WO2005026672A1

WO2005026672A1 - Apparatus and method for determining the light intensity of a traffic signal head

Info

Publication number: WO2005026672A1
Application number: PCT/NL2004/000624
Authority: WO
Inventors: Jan Varkevisser
Original assignee: Nederlandse Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek Tno
Priority date: 2003-09-12
Filing date: 2004-09-10
Publication date: 2005-03-24
Also published as: NL1024282C2

Abstract

An apparatus for determining the light intensity of a traffic signal head, wherein the apparatus (1) comprises a housing (2) placeable on the traffic signal head (20), which housing (2) is provided with an opening (3) for receiving light generated by the traffic signal head (20), the apparatus (1) being provided with an optical system (5, 6, 7, 8) and a light detector (9), the optical system (5, 6, 7, 8) being designed for depicting at least a part of the light (L) received via said opening (3) on the light detector (9), the optical system comprising at least a first lens (5) designed for focussing at least a part of the light (L) received via said opening (3). The invention further provides the use of such an apparatus, and a method for determining the light intensity of a traffic signal head.

Description

Title: Apparatus and method for determining the light intensity of a traffic signal head.

The invention relates to apparatus for determining the light intensity of a traffic signal head. From US patent 5,185,637 a portable apparatus is known for measuring the light intensity of a traffic signal head. The known apparatus is provided with a housing having an opening at its front side for collecting light emanating from the traffic signal head. In the housing of the known apparatus, behind the opening, successively, a screening window, a honeycomb optical grid, three photocells provided with colour filters and a parabolic focusing reflector are arranged. Between the honeycomb filter and the focusing reflector, a fourth photocell, not provided with a colour filter, has been placed. During use of the known apparatus, the housing is placed on the traffic signal head. The light generated by the traffic signal head falls, via the screening window, on the honeycomb filter and the three first mentioned photocells. With the aid of these three photocells, the colour of the incident light is determined. The collected light is transmitted by the honeycomb grid and then falls on the focusing reflector. The focusing reflector then focuses the light entering thereon on the fourth light detector for measuring the eventual intensity of this light. A drawback of the known apparatus is that it is relatively complex and expensive. For instance, the known apparatus has relatively many parts and a relatively complex control. Further, the known apparatus is relatively inaccurate. A relatively large part of the light to be measured, generated by the traffic signal head is, for instance, absorbed before the light falls on the fourth photocell. This light absorption is caused, for instance, by the use of the first three photocells arranged in the optical path of the light to be measured, by the edges of the honeycomb grid and by a rear side of the fourth photocell extending in the optical path. In addition, the light received by the known apparatus enters the fourth photocell relatively obliquely. This can lead to relatively large measuring deviations, in particular when the traffic signal head to be tested generates light which is not homogenously distributed. Further, the focusing reflector can scatter and/or absorb a relatively large part of the light incident thereon, which further increases the measuring error.

Furthermore, the focusing reflector can have a great influence on the spectrum of the light to be focused. In addition, the known apparatus is not well capable of testing whether a traffic signal head meets European and Dutch standards for traffic signal heads, for instance, the NEN-EN 12368 standard. Moreover, the known apparatus is not sufficiently capable of universally testing different traffic signal heads that can be provided with light sources witfci different spectra. The object of the present invention is to provide an improved apparatus for determining the light intensity of a traffic signal head, and in particular an apparatus with which this light intensity can be determined relatively accurately. To this end, the apparatus according to the invention is characterized by the features of claim 1. The apparatus provided by the invention comprises a housing, placeable on the traffic signal head, which housing is provided with an opening for receiving the light generated by the traffic signal head, the apparatus being provided with a light detector and an optical system. The optical system is designed for depicting at least a part of the light received via th.e opening, on the light detector. As the optical system comprises at least a first lens designed for focusing at least a part of the light received via said opening, the apparatus can focus the light in a simple manner without using a reflector to that end. Due to the use of this first lens, it can, for instance, be avoided in a simple manner that this detector is to be arranged with a rear side in the optical path of the incident light. Preferably, the detector comprises a spectrally calibrated detector, in particular a V(λ) corrected detector. According to the invention, it is advantageous when the optical system is designed for depicting on the light detector substantially only the light entering parallel to an optical axis of this optical system. As a result, only the light emitted substantially perpendicularly by the traffic signal head can be detected by the detector, at least with a correct positioning of the optical system relative to the traffic signal head, so that the light intensity of the traffic signal head can be tested particularly accurately and in a well reproducible manner. According to an advantageous elaboration of the invention, the apparatus is provided with a diaphragm for transmitting substantially only the light focussed by the first lens on an image point located in an image focal plane of that lens. What can thus be achieved in a relatively simple manner is that only a substantially collimated light beam, entering the first lens, is passed to the detector. Preferably, the diaphragm is designed for transmitting only the light of the traffic signal head focussed on an image point of the first lens. As a result, the light entering the first lens substantially perpendicularly and which emanates from the traffic signal head can simply be separated from the light entering from remaining directions in order that only the light emitted substantially perpendicularly by the traffic signal head can be passed the detector. The invention further provides the use of an apparatus according to any one of claims 1 - 15 for determining the light intensity of a traffic signal head. By using this apparatus for determining the light intensity of a traffic signal head, the light intensity of the traffic signal head can be tested in a relatively accurate, rapid and simple manner. Further, the invention relates to a method for determining the light intensity of a traffic signal head. Such a method is also known from US 5,185,637. This known method is relatively inaccurate, expensive and complex. The present intention contemplates a method with which the light intensity of a traffic signal head can be determined in a relatively easy manner. To this end, the method according to the invention is characterized in that light generated by the traffic signal head is collected by an optical system arranged in front of the traffic signal head, while the collected light is focused by a first lens of the optical system while, thereupon, the light is depicted on a light detector. In this manner, the light intensity of the traffic signal head can be determined in a simple manner near the traffic signal head, so that during light measurement, the traffic signal head can remain in a position of use. Preferably, only a light beam issuing substantially perpendicularly from the traffic signal head is depicted, preferably in collimated condition, by the optical system on the light detector. Thus, the light intensity of the traffic signal head can be determined relatively accurately while using the inverse square law for point sources. This inverse square law reads as follows: I=Er², wherein I is the light intensity of the light source (cd), E is the illuminance (lx) measured by the detector and r is the distance to the light source (m). This formula approximately holds for light sources arranged at a very large distance from the light detector. From the inverse square law it follows that the light intensity I is at least approximately directly proportional to the measured illuminance E at a determined fixed measuring distance. With the method according to the invention, for instance, it can be determined in a simple manner whether a traffic signal head meets the NEN-EN 12368 standard, without, to that end, a light detector needing to be arranged at a large distance from the traffic signal head, for instance in a darkened laboratory space. Further elaborations of the invention are described in the subclainαs. Presently, the invention will be clarified with reference to two exemplary embodiments and the drawing. In the drawing: Fig. 1 shows a schematic longitudinal cross-section of an exemplary embodiment of the invention in a position of testing; Fig. 2 shows a front view of the exemplary embodiment represented in Fig. 1; Fig. 3 shows a rear view of a second exemplary embodiment of the invention; and Fig. 4 shows a cross-sectional view along the line IV-IV of the rear view represented in Fig. 3. Unless expressly stated otherwise, in this application, "approximately", "virtually", "substantially" or such terms are at least understood to include a value deviating plus and minus 10% of the given respective value. In the present application, identical or corresponding parts are indicated with identical or corresponding reference numerals. Figs. 1 — 2 show a first exemplary embodiment of a portable apparatus 1 for determining the light intensity of a traffic signal head 20. SuLch a traffic signal head is known from practice. The traffic signal head 20 to be tested can, for instance, form part of a traffic signal light. The traffic signal head 20 can be designed in various manners and in various sizes. As shown in Fig. 1, the traffic signal head 20 comprises in particular an outer lens 21 for focusing and radiating light generated by a traffic signal head light source (not shown) in a desired direction. The traffic signal head shown is designed for radiating a light beam substantially perpendicularly, at least substantially parallel to an optical axis of the outer lens 21. Herein, the term "substantially parallel" is at least understood to include light making an angle with the optical axis of the outer lens 21 in the range of approximately -10° to + 10°, in particular in the range of approximately -5° to +5°. Various light sources can. be used in the traffic signal head 20, for instance one or more light bulbs, LEDs, halogen lamps and/or Krypton lamps. For the sake of clarity, such lights sources have not been represented in the drawing. The apparatus 1 comprises a housing 2, placeable, preferably manually, in a position of testing on the traffic signal head 20. On a rear side, the housing 2 is provided with a handle 14 for holding the housing 2. This position of testing is represented in Fig. 1. The housing 2 is of tubular design and comprises, in particular, a first tube part 11 and a second tube part 12. Preferably, an inside of the housing is of light- absorbing design, for instance of a mat black colour, so that undesired internal reflections can be prevented. On a front side 13, placeable on the traffic signal head 20, the first tube part 11 of the housing 2 is provided with an opening 3 for receiving substantially all the light emitted by the traffic signal head 20. In the first exemplary embodiment, this opening 3 is closed off by a transparent window 4 for protecting the inside of the housing 3 against pollution. The opening 3 of the housing 2 has a diameter which is at least as great as the diameter of the outer lens 21 of the traffic signal head 20 to be tested. In the second tube part 12 of the housing 2, a light detector 9 is arranged for determining the intensity of the light L received via the opening 3. The housing 2 is further provided with an optical system 5, 6, 7, 8 placed between the light detector 9 and the opening 3. This optical system is designed for depicting a part of the light incident via the opening 3 on the light detector 9. The optical system 5, 6, 7, 8 is particularly designed for depicting on the light detector 9 substantially only the light L entering on the light detector 9 parallel to an optical axis of the system. This optical axis is represented in Fig. 1 with a dotted line O. Herein, the term "substantially parallel" is at least understood to include light making an angle in the range of approximately -10° to + 10°, in particular in the range of approximately -5° to + 5° with the optical axis O of the optical system. Viewed from the opening 3 to the detector 9, the optical system is provided, successively, with a first positive lens 5, a first diaphragm 6, a second diaphragm 7 and a second positive lens 8. The first lens 5 is designed for focusing substantially all the light L received via the opening 3. To this end, the first lens 5 is arranged in the first tube part 11, adjacent the opening 3 in the housing 2, with the first lens 5 extending parallel to the opening 3. Alternatively, the first lens 5 can, for instance, be placed in the opening 3, while no separate screening window 4 has been provided. Further, the diameter of the first lens 5 is substantially equal to the diameter of the opening 3, and is therefore at least as great as the diameter of the outer lens 21 of the traffic signal head 20. Preferably, the first lens 5 is a Fresnel lens known from practice. Such a lens is relatively lightweight and flat. It is further advantageous when the first tube part 11 is detachably connected to the second tube part 12, in order that the dimensions of the housing 2 and/or the opening 3 of the apparatus can be adjusted to the dimensions of the traffic signal head 20. The first tube part 11 can for instance be supplied in different sizes, depending on the different elaborations of the traffic signal heads 20 to be tested. During use, each time, a first tube part 11 matching the traffic signal head 20 can be placed, with the associated first lens 5, on the second tube part 12. The detachable coupling between the tube parts 11, 12 can be designed in different manners, for instance as a clamping connection, a snap connection, screw connection or the like. For the purpose of the depiction mentioned, the first lens 5 is arranged in the housing 2 such that the first lens 5 is substantially parallel to the lens 21 of the traffic signal head 20 to be tested when the housing 2 has been placed on the traffic signal head 20 in a position of testing. The first diaphragm 6 is arranged behind the first lens 5 to reduce the exit pupil of the first lens 5. The first diaphragm 6 serves, inter alia, for stopping light scattered in the first tube part 11, and a part of the light entering non-parallel to the optical axis O of the optical system. The second diaphragm 7 extends in the optical center of the first lens 5, in the image focal plane B of this lens 5. The second diaphragm is designed for transmitting substantially only the light focused by the first lens 5 on one image point in the focal plane B of the lens 5. To this end, the second diaphragm 7 is provided with a relatively narrow light passage 7'. The second diaphragm 7 effects that substantially only a collimated light beam entering on the first lens 5 is transmitted to the light detector 9. The light passage 7' of the second diaphragm 7 has, for instance, a diameter of less than approximately 6 mm, more in particular less than approximately 3 mm. The diameter of the light passage 7' can for instance comprise l/100^th of the focal distance of the first lens. With such a narrow light passage 7', a relatively accurate light intensity measurement can be carried out. In the present exemplary embodiment, the above mentioned image point of the first lens 5, located in the image focal plane, comprises the image focal point B of this lens 5. This image focal point is the intersection of the optical axis O of the optical system and the image focal plane B of the first lens 5. Hence, in a particularly simple manner, only the light beams emitted perpendicularly by the traffic signal head 20, at least the light beams entering substantially parallel to the optical axis O of the optical system on the first lens 5, are depicted on the light detector 9. As a result, the light intensity of the traffic signal head 20 can be determined relatively accurately based on the inverse square law for the light intensity of a point source. The second positive lens 8 is designed and positioned for collimating the part of the light L transmitted by the second diaphragm 7 and focused by the first lens 5. To this end, an object focal point of the second lens 8 coincides with the focal point of the first lens 5. The first and second lens 5, 8 together form a collimator. The second lens 8 is designed such that the light beam issuing therefrom has a cross-section equal to or smaller than a light detection surface of the light detector 9 facing the second lens. As a result, substantially all the light L transmitted by the second diaphragm 7 can be projected substantially perpendicularly on the detector 9. The light detector 9 is designed to produce a light detection signal that is dependent on the intensity of the light detected by the light detector 9. The detector 9 is arranged relative to the optical system 5, 6, 7, 8 such that the light transmitted by the optical system falls substantially perpendicularly on the light detection surface of the detector 9. As a result, the apparatus is relatively insensitive to inhomogeneities of light emitted by the traffic signal head 20 to be tested. According to the invention, it is particularly advantageous when the light detector comprises a V(λ) corrected detector 9, in particular one V(λ) corrected light-sensitive cell 9. Such a detector is known per se from practice and can be designed in different manners, for instance with a semiconductor detector, a V(λ) corrected filter and the like. Through the use of one (λ) corrected light cell, the apparatus can be designed in a simple and inexpensive manner. During use, the V(λ) corrected detector 9 automatically measures the light intensity of the traffic signal such as observed by the human eye, at least according to the V(λ) standardized eye sensitivity curve, independently of the colour and the spectrum of the light emitted by the traffic signal head. As a result, the apparatus 1 can test various types and colours of traffic signal heads universally as to light intensity. The apparatus 1 is further provided with a control (not represented in Figs. 1 and 2), for instance a microcontroller, computer, PDA, luxmeter and/or the like, for receiving the detection signal of the detector 9 and process it into a test result. Preferably, such electronics are wholly or partly provided in a splash-proof space or splash-proof housing. Such a splash-proof space can for instance be a chamber or the like provided on the rear side of the housing 2. Furthermore, the apparatus can be provided with a display (not shown) for indicating a test result determined by the control. A feed for feeding this control, light detector, display and/or other parts of the apparatus 1, for instance one or more accumulators, batteries or the like, can also be provided in housing 2 and/or be detachably coupled to the housing. For the sake of clarity, such a feed is not represented in the drawing. On the front side, the housing 2 of the present exemplary embodiment is provided with three interspaced positioning sensors 10 designed to generate a positioning signal when the housing 2 has been positioned in the desired position of testing on the traffic signal head 20. These positioning sensors can be designed in various manners. In particular, these sensors 10 are each designed to detect the traffic signal head 20 at least under the influence of touch, in particular pressure. To this end, the positioning sensors 10 can comprise for instance, micro switches. Further, the positions of the positioning sensors 10 relative to the housing 2 can for instance be controllable or adjustable, which will be elucidated hereinbelow with reference to a second exemplary embodiment. Preferably, the positioning sensors 10 are coupled to the control of the apparatus 1, while the control is designed to start the measurement of light intensity only after receipt of the positioning signals of each of the positioning sensors 10. As shown in Fig. 1, during use, the housing 2 of the apparatus is placed on the traffic signal head 20 to be tested such that the optical system 5, 6, 7, 8 of the apparatus 1 is arranged in front of the traffic signal head 20 and the optical axis of the lens 21 of the traffic signal had 20 coincides with the optical axis O of that optical system. The light emitted through the outer lens 21 of the traffic signal head 20 is then completely collected via the opening 3 in the housing 2 of the apparatus 1. The light L entering via the opening 3 is depicted by the optical system 5, 6, 7 8 on the light detector 9 for determining the light intensity of the traffic signal head 20. The optical system 5, 6, 7, 8 and the detector 9 are then screened off from ambient light by the housing 2, so that the measurement cannot be influenced by ambient light. Due to the design of the optical system 5, 6, 7, 8 substantially the entire light beam issuing substantially perpendicularly from the traffic signal head 20 is depicted in a collimated manner, substantially perpendicularly to the light detector 9. The light emitted by the traffic signal head 20 in other directions is substantially collected by the optical system 5, 6, 7, 8. Moreover, a lens 5 is used for focussing the light in the housing 2. Therefore, the measurement can be carried out accurately. Preferably, the light intensity measurement is carried out automatically by the control of the apparatus 1 once the housing 2 has been positioned perpendicularly on the traffic signal head 20 and the control of each of the positioning sensors 10 receives a positioning signal. During measurement, the control receives the light detection signal from the Light detector 9. Then, the control can transmit the measured light detection signal, processed or not processed, to a user. When using a V(λ) corrected ligh-t detector 9, the light value measured via the light detector 9 can simply be multiplied by one constant already determined experimentally by calibration, for calculating the light intensity of the traffic signal head. This constant appears to be substantially independent of the colour and/or the spectrum of the measured light, so that different types of traffic signal heads can simply be tested with the same apparatus 1. Then, based on the measured light detection signal and/or calculated light intensity, it can be determined whether the light intensity of the traffic signal head meets the requirements imposed thereon. Figs. 3 and 4 show a second exemplary embodiment, differing from the exemplary embodiment represented in Figs. 1 and 2 in that the positioning sensors 10 have been provided on telescopically slideable, axially adjustable legs 151. These legs 151 are slideably arranged in longitudinal tubes 150 coupled to the housing 102. In the exemplary embodiment, four such legs 151 and associated longitudinal tubes 150 are provided, naturally, also a different number thereof can be used. Preferably, at least three of these four slideable legs 151 are provided with an associated positioning sensor 110. The position of a part of one of these legs 151 has been indicated in Fig. 4 in dotted lines. In this manner, the positioning sensors 110 can be positioned in axial direction X. Due to such an axial adjustment of the positions of the positioning sensors 110 relative to the housing 110, a space suitable for measurement can be created between the housing 110 and the traffic signal head 20 to be measured. Each of the sensors 110 can, for instance, be axially adjustable in only a limited, discrete number of positions, for instance two or more. Further, adjacent the front, at the top side, the second exemplary embodiment is provided with two spacers 119, provided eccentrically adjacent ends of the two upper slideable legs 151. The spacers 119 which, in the exemplary embodiment, are of rectangular design, are pivotally coupled to the respective legs 151 in a direction of rotation indicated in Fig. 3 with arrows . During use, the position of each spacer 119 can be adjusted for placing the apparatus 101 in a centered manner on a traffic signal liead, while the spacers 119 bear against, for instance, a sunshade of th.e traffic signal head. Further, it is preferred that each spacer is provided with information, for instance text, comprising names of different brands or manufacturers of traffic signal heads, for indicating in which position the spacer 119 is suitable for use with which type of traffic signal head. Further, the second exemplary embodiment is provided with a holder 113 mounted on the housing 102, which holder is designed for detachably coupling a control 114 to the apparatus 101. Preferably, this control comprises a portable PDA (personal digital assistant) known per se, integrally provided with a display, a computer, input means, communication means and the like. Preferably, the PDA 114 is provided in a watertight housing 118. By means of one or more electronic connections (not represented), the PDA 114 is coupled to the positioning sensors 110 and the light cell 109. Preferably, the position of the PDA is adjustable. To this end, the PDA can be connected via a ball and socket joint 116 or the like to the housing 2 such that, during use, the display of the PDA can be brought to a position in which it can be read by a user. During use, the PDA 114 can serve for giving the user indications, for controlling the measuring process and/or monitoring the measuring process. The PDA can, for instance, give information to a user via its display and/or by means of sound signals and/or speech. The PDA can, for instance, indicate whether and when the positioning sensors 110 have been positioned on the traffic signal head 20 in the correct manner. Further, the PDA can, for instance, automatically start measuring when the positioning sensors 110 have been positioned correctly on the traffic signal head. In the PDA, various computer programs can be loaded. The PDA can for instance be loaded with a calibration computer program for rendering the PDA suitable for calibrating itself to a particular type of traffic signal head when the apparatus 101 has been placed on a specimen of that type. With such a calibration, the light intensity of the traffic signal head is for instance recorded in a calibration data file. Preferably, the calibration is carried out in conditioned laboratory surroundings. The calibration program can for instance control the apparatus 101 so as to carry out a measurement according to the European standard NEN 12368. Such a calibration computer program can for instance be secured with a password, and further be designed for inputting and adding identification data such as type and brand about a type of traffic signal head to be measured to the calibration data file. Further, the PDA can for instance be loaded with a program for back-up and restore possibilities, to prevent calibration data and/or computer programs present from becoming lost. Further, the PDA can for instance be loaded with a user computer program, in order that a user can use the apparatus 101 in an outside situation for checking traffic signal heads. The user program can for instance be designed to use the abovementioned data file containing calibration data. The user program can then for instance render the PDA suitable for giving a user only reading access to that data file, in order that the user can first search and select a type of traffic signal head to be measured in the file, to then test a specimen of the traffic signal head with, the apparatus while using the associated calibration values already stored. Further, the user program can for instance be designed to indicate to a user in which position the sliding legs 151 are to be brought and in which position the spacers 119 are to be brought for measuring a particular type of traffic signal head. It is self evident that the invention is not limited to the exemplary embodiments described. Various modifications are possible within the framework of the invention as outlined in the following claims. For instance, the optical system can be provided with various optical elements, for instance one or more lenses, diaphragms, glass fibre cables, mirrors or the like. Such optical elements can each be designed in various manners. Further, the housing 2 of the detector can be manufactured from various materials. The housing 2 can for instance be stably designed in one piece. The housing 2 can further for instance be manufactured from carbon fibre. Further, various parts of the apparatus can for instance be of replaceable design, for instance for replacing a damaged part. The mirror 4 for instance can be designed so as to be replaceable. Further, in the housing 2 for instance, a substantially light-tight sealed off compartment can be provided such as a mat black inside, in which compartment a second lens 8 and a light cell 9 have been provided. Such a compartment is represented in Fig. 4 with reference numeral 130. In this manner, the light cell 9 can be screened off well against scattered light from the first lens 5. Further, the apparatus can be provided on various locations with handles 14, 114 for manually positioning the apparatus. Also, one or more handles can be provided on a longitudinal side of the housing 102, as is represented in Fig. 3. The handles 14, 114 can also be manufactured from various materials, for instance plastic and/or other materials. Further, for instance a holder can be provided, which is specifically designed for storing and/or transporting the present apparatus in a safe and stable manner. Such a holder, which is not represented in the Figures, can further be designed specifically for storing the control of the apparatus and/or the instructions for use. The above-mentioned control can be designed in various manners, for instance as PDA or otherwise, and be loaded with various types of computer programs and data files so as to render the apparatus 1, 101 suitable for a desired calibration, traffic signal head test or the like.

Claims

1. An apparatus for determining the light intensity of a traffic signal head, wherein the apparatus (1) comprises a housing (2) placeable on the traffic signal Jαead (20), which housing (2) is provided with an opening (3) for receiving light generated by the traffic signal head (20), the apparatus (1) being provided with an optical system (5, 6, 7, 8) and a light detector (9), the optical system (5, 6, 7, 8) being designed for depicting at least a part of the light (L) received via said opening (3) on the light detector (9), the optical system comprising at least a first lens (5) designed for focussing at least a part of the light (L) received via said opening (3).

2. An apparatus according to claim 1, wherein the optical system

(5, 6, 7, 8) is designed for depicting on the light detector (9) substantially only the light (L) entering parallel to the optical axis (O) of the optical system.

3. An apparatus according to claim 1 or 2, wherein the optical system is provided with a diaphragm (7) for transmitting substantially only the light focused by the first lens (5) on one image point located in an image focal plane (B) of tliat lens (5).

4. An apparatus according to claim 3, wherein said image point of the first lens (5) is located in a light passage (7') of said diaphragm (7), which light passage (7') has a diameter of less than approximately 6 mm.

5. An apparatus according to claim 3 or 4, wherein said image point is an image focal point of the first lens (5).

6. An apparatus according to any one of the preceding claims, wherein said opening (3) of the housing (2) is designed for receiving substantially all light emitted by the traffic signal head (20), while the first lens (5) is designed for focusing substantially all light received via said opening (3).

7. An apparatus according to any one of the preceding claims, wherein said first lens (5) is arranged in the housing (2) adjacent or in said opening (3).

8. An apparatus according to any one of the preceding claims, wherein the first lens (3) comprises a Fresnel lens.

9. An apparatus according to any one of the preceding claims, wherein said first lens (5) has a diameter which is at least as great as the diameter of an outer lens (21) of the traffic signal head (20).

10. An apparatus according to claim 9, wherein said first lens (5) is arranged such that during use of the apparatus, the first lens (5) is substantially parallel to the outer lens (21) of the traffic signal head (20).

11. An apparatus according to any one of the preceding claims, wherein the optical system is provided with a second lens (8) designed and positioned for collimating at least a part of the light (L) focused by the first lens (5).

12. An apparatus according to any one of the preceding claims, wherein said light detector comprises a V(λ) corrected detector (9), the light detector (9) comprising, in particular, one V(λ) corrected light-sensitive cell.

13. An apparatus according to any one of the preceding claims, wherein the housing (2) is provided with positioning sensors (10) designed for generating a positioning signal when the housing (2) has been positioned on the traffic signal head (20) in a desired position of testing.

14. An apparatus according to claim 13, wherein the positioning sensors (10) are each designed to detect the traffic signal head (20) in particular at least under the influence of touch, more in particular of pressure.

15. An apparatus according to any one of the preceding claims, wherein said optical system (5, 6, 7, 8) is arranged in the housing (2) of the apparatus (1), while said detector (9) is, preferably, also arranged in said housing (2).

16. The use of the apparatus according to any one of the preceding claims for determining the light intensity of a traffic signal head.

17. The use according to claim 16, wherein said housing (2) is placed on the traffic signal head (20) such that light generated by the traffic signal head (20) is received via said opening (3) in the housing (2), while at least a part of the light (L) collected via said opening (3) is depicted by the optical system (5, 6, 7, 8) on the light detector (9) for determining said light intensity.

18. The use according to claims 16 or 17, wherein the dimensions of said opening (3) and/or said housing (2) are adjusted to the dimensions of the traffic signal head (20).

19. A method for determining the light intensity of a traffic signal head, wherein light generated by the traffic signal head (20) is collected by the optical system (5, 6, 7, 8) arranged in front of the traffic signal head (20), while the collected light (L) is focused by a first lens (5) of the optical system, the light (L) thereupon being depicted on a light detector (9).

20. A method according to claim 19, wherein only one light beam issuing substantially perpendicularly from the traffic signal head is depicted, preferably in collimated condition, by the optical system (5, 6, 7, 8) on the light detector (9).

21. A method according to claim 19 or 20, wherein a diaphragm (7) is arranged between the first lens (5) and the detector (9), while an image focal point of said first lens (5) is in a light passage (7') of the diaphragm (7), said light passage (7') preferably having a diameter of less than approximately l/100^th of the focal distance of the first lens (5).

22. A method according to any one of claims 19 - 21, wherein a second lens (8) is arranged between the first lens (5) and the detector (9), while the light (L) focused by the first lens is collimated by a second lens (8).

23. An apparatus according to claim 13 or 14, wherein positions of the positioning sensors (110) relative to the housing (102) are controllable or adjustable.

24. An apparatus according to any one of claims 1 - 15, provided with a computer control (118) which control preferably comprises a PDA (personal digital assistant).