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Headlights Part 1: The Basics

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Story and Photos by Jim Clark, The Hot Rod MD

Headlights, technically referred to as headlamps, are standard equipment on any new vehicle and only require that you know where the controls are located for operating them. However, when building or modifying a hot rod knowing how to select new headlights and mount them is necessary.

The two standards governing the authorized use of headlights are the International ECE Regulation and North American SAE Regulations contained in FMVSS / CMVSS 108. Federal and Canada Motor Vehicle Safety Standard 108, which incorporate SAE technical standards, regulate the design, performance and installation of all motor vehicle lighting devices in North America. ECE internationalized regulations are in force elsewhere in the world either by reference or by incorporation in individual countries' vehicular codes. Some states simply adopt the FMVSS 108 SAE standard without a lot of additional requirements but some jurisdictions like California impose additional regulations for vehicles registered there.

The primary differences between the two standards have to do with how much light can be directed toward the oncoming traffic on low beam and the total amount of light allowed to be projected under all conditions. (1) ECE low beams have a distinct horizontal "cutoff" line at the top of the beam. SAE low beams may or may not have a cutoff and instead control the low beam projection by placement of the filament in relation to the reflector. (2) In North America high beam intensity is now 150,000 candela (cd) per vehicle, double the nonhalogen limit of 75,000 cd but still well shy of the international European limit of 225,000 cd.

A Little History
The first modern vehicle electrical system was debuted by Cadillac in 1912. It integrated the Delco electrical ignition system with their new lighting system. In 1924 Bilux introduced the first headlamp with a single bulb that emitted both the high and low beam. In 1925 Guide Lamp introduced a similar design.

By the end of the 1920s headlamps were pretty much standardized in design with a parabolic (cone-shaped) reflector, dual-filament bulb in the center and a glass lens at the front. This bare-bulb configuration continued to be in common usage throughout the 1930s until the 7-inch in-diameter; round sealed beam headlamp was introduced in 1940. The sealed beam headlamp was soon required as the standard on all vehicles sold in the United States.

Adoption of the sealed beam headlamp solved the problem of contamination from the elements experienced with bare-bulb type units. The sealed beam headlamp also reduced the dimming effect (about 30% after 1000 hours of use) created in a bulb when the tungsten in the filament boils off and condenses on the inside of the glass bulb, blackening it. The sealed beam headlamp experiences the same problem because it also has a tungsten filament but to a much lesser degree due to its much larger interior glass surface.

Definition of a Filament
A wire coil with current run through it, making it glow. Eventually the filament burns out. The device was not practical as a lighting source until Thomas Edison housed the filament in a glass vacuum bulb that dramatically extended the life of the filament.

Sealed beam headlamps continued to be the standard in North America until 1983 when newly revised standards allowed the use of other shapes than round and replaceable-bulb headlamps.

Types of Headlamps:

A three-part assembly consisting of a light bulb, reflector and lens. Fresnel (multi-faceted) and prism optics are molded into the lens to refract (shift) parts of the light both laterally and vertically to adjust the light distribution.

Early vehicles (prior to 1940) were equipped with headlights that consisted of a housing, reflector, bulb and socket, fronted by a fresnel and prism glass lens.

Sealed Beam
A tungsten filament sealed inside a glass reflector/lens assembly. The integral lens has the same fresnel and prism optics as the freestanding lens.

An integral and indivisible optical assembly including the light source with “SEALED BEAM” molded in the lens.

This is the standard 7-inch round sealed beam headlamp with fresnel and prism optics molded into the inside surface of the integral lens. Standard three-prong molded plug fits over the contacts permanently attached to the filament wires coming out of the sealed unit.

Halogen Sealed beam
A sealed beam lamp with halogen gas inside the reflector/lens assembly. The integral lens has the same fresnel and prism optics as the freestanding lens.

Halogen sealed beam headlamps like this rectangular unit basically replaced the standard sealed beam headlamp because they were brighter and had a longer life cycle. They are available in 7-inch round, 5-3/4-inch round, in addition to the 8 x 6-inch and 6 x 4-inch rectangular units.

Replaceable-bulb Composite Headlamps
U.S. headlamp regulations were amended in 1983 to allow replaceable-bulb, nonstandard-shape, architectural headlamps with aerodynamic lenses that could for the first time be made from plastic. These may control distribution of the light source with lens optics or with nonparabolic, complex-shape reflectors with clear protective lenses.

FMVSS 108 Definition: Replaceable Bulb Headlamp
A headlamp comprising a bonded lens and reflector assembly and one or two replaceable light sources, except that a headlamp conforming to paragraph S10.18.8 or paragraph S10.18.9 of FMVSS No. 108 may have a lens designed to be replaceable.

Replaceable-bulb composite headlamps like the 7-inch round unit from United Pacific shown here features lens optics and a replaceable bulb that installs at the rear of the reflector. The most popular replaceable bulb in use in the US is the Tungsten-halogen (quartz-halogen) H4 type bulb with standard three-prong contacts like the ones on sealed beam headlamps. Other tungsten halogen replaceable-type bulbs are available with a different type of connector and a different H-number designation. Replaceable-bulb composite headlamps can have clear lenses without lens optics like the unit shown with the blue-dot in the center (not for highway use in every jurisdictions).

LED Headlamps
Automotive headlamp applications using light-emitting diodes (LEDs) have been installed on a few new high-end vehicles but as of November 2011, DOT/SAE have not published the guidelines for LED Headlights. Contrary to popular belief LEDs actually produce a significant amount of heat per unit of light output and unlike incandescent bulbs an LED's heat is produced at the rear of the emitters. LEDs are damaged by high temperatures and during prolonged high-temperature operation will permanently degrade the LEDs and ultimately shorten the device's life. Some aftermarket LED headlamps are available but you should make sure that they are authorized for use where your vehicle is registered. A basic sealed beam headlamp will cost less than $7 while an LED headlamp for the same application will cost from $250 to $400 dollars and require additional components to make them function.

Shown are two 7-inch round LED replacement headlamps available from United Pacific. One with 11 LEDs and the other with 16 LEDS. They will fit into most of the popular headlight housings used on early hot rods but may not be currently legal for use in your jurisdiction.

See this typical warning from United Pacific.
WARNING: United Pacific LED Headlights (UP Item # 31354) Meets Current DOT/SAE Photometric Requirements for Headlights. DOT/SAE is in the Process of Drafting Guidelines Specifically for LED Headlights. However, as of November 2011, DOT/SAE Have not Published the Guidelines for LED Headlights. United Pacific Industries Recommends these Headlights for Show/Off-Road use until DOT//SAE Establishes Formal Guidelines for LED Headlights.

Projector (polyellipsoidal) lamps
Projector headlamps are the type seen on most new vehicles. They use an elliptical reflector and shield near the image plane to control the light distribution. The lens can be clear or have a slight fresnel surface. The light source is provided by a bulb.

These high power projector headlamps from United Pacific are typical of the ones that come standard on many new vehicles and usually have separate high and low beam units. They don’t fit in most traditional hot rod style housings making them undesirable for the application.

Selecting headlights for hot rods used to be a simple process, but the introduction of different headlamps to fit in the traditional housings has made that process more complicated. Original bare-bulb style headlamps can be used on some early vehicles, where local laws allow it, but most can be upgraded with Halogen bulb conversions.

Two types of sealed beam headlamps (standard or halogen) can be used in housings like the Dietz, King Bee and Guide teardrop style housings. Replaceable-bulb Composite Headlamp units that use modern style bulbs are also available to install in the traditional housings. The fourth possible choice is the LED replacement unit that fit in the traditional style housings, but may not be legal in all jurisdictions yet and is quite expensive when compared to the other options.

FMVSS 108 is available on the Internet and states what you shall do (legal for must do) when installing lighting devices on a vehicle. It is hundreds of pages long beginning with definitions, specifications for materials, test parameters and procedures, and equipment mounting and performance requirements. Like all regulations FMVSS 108 doesn’t tell you what you cannot do; if it is not addressed there it is not authorized. This quote from FMVSS 108 (Each vehicle shall be equipped with at least the number of lamps, reflective devices, and associated equipment specified in the Standard, and required equipment shall be designed to conform to the SAE Standards or Recommended Practices referenced in the Standard as applicable.) makes this very clear. Here we have covered the basics. In “Headlights Part-2” we will cover how high and low beam functions are achieved and how the proper aiming of headlights is accomplished.

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