BACKGROUND OF THE INVENTION Motorists and pedestrians have been relying on optical display systems such as signage or traffic lights for managing and directing flow of automobiles or humans for decades. Hitherto, incandescent or halogen lamps have been the optical sources for such display systems as they are inexpensive and reasonably easy to maintain and operate. Since the middle 1990s, display systems employing LED devices have started to replace traditional display systems using incandescent or halogen lamps because the former consume only a fraction of the energy of the latter. For instance, one red LED traffic signal head that contains 196 individual LEDs operates on a 120 Volt cE consumes less than 10 Watt, as compared with a 150 Watt incandescent lamp it is intended to replace. Published estimates of the energy savings that can be achieved due to the replacement of a red signal head with an LED unit range from 82% to 93%. See, Optimizing the Design and Use of Light- Emitting Diodes for Visually Criircal Applications in Transpotfabon and

Architecture ; Lighting Research Center, Rensselaer Polytechnic Institute ; November 1999.

LEDs have much longer rated lives than the incandescent lamps used in haMc signals systems: LEDs have average rated lives of over 100, 000 hours as compared to 8,000 hours rated live of an incandescent traffic lamps. In terms of maintenance and operating costs, LEDs have a distinct advantage over the existing incandescent lamps.

Although LEDs appear to have certain advantages over incandescent lamps, incandescent lamp has a significant edge in terms of cost it cost $2.50-$3.00 to replace an incandescent lamp, while a red LED head is about $100. Green LED head cost between 2.5 and 5 times more than the red ones. Across the United States a number of elechic utilities and public agencies have offered financial incentives to municipalities to install LED traffic signals with mixed success. The high initial cost appears to be a significant barrier to LED's wider adoption.

A further key to the possible wider acceptance of LED traffic signal system is the ability of the LED industry to address the visibility issue. LEDs are sensitive to temperature changes in two significant ways: actual live and luminous output. Existing LEDs are rated for operating temperatures of 25 degree C; at lower temperature they produce more light and at higher temperature, less. For example, at-40 degree C AlInGaP LEDs have an output that is 192% of the output at 25-degree C; at 55 degree C, the luminous output is 75% of that at 25 degree C.

One option LED manufacturers develop is to optimize for operation at higher temperature than those currently designed. Another potential alternative is to vent the signal head or include a heat sink to decease the interior temperature.

OBJECT OF THE INVENTION tt is an object of the present invention to enhance the performance of LED display system without imposing any physical or electrical overhead to LED traffic signal system.

It is another object of the present invention to reduce the fluctuation of temperature within the LED traffic signal heads in order for the LED to achieve optimal visibility criteria It is yet another object of the present invention to isolate the optical module from the power module of LED traffic signal heads in order to prolong the rated live of LED optical module.

It is a further object of the present invention to segregate the optical module from the power module of LED traffic signal heads such that the power module can be replaced with more energy efficient transformers without retailing the optical module.

It is an additional object of the present invention to seal the optical module of a LED traffic signal head for all weather conditions and attaining superior performance.

SUMMARY OF THE INVENTION By segregating the optical and power modules of a LED traffic signal head and sealing each against all weather conditions, an improved LED display system is realised that can achieve the expected energy saving and visibility performance consistently without increasing the payback period of the system.

The isolation of the optical module minimizes the fluctuation in temperature of the LED devices and hence ensures a greater luminous output from the optical module. As such, the chances of motorists and pedestrians missing the appropriate tSc signals in inclement weather conditions are reduced. The compartmentalization of the power module not only offers better thennai management of the LED display system but allows upgrade of the transfonner without the need to retire the optical module. It follows that the present invention permits the LED display system to meet ever stringent energy efficient standards and yet retaining its initial investment.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective elevational view of an embodiment of a LED display employed in a traffic light module.

Fig. 2 is a side elevational plan view of the embodiment in Fig. 1 highlighting the segregation of the optical and power modules.

Fig. 3 is a exploded, perspective elevational view of the components of a traffic light module incorporating the present invention.

DETAILED DESCRIPTION OF THE INVENTION Central to the present invention is the concept of improving the thermal management of LED display by separating the optical module from the power module. Unlike prior art disclosed in U. S. Patent No. 5,636,057 where the optical and power module are integrated as one unitary head, the present invention is less susceptible to fluctuation of internal temperature of the optical module.

Furthemmore, the present invention seals the optical and power modules against all weather conditions. In contrast with prior art such as that taught in U. S.

Patent No. 6,036,336 where the LED display head is exposed to ambient temperature, the present invention is more likely to achieve the expected rated live and luminous output.

PREFERRED EMBODIMENT OF THE INVENTION Fig. 1 is a perspective elevational view of an embodiment of a LED display employed in a traffic light head. A LED display head 5 comprises at least one protective cover 2; at least one optical board 6 and at least one optical module cover 8. The protective cover 2 can be made of any synthetic material that is transparent In the preferred embodiment of the present invention, the protective cover 2 is made of dear, dirt and scratch resistant polycarbonate material. The optical board 6 can be made of any surface mount material or simifar substrate. In the preferred embodiment of the present invention, the optical board 6 is a printed circuit board made from FR4 material and having a plurality of LED devices 38 inserted through said board. Fnally, the optical

module 5 is sealed with the cover 8; in the preferred embodiment of the present invention, the optical module cover is made from polycarbonate material. The power module is shown only partially with a power module cover 12 coupled to the optical module. Details of the power module will be elaborated further below in connection with the description of Figs 2 and 3 respectively.

Fig. 2 is a side elevational plan view of the embodiment in Fig. 1 highlighting the segregation of the optical module 9 and the power module 15 with a plurality of spacers 40. Prior aft LED signal heads integrate the optical module with the power module as manufacturers of LED traffic signal systems did not appreciate the effects of temperature have on low-power and tow-current semiconductor devices such as LEDs. As mentioned briefly in the background section above, the luminous output of LEDs changes with temperature: the lower the temperature, the higher the luminous output and vice versa. It was therefore not surprising that few of the LED signals heads in production approached the performance benchmarks of the prototype LED signal heads.

Referring again to Fig. 2, the LED optical head 5 of the present invention comprises separate optical module 9 and power module 15. Furthermore, the optical module 9 isolates the LED board 6 from the environment by encasing it with the protective cover 2 and the optical module cover 8. A plurality of legs 42 also raise the LED board 6 away from the optical module cover 8.

Disposed on the surface opposing the LED board are a plurality of spacers 40 for coupling detachably the optical module 9 with the power module 15.

Aga ! n in Fig. 2, the power module 15 comprises at least one transformer (not shown in Fig. 2 but in Fig. 3) that is encased with a power module cover 12 and a power module housing 24. A power cord 44 provides sources of electric power to the power module 15. Although the power module emits comparatively large amounts of heat, the effects it has on the optical module 9 are minimized because of the isolation of the two modules and the sealing of the two modules.

Fig. 3 is an exploded, perspective etevationat view of the components of a LED signal head 5 incorporating the present invention. As mentioned above, the LED signal head 5 comprises two distinct modules : the optical module 9 and power module 15, respectively. The optical module 9 further comprises the protective cover 2, an 0-ring 4, the LED board 6 and the optical module cover 8. The optical module 9 is sealed such that it is weather proof against moisture, air and dust by encapsulating the LED board 6 with the protective cover 2 and the optical module cover 8 with the 0-ring 4. Power to the LED board 6 is provided by an optical power cord 10 that terminates at the optical module cover 8. The sealing of the optical module minimizes fluctuation of operating temperature of the LED. The immediate benefit is the greater luminous output of the LEDs-a key factor in determining the visibility of LED traffic signal in indement weather. As compared with traffic signals employing either incandescent or halogen lamps, LED traffic signal do not suffer from sun- phantom effect. Sun-phantom effect arises when reflection of the sun on the reflectors of the traffic signals using incandescent lamps creates an illusion that the traffic light is activated when it is not A secondary and long term advantage is the ability of the optical module to achieve the rated live-another key financial consideration in determining the acceptance and also payback period

of adopting LED display for traffic signal systems. Finally, the stability in temperature within the optical module 9 also minimizes condensation therein irrespective of any fluctuation in ambient temperature.

Referring again to Fig. 3, the power module 15 of the LED signal head 5 further comprises the power module cover 12, a membrane 14, insulators 16, a transformer 20, an optional washer 18, a driver board 22 and a power module housing 24. In the preferred embodiment of the present invention, the power module 15 is sealed securely with the membrane 14. Power to the power module 15 is provided by the power cord 26 via the driver board 22. Unrike prior art LED signal heads, the power module of the present invention is segregated from the optical module 9 in order to reduce the thermal load the LED board 6 is subjected. One of the major advantages of having a separate power module is the ability of cities and/or municipalities to upgrade the power module without retiring the optical module to meet ever stringent energy efficiency regulations such as the harmonic current emissions standards in major industialsed countries.

It should be understood by one skilled in the art that the LED signal head of the present invention is adapted for repladng existing traffic signals using incandescent or halogen lamps. Furthermore, the replacement can be accomplished in a piece meal fashion : for instance, in view of the higher costs of green LED signal heads, some cities or municipalities may elect to replace the red traffic signals initially before deciding on those for amber and green itafiic signals.

The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all changes which come within the meaning and range of equivalency of the daims are, therefore, to be embraced therein.