turning now to the drawings , fig1 is a schematic front elevational view of a traffic signal head incorporating the invention . as seen in this fig ., a traffic signal head generally designated with reference numeral 10 has a generally circular housing 11 for containing a plurality of individual led light sources 12 . light sources 12 are arranged in a suitable pattern within the periphery of housing 11 and are preferable mounted in an essentially co - planar manner . light sources 12 may be one of two different types - a first type consisting of an individual encapsulation package having two led elements — one red and one green ; and a second type consisting of an individual encapsulation package having three led elements - one red , one yellow , and one green . preferably , all sources 12 in a given housing 11 are identical . fig2 illustrates the first type of light source 12 . as seen in this fig ., an individual source 12 has a conventional light transparent envelope 15 which contains two separate led elements 16 , 17 . element 16 is a red light generating led , while element 17 is a green light generating led . the anodes of elements 16 , 17 are individually connected to conductive lead - out wires 21 , 22 , respectively . the cathodes of elements 16 , 17 are tied together and connected to common lead - out wire 23 . source 12 is preferably a type ssl - lx 50971gw dual led device available from lumex corp . of chicago , ill . as schematically depicted in fig2 in the lower portion thereof , when lead - out wires 21 and 23 are operatively connected to a driving circuit ( shown in fig3 ), only led 16 is activated and the source 12 generates red light only . when lead - out wires 22 and 23 are operatively connected to a driving circuit , only led element 17 is activated and the source 12 generates green light only . when all three lead - out wires 21 - 23 are operatively connected to a driving circuit , both elements 16 and 17 are activated to generate both red and green light . the combination of these two primary colors results in the generation of yellow light by source 12 . it is noted that envelope 15 acts as a focussing lens for light generated by led elements 16 , 17 , so that the light emanating from source 12 is emitted in a parallel fashion . consequently , the emitted light as viewed when both led elements 16 , 17 are activated is truly yellow . fig3 illustrates a driving circuit for a signal head using a plurality of led sources 12 of the first type shown in fig2 . as seen in this fig ., a plurality of sources 12 are connected in parallel with the anodes of led elements 16 connected to a first bus conductor 25 , the anodes of led elements 17 connected to a second bus conductor 26 , and the cathodes of both led elements 16 and 17 connected to a third bus conductor 27 . third bus conductor 27 is coupled to one terminal of a d . c . power source ( not shown ). first and second bus conductors 25 , 26 are individually coupled to the emitter of an associated power control transistor 30 , 31 , respectively . conventional traffic control signals termed “ red ”, “ yellow ”, and “ green ” are coupled via isolation diodes 33 - 36 to the control electrodes of transistors 30 , 31 , as shown . when a “ red ” control signal is active , transistor 30 is switched on , thereby permitting current to flow through led elements 16 to cause the generation of red light . when a “ green ” control signal is active , transistor 31 is switched on , thereby permitting current to flow through led elements 17 to cause the generation of green light . when a “ yellow ” control signal is active , both transistors 30 and 31 are switched on , thereby permitting current to flow through led elements 16 and 17 to cause the generation of yellow light . in order to provide a hue adjustment , a variable resistance 38 is provided in one of the first and second bus conductors 25 , 26 . by varying the resistance 38 , the relative amounts of current flowing through the two main circuit branches can be changed to provide a yellow color of the desired hue . if desired , individual variable resistances may be inserted in series with the individual led elements 16 , 17 to accommodate variations in the intensity of light generated by these elements , although present day leds usually do not require such fine tuning . fig4 illustrates an individual light source 12 of the second type . as seen in this fig ., each source 12 includes three separate led elements in a single encapsulation package : a red light generating led element 41 , a green light generating led element 42 , and a yellow light generating led 43 element . all three led elements 41 - 43 are physically mounted in close mutual proximity so as to simulate a pinpoint source . the cathodes of led elements 41 - 43 are coupled together and connected to a common lead - out wire 45 , which is coupled to power ground ( not illustrated ). the anodes of led elements 41 - 43 are individually coupled to lead - out wires 46 - 48 so that power can be applied independently to each of these elements . in a signal head using the fig4 type source , the driving circuit is configured to enable independent operation of groups of red led elements 41 , groups of green led elements 42 , and groups of yellow led elements 43 . the design of such a driving circuit is deemed well within the skill of the ordinary practitioner in the art . as will now be apparent , signal heads incorporating the invention enjoy a number of advantages over known signal heads . firstly , in the single signal head compartment application of the invention the size and weight of the overall device are substantially reduced due to the requirement for only one enclosure compartment for all light sources . more specifically , unlike known signal heads which require at least three separate light compartments , signal heads according to the invention may use only one compartment to house the light sources for all three colors . in addition , signal heads employing the first type of light source illustrated in fig2 reduce the number of individual led light elements required to provide a properly operative signal head , thereby saving space , weight and cost . further , signal heads utilizing the invention are fully compatible with existing traffic control signal generating equipment , using the same three color control signals which are present in all traffic control systems . both embodiments of the invention may be designed and used as universal light modules for signal heads . more particularly , as illustrated in fig5 standard signal heads typically have three compartments for . containing the light sources - one for the red source , one for the yellow source , and one for the green source . either embodiment of the invention can be physically installed in all three compartments of a standard signal head and operated as a dedicated red source , yellow source or green source by simply coupling the modules to the proper color control signal . thus , for example , the led elements of both types of light source can be electrically connected to provide red color signals alone , yellow color signals alone , or green color signals alone . the advantage of this application of the invention resides in the fact that only one single module need be inventoried for all the light source needs of a traffic control installation . the invention can also be configured to provide directional visual advisory signals , such as left or right turn arrows of any desired color or straight through arrows . fig6 illustrates a left turn arrow configuration . as seen in this fig ., a group of individual light sources 12 is geometrically arranged as a left - pointing arrow . to illuminate the arrow , the individual led light elements of the desired color in each individual source package are simply activated using the electrical control signal provided by the traffic control system . in addition , arrowed signals of two or three colors can be generated so that the left turn lane can exhibit red , yellow , and green arrows , if desired . fig7 illustrates one variation having a bi - directional red arrow and a bi - directional green arrow . as seen in this fig ., a signal head 10 is coupled to a power source by means of power conductors 51 , 52 . red , yellow , and green . color control signals are supplied to signal head 10 via control signal conductors 53 - 55 . color arrow control signals are supplied to signal head 10 via control signal conductors 56 - 59 . to activate a right - pointing red arrow advisory signal , a control signal is applied to conductor 56 . to activate a left - pointing red arrow advisory signal , a control signal is applied to conductor 57 . to activate a right - pointing green arrow advisory signal , a control signal is applied to conductor 58 . to activate a left - pointing green arrow advisory signal , a control signal is applied to conductor 59 . as will be apparent to those of ordinary skill in the art , the arrangement of fig7 may be expanded to include yellow arrow advisory signals , and may be contracted to provide single headed arrows , arrows of only one color , and the like . another modification to the arrow head module of fig6 is as follows . any of the modules can be configured to display directional arrows or other informative symbols by hard . wiring together the anodes of individual ones of the light sources 12 , and providing a dedicated lead out wire for receiving a control signal for activating the grouped sources . a standard module can thus incorporate several display functions , and the system user can select those functions which are appropriate for a given module installation by simply providing a suitable control signal to the control terminals for the functions to be activated and ignoring the rest of the built - in functions . thus , light sources according to the invention are user programmable . although the above provides a full and complete disclosure of the preferred embodiments of the invention , various modifications , alternate constructions and equivalents will occur to those skilled in the art . for example , other geometrical configurations than the circular shape described above may be employed , as desired . further , many symbols and legends may be implemented by installing an opaque mask with appropriate translucent areas in front of a module . in addition , modules may be fabricated using integrated circuit processing technology to provide a plurality of individual light sources distributed throughout the viewable area of the module , with each light source having the two - color or three - color light elements with cathodes coupled in common and individually connectible anodes . in such an embodiment , the individual encapsulation packages can be replaced with a single transparent or translucent cover having lens regions located over the individual light sources . therefore , the above should not be construed as limiting the invention , which is defined by the appended claims .