3

Signal Lamp Design Process

The design of an LED signal lamp consists of four inde

pendent but interrelated designs: optical design, mechanical

design, thermal design, and electrical design.

The optical design is needed in order to design the secondary

optics elements, such as reflectors or lenses, which are

mounted in front of the LED emitters. In addition, the outer

pillow lens needs to be designed in order to generate the

desired output beam pattern. The optical design of an LED

signal lamp is not unlike that of an incandescent signal lamp,

except that the LED emitters have a much smaller geometry

and a different optical radiation pattern.

A mechanical design is needed in order to generate the

desired mechanical drawings for the outer case, outer lens,

and possibly internal secondary optics. The mechanical

design would also include the selection of materials used for

the signal lamp assembly. The mechanical design is not

unlike the mechanical design of an incandescent signal lamp.

The purpose of the thermal design is to evaluate the heat

flow from the LED emitters to the ambient air and to reduce

the thermal resistance as much as possible. For best results,

the LED signal lamp should be designed to minimize self

heating of the LED emitters. SuperFlux LEDs are limited to a

maximum junction temperature of 125°C. In addition, all

LEDs experience a reduction in light output at elevated

temperatures. This phenomena is fully reversible, such that

the light output returns to its original value when the in the

temperature returns to its initial value. However, selfheating

causes an undesirable reduction in the luminous flux output

of the LEDs. The thermal design of an LED signal lamp

differs from that of an incandescent design. For an incandes

cent design, the design focus is to choose plastic materials

that can withstand the heat generated by the bulb. For the

LED lamp design, the focus is to protect the LEDs from high

temperatures and to optimize the optical performance.

The purpose of the electrical design is to choose the appro

priate forward current through the LED emitters and ensure

that this current stays within an acceptable range during

worstcase operation at the extremes of ignition voltage and

temperature. Also, the electrical circuit configuration deter

mines the luminous intensity matching between the emitters

within the LED signal lamp. In addition, the electrical design

can also protect against EMC transients, and highvoltage

and lowvoltage transient conditions. In many cases, an elec

trical design is not needed for an incandescent signal lamp

since the bulb can be driven directly from the ignition voltage.

These four design processes are interrelated. For example,

the mechanical drawings used to construct the signal lamp

cannot be completed until the optical, thermal, and electrical

designs are finished. Since these different design processes

are interrelated, it is not uncommon to design the LED signal

lamp using estimates for these different factors and to iterate

the optical, mechanical, thermal, and electrical designs

based on bench testing of prototype signal lamps.

A flow chart of the basic design process for an LED signal

lamp is shown in Figure 1.1 and consists of the following

steps:

1.

Define external operating parameters for the signal lamp.

These parameters are usually specified by the car manu

facturer or defined in various automotive specifications.

These parameters include:

• Operating and storage temperature requirements for

the signal lamp.

• Photometric test conditions of the signal lamp (i.e.,

whether testing is done at initial turnon at room

temperature, after a 30 minute warmup at room

temperature, or over some operating temperature

range).

• Design voltage (the voltage at which the photometrics

will be tested).

• Operating voltage range (i.e., 9 V to 16 V).

• Transient operating voltage range (i.e., 24 V for 1

minute).

• EMC transients applied to the signal lamp (i.e., SAE

J1113 pulses 1 through 7 and theamplitude and dura

tion of each pulse).

• Whether any additional photometric guard band is

required above the minimum photometric require

ments defined by the SAE or ECE standards.

Please refer to AB206 for a summary of environmental

strife tests that have been used to validate Super Flux

LEDs as well as suggested assembly validation tests for

automotive applications.

SuperFlux LEDs in Automotive

Application Brief AB201 (5/04)