Autolamps-Online

Light and Vision: An incandescent lamp emits light

We perceive the light of the lamp as "white light", this is in truth a mixture of light of different colours that are emitted at the same time.
The "white light" of an incandescent lamp can be split up into its components by means of a prism, just as the light of the sun is split up into the colours of the rainbow.

The way the eye sees

An incandescent lamp emits a light of all colours, that’s called "continuous spectrum".    
The human eye is especially sensitive to the yellow light (wave length around 550 nm)
That means a source emitting 10 Watts of yellow light will seem brighter than 10 Watts of blue light, since the eye is a lot more sensitive in the yellow region than in the blue.

Age and Vision

The eyes grow less sensitive with age:
A 20 year old needs 50% more light than a 10 year old to do the same task.
A 60 year old needs 11 x the amount of light as a 10 year old to do the same task!.
The older you get, the more important it is to have sufficient lighting
Since every age group is involved in driving, good lighting is essential for good safety and comfort.

Technical Vocabulary

A lamps components and material

What is a lamp made of?:
Cap = Metal (Nickel plated brass, or other)
Lead in wires = Copper coated steel, chemically treated or molybdenum
Filling gas = Vacuum, Argon, Krypton, Xenon
Bulb = Glass  
Filament= Tungsten wire

Where does light come from

The "heart" of the lamp:
The filament is made of tungsten wire
Electricity flows through the filament
The electrical resistance of the wire makes it heat up
Tungsten resists heat very well (melting point : 3370 °C, iron: 1535°C)
When hot, the metal of the wire emits light = Incandescence.
You’ve seen this before:
When a hot plate of an electric stove  starts to glow
The hotter the metal, the "whiter" the light that comes out. During normal operation the lamp’s filament is "white hot".

Output from a lamp

What a lamp produces, Light & Heat:
Incandescent bulbs generate more heat than they do light
More efficient sources of light are often a lot more complicated and more expensive i.e. Gas Discharge
A too high wattage bulb may lead to too much heat being generated and could  damage the headlight.

The life cycle of a vacuum filled lamp:

Electric current goes through the filament and heats it till its white hot and it emits light
When something is hot, it means its atoms are moving a lot. The filament’s atoms move so much, that they tear loose from the other atoms and leave the surface of the metal = Tungsten evaporates.
You’ve seen this happen elsewhere: When you heat water , the water atoms move so quickly, that they evaporate
The tungsten that has evaporated deposits itself on the "cool" glass of the bulb
The tungsten leaving the filament, makes it thinner and more fragile
At one point in time the filament is too thin and breaks = End of lamp’s normal life.

A lamp at the end of its life

What does a lamp look like at the end of its life:

The lamp  must be replaced
No quality defect, but just like a car’s tyre that has been used, a bulb must be replaced when it is "worn out". Different lamps and different manufacturers have different lives.
The bulb is "black" due to the filament’s tungsten, which evaporated and was deposited on the glass wall
The "older" the lamp, the "blacker" it is. The "blacker" the bulb, the less light comes out. Less light may compromise safety. That is why lamps should always be changed in pairs, as even though the other lamp might still be functioning, it probably will be giving a lot less light than it did when it was new.

Flux maintenance

Reduction in performance over time :

When new, the lamp gives its maximum light output
When older, the lamp emits less light (due to blackening of the bulb).
The ratio between new/old measured at 75% of its nominal life time is called flux maintenance
Flux maintenance is given in %.

The effect of filling gases in a lamp

Heavy gases instead of a vacuum :
 
The mechanism is similar to that of vacuum lamps: The tungsten evaporates from the hot filament, causing it to thin and in the end to break.
Inert gas atoms mechanically "get in the way" of the tungsten atoms that want to break loose from the filament. Therefore a filling gas slows the filament’s thinning and increases life time.
The bigger and heavier the gas atoms, the "better" they slow the evaporation of tungsten.

Lamp filling and atom size

How filling gases differ :

The size of the gas atoms is different: Xenon is bigger than Krypton is bigger than Argon. The bigger and heavier the gas atoms are, the "better" they slow the evaporation of tungsten.
Gas fillings  change the performance in one or both of two ways :
Increase light output, by increasing filament   temperature, while keeping tungsten evaporation moderate
Improve life time significantly, by lowering filament evaporation
Lamp design = Filament design + choice of gas
To design a new filament, the lamp’s gas filling has to be taken into consideration.

What halogen filling does

The halogen cycle :

A tungsten atom evaporates from the filament and "wants to deposit itself on the bulb".
The atom is "caught" by the halogen gas and forms a molecule that no longer "wants to deposit itself on the bulb".
When this halogen/tungsten molecule gets near the very hot filament, it decomposes breaking up into A tungsten atom, that is deposited on the filament and "strengthens" the filament at this place.
A halogen gas atom, that is back in the cycle to "capture" a free tungsten atom
Thus, the halogen cycle hinders the evaporation of the filament, by catching the tungsten atoms and putting them back on the filament.

Identify lamp failures

Typical failure features :

Normal end of life
The filament has grown thinner, due to the normal evaporation of tungsten. When this filament becomes even thinner in one section, its fragility will make it break.
Mechanical shock
This lamp has been subject to strong acceleration. The sharp edges of the broken filament show this.
Over voltage
Connecting a lamp to a tension it is not designed for can lead to the melting of the filament

Lifetime formula

Voltage to life calculation :

The supply voltage is critical to the life of a lamp!
Its a key factor to incandescent lamp performance
The life of a lamp is especially dependent on voltage.
4% higher voltage can reduce the life of a lamp by 40% !
4% lower voltage can increase it in a similar way

Different headlight designs

How to recognize different headlights :

Ellipsoidal
Small round bulls eyes or you see round lenses in the head light
Parabolic
Large reflector and scatter lens on glass front. Traditional type used for the last 30 years
Complex shape
Often little or no scatter lenses on glass front. Either "steps" or smooth reflector surface. Requires very precise positioning of lamps, therefore use of H7 or H1 Spot.
The efficiency has increased with the introduction of complex shape reflectors