Hence, object of the present invention is to provide a solution to the problems left open by the known art, by providing a measuring method as defined in independent claim 1.

A further object of the present invention is to provide a measuring device, as defined in claim 14.

Secondary features of the present invention are instead defined in the corresponding dependent claims thereof.

As will be illustrated hereinafter in the description, the present invention entails several advantages.

In particular, the present invention allows to assess the efficiency conditions of a high number of ALS, with a single acquisition, setting aside the need to perform individual measuring on each ALS, further allowing to examine ALS photometry up to a distance of several hundreds of meters and also in daytime. In particular, the presence of a consistent background luminosity also allows an easier calibration of the system. It should be stressed that while in currently employed systems both background luminosity and ALS contribution, comprised in the measuring field of the sensors, constitute a hardly assessable systematic error, in the present invention they are measured and utilized.

The determining of photometric quantities of interest occurs through a processing of the acquired digital image correlated with the measuring of a calibrated instrument.

Therefore, the present invention sets itself not merely as a more flexible and efficient alternative to the existing movable devices, but, also, as sole movable measuring device capable of performing photometric measuring on ALS of the precision approach lighting system.

These and further advantages, as well as the features and operation modes of the present invention, will be made evident in the following detailed description of an embodiment thereof, given by way of example and not for limitative purposes. Reference will be made to the figures of the annexed drawings, wherein:

Fig. 1 is a flow chart of a method according to the present invention; and

Fig. 2 is an exemplary block diagram of a system according to the present invention.

The present invention will hereinafter be described making reference to the above- indicated figures.

The proposed methodology is based on the combined use of an illumination measuring instrument, calibrated according to CIE Standards, and of a digital image acquisition device. According to the preferred embodiment of the present invention, a luxmeter and a reflex camera having a 15Mpixel CMOS sensor and comprising a 60mm fixed-focal lens were utilized.

The aim is the measuring of the luminous intensity of illuminating objects, in particular of ALS constituting Aeronautical Ground Lights (AGL).

The method according to the present invention is based on an acquiring of a digital image of an airport scene inside which, e.g., a portion of a precision approach lighting system is present, and a concomitant measuring of the luminosity, by means of the luxmeter, of part of the scene depicted in the digital image.

As is known, the sensor of a digital camera outputs a signal which is linearly proportional to the illumination of the sensor itself and provides a scale measuring of the illumination to which the various photosites (photosensitive elements which on the image correspond to pixels) making up the sensor are subjected.

The same principle regulates the operation of an illumination measuring probe (luxmeter), with the substantial difference that the luxmeter outputs a quantity expressed in International System units and calibrated according to CIE Standards.

The present invention aims at combining the two measurements, coming from the two instruments, so as to calibrate each acquired image on the measurement provided, simultaneously with the taking, by the luxmeter. Thus, it is eliminated the measuring uncertainty present in case a calibration of the camera sensor is performed separately from acquisition of the digital image of the airport scene, uncertainty due to the stochastic variation of the system sensitivity; such a variation is due to several factors, e.g., among others, sensor temperature variation and variation of the actual image-taking time.

To this end, the present invention envisages to limit the angle of sight of the luxmeter to a value lower than the field angle of the camera-mounted lens, in a manner such as to measure illumination related to a well-defined, predetermined portion of the scene acquired with the digital camera. Hereinafter, this limiting of the luxmeter will also be referred to as "ducted luxmeter".

By mechanically fixing the two instruments (camera and luxmeter) integrally to each other, through geometric relationships and subsequent calibration tests there may be determined which of the pixels of the camera-acquired digital image portray the portion of the scene which is comprised inside the angle of sight of the luxmeter.

Upon determining the surface of the image comprised in the area of sight of the luxmeter, it is possible to weigh the relative scale of illumination measured through the photo image with the measurement obtained through the luxmeter.

Calculation of the ratio between illumination (measured in lux) and luminosity of pixels of the image is obtained by dividing the value read on the luxmeter by the total luminosity of the digital image pixels comprised in the area of sight of the luxmeter.

Thus, the value in lux of sensor contribution to illumination can be calculated; said contribution value is produced by any one portion of the image, e.g., that corresponding to the presence of an ALS, as the ratio between illumination in lux of any one photosite and the luminosity value of the corresponding pixel is constant over the whole photo image (the calculation formula is explained below).

To be able to perform a correct and effective measuring of an ALS-related illumination and a subsequent calculation of luminance expressed in candles (as required by rules, e.g. ENAC - Ente Nazionale dell'Aviazione Civile - ones), it a subsequent processing of information contained in the digital images is provided, the measuring being performed with the ducted luxmeter and information related to the configuration of the approach lighting system subject of measuring.

Such a processing, performed by means of a software, envisages first of all a reading of the raw file of the acquired digital image (RAW file). This reading step comprises a stage of coding the image in a color space, allowing color representation on the basis of numeric variables, e.g. the sRGB system.

This operation is particularly delicate, as at this stage it is necessary to keep unaltered the characteristic of linear response of the CMOS sensor to illumination. Therefore, it is preferable that the conversion procedures apply no gamma correction (something that instead is usually done by the most widespread image processing programs to compensate for the visibility characteristic of human eye).

Depending on the color space utilized (e.g., sRGB) the polychromatic image can be converted into a monochromatic one, weighing each color by a coefficient characteristic of the color space in which the image has been coded. E.g., in the sRGB space, luminous intensity is obtainable as L=.0.2990R+0.5870G+0.1140B. Thus, the intensity of each pixel (on an 8- or 16-bit scale) will correspond to an illumination value to which the corresponding photosite has been subjected during the taking. For instance: if photosite A is subjected to an illumination equal to 0.01 lux and photosite B to 0.03 lux, then the L value of the pixel corresponding to photosite B will be 3 times that of the pixel corresponding to photosite A.

Always depending on the color space utilized for each photosite, color may be calculated, preferably according to CIE (International Commission on Illumination) provisions.

Then, the digital image is subjected to a segmentation process, in order to separate (extract) the objects of interest (in this case, ALS) from the background. Such a segmenting step comprises a first stage in which a luminosity threshold is defined. The luminosity threshold may be defined through known thresholding algorithms for defining the optimum threshold (like the Otsu algorithm), or empirically. All pixels having a luminosity greater than that luminosity threshold are set equal to 1 , the others equal to 0.

The binary image thus obtained is subsequently processed, according to known filtering algorithms, to reduce disturbances and noise.

Lastly, an algorithm, selected among known ones, has the task of separating (extracting) the individual ALS, assigning to each of them an identification label represented by a numeric code. The algorithm generates an indexed image, assigning 0 to the background and different numeric values to each cluster of pixels different from 0, and, therefore, in the specific case to each ALS.

A further step of the method according to the present invention provides than each ALS comprised in the binary image be recognized and identified, preferably through comparison with a database, pre-existing or previously created ad hoc. By 'ALS database' a list of all ALS is meant, each of which characterized by its coordinates (X₁Y₁Z) related to a suitable reference system (e.g., that integral to the runway with x- axis coincident with the center line, z-axis upward and y-axis according to a right- handed Cartesian triad, centered in the center of the runway) and by an identification number.

Identification occurs through recognition of a label placed in the vicinity thereof. The label is positioned before the measuring campaign, or it can be permanent.

Then, to each ALS an ID and its spatial coordinates are associated.

Through a photogrammetric triangulation process, the position of the measuring apparatus (camera and ducted luxmeter) is calculated in terms of spatial coordinates (in the runway reference system) and orientation angles (elevation, azimuth and rotation). The photogrammetric triangulation process consists in the solving by iteration of a spatial coordinates variation system in which it is calculated the position and orientation of a movable reference system (integral to the measuring instrumentation) with respect to the stationary reference system (integral to the runway and described above). Calculation is performed through knowledge of the position of at least three ALS, both in the stationary reference system and in the photographic reference system. Actually, the position of each ALS on the photo provides the equation of the half line traversing the origin of the movable system (coinciding with the lens focus) and the point on the sensor in which there is the photosite corresponding to the pixel singled out as center of the ALS found with the segmentation algorithm. In other words, mathematical equations are solved in order to know the accurate geometry of the measuring system with respect to the various ALS.

This procedure of self-determining the position and the relative orientation of the measuring system with respect to ALS may be replaced by a predetermining, e.g., topographic, of the same orientation and position; however, the former allows a greater speed of execution of the measuring, reducing the runway engagement times.

Upon knowing the position of the measuring apparatus, it is possible to single out in the image the ALS meeting some predetermined requisites, e.g. a given distance from the camera, a given position on the image, absence of overlapping with other lamps due to perspective effect. This stage is based on a calculation of coordinates (in pixels) on the photo reference of each ALS present in the database.

Upon singling out the ALS of interest, for each of them it is possible to calculate the sum of the luminous intensity of the pixels constituting it.

Referring to said sum as S, the method provides a calculation of the ratio K between the value in lux measured at the time of image acquisition and the sum S' of the luminous intensities of all pixels corresponding to the area enclosed in the cone of sight of the luxmeter,

K=Lum/S\

Upon knowing this parameter K (potentially different at each acquisition, due to the reasons highlighted above with regard to uncertainties related to the individual photographic take) it is possible to calculate the illumination contribution of the photographic sensor related to each ALS present in the photo.

Then, if the sum of the luminous intensities of the pixels constituting an ALS of interest is S, then the illumination contribution Lum(S) due to that ALS will be:

Lum(S) = S*K.

Through simple geometric relationships, upon knowing the nature of the photometric quantities that are utilized it is possible to calculate the value in candles of the luminous intensity related to each ALS of interest. This quantity, however, is referred to the angular position (in elevation and azimuth) at which the measuring apparatus lies with respect to the ALS (in the ALS reference system). In practice, luminous intensity lnt is calculated according to the known lighting technique relationship:

lnt = Lum(S) * R² / cos (ø)

Where: R is the distance between ALS and measuring apparatus, and φ is the orientation angle of the measuring apparatus with respect to the ALS.

To determine the color of beam light, weighted mean will be calculated according to the luminance of each single photosite of the chrominances of all photosites comprised in the segmented area.

Therefore, by performing subsequent measuring sessions (image acquisition and illumination measuring) with the measuring apparatus differently positioned, isocandela curves may be determined for each ALS.

At each acquisition a value of luminous intensity is obtained, in candles, of a high number of ALS, each referring to a pair of elevation and azimuth values.

The matrix of the intensity values, in candles, of each ALS (where azimuth angle on the X-axis and elevation on the Y-axis are reported) is updated with the value calculated at the preceding point. In practice, in each measuring session, for some ALS (those in the useful measuring field) luminous intensity values are calculated in a specific direction (elevation and azimuth) with respect to individual ALS positions. After plural measuring sessions, in the database of each ALS there will be found a series of values of luminous intensity, corresponding to specific directions in elevation and azimuth, constituting by points the measure of the photometric solid of the ALS itself.

Lastly, though an aspect to be considered of particular relevance, the method according to the present invention provides a stage of verifying the acceptability according to predetermined rules, in particular according to the provisions in force, of the signaling efficiency of the individual ALS and/or of constructing the actual isocandela curves.

In practice, after having carried out a battery of measurings with a sufficiently dense and complete direction distribution, data from the measuring, recorded in the database for each ALS mentioned above, need to be processed, through interpolations, to obtain intensities and colors in the reference directions, according to what has been set by efficiency checking provisions, or to reconstruct the isocandela curves with the desired resolution.

Evidently, a system according to the present invention further comprises means for processing data acquired by said acquisition and measuring apparatus.

Said data processing means comprising, e.g., a processor, on which there could be executed one or more software procedures, overall implementing a processor program, stored on CD, hard disk or any other storage medium, apt to implement a method as the one described hereto.

The present invention has been hereto described with reference to a preferred embodiment thereof. It is understood that other embodiments might exist, all falling within the concept of the same invention, and all comprised within the protective scope of the claims hereinafter.

Claims

1. A method for measuring photometric quantities related to illuminating objects, comprising the steps of:

providing an acquisition and measuring apparatus, comprising a digital image acquisition device and a illumination measuring device;

acquiring, by means of said digital image acquisition device, a digital image, comprised of a plurality of pixels, reproducing one or more of said objects; determining a relative position between acquisition and measuring apparatus and illuminating objects;

performing a measuring with the illumination measuring device, concomitant to image acquisition, said measuring device having an angle of sight limited in a manner such as to measure exclusively the illumination of a portion of the scene corresponding to the acquired image, obtaining a value (Lum) expressed in lux;

processing said acquired image for extracting an information on the luminous intensity of each pixel of said digital image, obtaining a binary image;

calculating a ratio K between said illumination value (Lum) and the sum of the luminous intensities of all pixels of the image corresponding to said scene portion;

extracting one or more objects of interest from said binary image, and for each object of interest extracted:

• calculating the sum S of the intensities of the pixels belonging to the object of interest;

• calculating the illumination (Lum(S)) related to the (recognized) object of interest, as Lum(S)=S*K, and

• calculating a value of luminous intensity (Int) corresponding to Lum(S), expressed in candles, through predetermined geometric relationships, based on said relative position

• calculating the color of emitted light.

2. The measuring method according to claim 1 , wherein said step of processing said image for extracting the information on the luminous intensity and the color of each pixel of said digital image, obtaining a binary image, comprises the stages of:

coding the acquired digital image in a color space, obtaining a polychromatic image;

converting the polychromatic image into a monochromatic image, weighing each color with a coefficient peculiar to the color space, and

segmenting the monochromatic image to separate from the background the objects of interest;

calculating for each object of interest the color as weighted mean of the color of each pixel.

3. The measuring method according to claim 2, wherein said stage of segmenting the monochromatic image provides defining a luminosity threshold and setting all pixels having luminosity greater than that threshold equal to 1 , the others equal to 0.

4. The measuring method according to claim 2 or 3, wherein said color space is a standard color space.

5. The measuring method according to any one of the preceding claims, further comprising a step of filtering said binary image, to reduce disturbance and noise.

6. The measuring method according to any one of the preceding claims, wherein said step of extracting one or more objects of interest from said image, comprises the stages of:

associating, in a database, to each illuminating object, an identification label and the respective spatial coordinates;

recognizing each object of interest reproduced in the image, through a search in said database;

7. The measuring method according to claim 6, wherein each of said labels is placed in the vicinity of a respective illuminating object and comprises a graphic code.

8. The measuring method according to any one of the preceding claims, further comprising a step of determining the position of the acquisition and measuring apparatus, with respect to the recognized objects of interest.

9. The measuring method according to claim 8, wherein said determining is performed by photogrammetric triangulation, calculating spatial coordinates and orientation angles of the acquisition and measuring apparatus.

10. The measuring method according to any one of the preceding claims, further comprising a step of determining respective isocandela curves of each recognized object of interest.

11. The measuring method according to any one of the preceding claims, wherein the calculation of the color of the emitted light is performed for each illuminating object.

12. The measuring method according to claim 10 or 11 , wherein said determining is performed by means of analysis of a series of acquisitions and measuring performed from different positions of the acquisition and measuring apparatus.

13. The measuring method according to any one of the preceding claims, further comprising a step of comparing the luminance and color values obtained for each object of interest with predefined values, in order to verify the acceptability thereof on the basis of predetermined rules.

14. A system for measuring photometric quantities related to illuminating objects, comprising an acquisition and measuring apparatus comprising a digital image acquisition device apt to acquire a digital image of a scene, and a device for measuring the illumination of said scene, made integral therebetween.

15. The system according to claim 14, wherein said digital image acquisition device comprises a digital camera.

16. The system according to claim 15, wherein said digital camera is of reflex type.

17. The system according to claim 15 or 16, wherein said digital camera has a 15Mpixel CMOS sensor.

18. The system according to one of the claims 14 to 17, wherein said digital image acquisition device comprises a 60mm fixed-focal lens.

19. The system according to one of the claims 14 to 18, wherein said measuring device comprises a luxmeter.

20. The system according to one of the claims 14 to 19, wherein said measuring device has an angle of sight limited in a manner such as to measure exclusively the illumination of a portion of the scene corresponding to the image acquired by means of said digital image acquisition device.

21. The system according to one of the claims 14 to 20, further comprising means for processing data acquired by said acquisition and measuring apparatus said data processing means comprising a processor and one or more software procedures apt to be executed on said processor for implementing a method according to any one of the claims 1 to 13.

22. A processor program stored on a storage medium, apt to implement a method according to any one of the claims 1 to 13.