UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 13th Jun 2019, 9:52 pm   #1
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Lamp bulbs - what causes 'mirroring' inside?

Stuck in traffic behind an immaculately clean car this morning I noticed I had a front sidelight bulb off, so changed it, only to realise that the other bulb, although working, was only about one third as bright as the new bulb I had just replaced.

Both bulbs, the blown one and the dim one, had a heavily built up 'mirror' finish on the inside of the glass. That was what was making the 'working' bulb so dim.

I've seen this a few times before but I'm fairly sure I only ever see it on automotive bulbs, perhaps due to the combination of low voltage / high current / long periods of use which are not really replicated anywhere else. I don't think I've ever seen this happen with torch bulbs, probably because they aren't usually run for several hours at a time.

What's the process by which this mirror finish is created, does anyone know?
SiriusHardware is online now  
Old 13th Jun 2019, 10:35 pm   #2
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,787
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Some bulbs have a deliberate mirror coating on the end. They are supposed to be fitted in specific reflector designs but sometimes blown bulbs get replaced with whatever is to hand that will fit. Bulbs become darkened after a lot of use, but not silvered.
paulsherwin is offline  
Old 13th Jun 2019, 10:40 pm   #3
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Evaporation of the tungsten filament and condensation on the inside of the bulb. The filament gets thinner and eventually fails.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 13th Jun 2019, 10:58 pm   #4
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: Lamp bulbs - what causes 'mirroring' inside?

I know the type you mean, like H7 headlight bulbs.

This is not one of those types, new bulbs of the same type are entirely transparent and clear. But - whether the correct description is silvered or darkened - what is it that is being deposited on the inside of the glass? It definitely looks metallic, like Mercury.

When I've seen this in the past the bulb has always been blown so I thought the coating must be part of what happens when the bulb blows - but now that I've caught one which has the coating but is still working, I'm not so sure about that.

Edit: Just caught David's response, thanks David. But why doesn't it seem to happen so much with mains powered filament bulbs?
SiriusHardware is online now  
Old 13th Jun 2019, 11:10 pm   #5
Biggles
Rest in Peace
 
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
Default Re: Lamp bulbs - what causes 'mirroring' inside?

I have seen this effect occasionally on small bulbs such as MES, and the low wattage car bulbs such as sidelights, but not really on domestic lightbulbs. I thought it may be due to them being over-run and the filament getting too hot causing metal of some kind to be sputtered? if that's the correct term, onto the glass. I think David's explanation is correct.
Alan.

Last edited by Biggles; 13th Jun 2019 at 11:14 pm. Reason: extra info
Biggles is offline  
Old 13th Jun 2019, 11:22 pm   #6
Jonster
Heptode
 
Join Date: Apr 2008
Location: Eastbourne, East Sussex, UK.
Posts: 671
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Check your battery voltage while the engine is running as it's possible the alternator is overcharging and giving too much voltage. If this is the case then get the problem sorted before other damage to the cars electrical system occurs
Jonster is offline  
Old 13th Jun 2019, 11:24 pm   #7
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Biggles - It sounds very logical, but I'm wondering if it is not so much the way the bulbs are run as the physical proximity of the filament to the glass in certain types of bulb?

If the glass enclosure is wrapped closely around the filament, as it is in MES lamps and small car sidelight bulbs, it happens, but in a lamp style with more physical distance between the filament wire and the glass, it doesn't?

Jonster - I happen to know the car's regulator (or whatever it is these days) is working, as my main mobile amateur radio indicates the supply voltage, among other things. This car (off topic for this forum, so don't ask) is quite gentle on bulbs, I normally only replace one somewhere on the car about once a year. These bulbs may actually have been the original ones, as I have not changed them before during my long (8 year) ownership of it.

Last edited by SiriusHardware; 13th Jun 2019 at 11:31 pm.
SiriusHardware is online now  
Old 14th Jun 2019, 12:10 am   #8
broadgage
Nonode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Some automotive bulbs are deliberately manufactured with a mirrored finish for "styling" purposes. Not all such bulbs are legal for use on public roads since the light output is reduced, and the light distribution is different to that from a normal bulb due to internal reflection.

Apart from these special bulbs, other types can develop a mirrored finish towards the end of life.
I believe that it is caused by simple evaporation of the tungsten filament and condensation upon the glass.
In most cases this results in a very thin coating that looks grey or black, but in some cases sufficient metal vapour is deposited to give a metallic mirrored effect.
This is more likely to occur in bulbs that have a high wattage in proportion to the size of the bulb, because the evaporated filament material is spread over a smaller surface area.
As well as in vehicle bulbs, I have observed this effect in 1.1 watt but physically small Christmas light bulbs, only on that part of the glass closest to the filament.
I have observed this in some torch bulbs, but only the relatively high wattage types such as 4.75 volt 0.5 amp and up. Lower wattage torch bulbs don't contain enough tungsten to mirror coat the bulb.
Only a minority of bulbs develop this mirror finish, I suspect that MOST bulbs fail after only a small proportion of the filament has evaporated. A minority last longer because the evaporation is spread very evenly over the whole filament which therefore survives for longer.

12 volt halogen capsule lamps sometimes develop a mirror finish before a very early failure. I suspect defective manufacture whereby the halogen mixture was not admitted to the bulb.
broadgage is offline  
Old 14th Jun 2019, 12:16 am   #9
G8UWM-MildMartin
Heptode
 
Join Date: Jan 2004
Location: Stockport, Greater Manchester, UK.
Posts: 827
Default Re: Lamp bulbs - what causes 'mirroring' inside?

My car is twice the age of yours and replacement sidelamp and number plate bulbs only last a year or two, headlamps longer, and all the rest are original apart from the amber indicator bulbs that lost their colour after 14 years.
I seem to remember reading that small ones, such as scale lamps and car side-lights are supposed to last longer on a.c. than d.c., but it does indeed seem that the size/proximity is a major factor.
G8UWM-MildMartin is offline  
Old 14th Jun 2019, 1:23 am   #10
Terry_VK5TM
Nonode
 
Terry_VK5TM's Avatar
 
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,324
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Don't forget that automotive lamps have heavy duty filaments for rough duty as opposed to domestic and many torch lamps, so they have the 'ability' to migrate more tungsten from the filament before failing.
__________________
Terry VK5TM
https://www.vk5tm.com/
Terry_VK5TM is offline  
Old 14th Jun 2019, 8:28 am   #11
KeithsTV
Nonode
 
KeithsTV's Avatar
 
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,571
Default Re: Lamp bulbs - what causes 'mirroring' inside?

One car we had used separate tail light bulbs which, from memory, were 10W. One used to fail regularly activating the bulb failure indicator. The filament hadn't failed but the glass was black. I'd replace it only for it to fail 3 months later with exactly the same symptom. It was only when the other tail light bulb failed, again with the glass blackened, I realised what might be the problem, they were running too hot in the small enclosure and the evaporation from the filament was reducing the bulb current and activating the failure system. I replaced them with 5W bulbs and never had another problem.

Keith
KeithsTV is offline  
Old 14th Jun 2019, 8:39 am   #12
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: Lamp bulbs - what causes 'mirroring' inside?

It's worth reading up on tungsten-halogen bulb technology. The presence of halogen gas acts to return most evaporated tungsten to the filament, so the loss rate is reduced and the bulb lasts longer. This can be traded off into running the filament hotter which gives a higher colour-temperature and better efficiency.

Unfortunately, the returned tungsten doesn't go back exactly where it came from, so high spots and thin spots form leading to its eventual demise.

Cars run with very thick filaments for rough service, and also because they run at low voltage and high current.

Cine and slide projector designers cottoned onto the lifetime advantage of low voltage bulbs, and later projectors went over to low voltage bulbs.

Lorries with 24v systems suffer more bulb failures than 12v cars. Partly due to thinner filaments and partly due to longer operating hours per week. Notice that they usually have multi-bulb tail lights.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 14th Jun 2019, 9:44 am   #13
Jon_G4MDC
Nonode
 
Jon_G4MDC's Avatar
 
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
Default Re: Lamp bulbs - what causes 'mirroring' inside?

21W/5W Stop/Tail light bulbs in my Discovery TD5 did this every 6months or so.
It liked to eat them.

The silver mirroring built up on the side of the bulb where some pressed metal "tracks" ran past the glass envelope. These tracks were riveted to the plastic base to form a sort of PCB which made the various interconnections from the plug/socket to the bulb holders.

I always promised myself to work out whether it was the +Ve or -Ve tracks which caused the effect. Unfortunately I was always in too much of a hurry to fix it and get the car back on the road to investigate so that is still a mystery.

In one particularly bad case the support wire for the upper filament, which sat between the lower filament and the side track, was completely eaten through. There was an air gap between the upper part, still hanging free on the filament end, and the stump coming up from the glass seal. I should have kept a photograph of it.
Jon_G4MDC is offline  
Old 14th Jun 2019, 10:11 am   #14
Lucien Nunes
Rest in Peace
 
Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Quote:
The silver mirroring built up on the side of the bulb where some pressed metal "tracks" ran past the glass envelope. These tracks were riveted to the plastic base to form a sort of PCB which made the various interconnections from the plug/socket to the bulb holders.

I always promised myself to work out whether it was the +Ve or -Ve tracks which caused the effect
This was the behaviour, or at least the relationship between the polarity of a lamp terminal and the rate of carbon deposition on the glass nearby, that prompted Edison and his team to attempt to control it with a charged electrode. The discovery that the resulting emission current was unidirectional, at first termed the 'Edison effect', led via Fleming's work to the commercial development and application of the thermionic valve.
Lucien Nunes is offline  
Old 14th Jun 2019, 10:52 am   #15
SiriusHardware
Dekatron
 
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Quote:
Originally Posted by G8UWM-MildMartin View Post
My car is twice the age of yours...
I doubt it. I said I had owned it for 8 years, but it was 11 years old when I bought it.
SiriusHardware is online now  
Old 14th Jun 2019, 11:27 am   #16
The Philpott
Dekatron
 
The Philpott's Avatar
 
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
Default Re: Lamp bulbs - what causes 'mirroring' inside?

12v auto lamps have to have a reasonable light output over the 11v to 15v range, the result being that at an (engine running) voltage of 14.4v they are being pushed quite hard and have a correspondingly short life. This seems to affect some more than others (21/5w being a good example, where the 21w filament is also compromised by being offset to one side)

As regards the deposition on the glass the small 5w bayonet or wedge lamps seem to be the worse affected ones, have seen quite a few silvered on the inside. The initial low temperature of the envelope on a cold start of a winter's night might encourage more rapid and obvious deposition than you would get with a lamp that is normally switched on/off when the ambient is 20c..? Similarly i wonder if the rapid cooling after reaching your destination and switching off has the same effect.

Dave
The Philpott is offline  
Old 14th Jun 2019, 4:55 pm   #17
Maarten
Dekatron
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,185
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Non-halogen bulbs can have some argon or xenon in them. The exact composition and pressure of the vacuum can also affect tungsten deposits.

The same goes for halogen bulbs with the quantity and pressure of the halogen. It took years of research to get it just right. Small deviations can cause different failure modes.
Maarten is offline  
Old 14th Jun 2019, 9:05 pm   #18
pmmunro
Octode
 
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
Default Re: Lamp bulbs - what causes 'mirroring' inside?

Unlike headlamp bulbs, small automotive "signal" lamps such as sidelight, stop/tail and direction indictor bulbs are often vacuum lamps and this type will show blackening of the envelope due to evaporation of tungsten from the filament. The vacuum lamp has two principal failure modes: filament failure and unacceptably low output due to blackening.

There is a compromise to be struck in filament lamp design between thermal losses and operating temperature and the design choices depend on the application. The output of a lamp increases rapidly as the working temperature of the filament increases but, in a vacuum, evaporation also increases. To allow high filament temperatures the rate of evaporation needs to be reduced by gas filling but this gas conducts heat away from the filament reducing the luminous efficacy (the efficiency of conversion of heat to light).

Thermal losses from the filamant were studied around 1918 by Irving Langmuir who found that a thick wire showed proportionately lower losses than a thick one. He also found that coiling the filament gave it the approximate characteristics of a thick wire whose diameter was the same as the outer diameter of the coil.

Headlamp bulbs need to give as high a light output as other characteristics, such as filament life, will allow and so run at high filament temperatures and are gas filled to limit evaporation; the tungsten halogen technique further enhances the perfomance.

Signal lamps are run at lower temperatures and don't need to have such high luminous efficacies. However, to maintain acceptable luminious efficacy, the heat loss and consequent energy loss, from the filament is minimised by having either a vacuum, which doe not conduct heat, or lower pressure of gas filling than for a headlamp bulb. Lower temperatures are less demanding on the housing too.

PMM
pmmunro is offline  
Old 14th Jun 2019, 9:17 pm   #19
slidertogrid
Octode
 
slidertogrid's Avatar
 
Join Date: Apr 2005
Location: Peterborough, Cambridgeshire, UK.
Posts: 1,897
Default Re: Lamp bulbs - what causes 'mirroring' inside?

I have noticed this effect particularly with the 5 watt cap less bulbs used for front position and number plate lamps.
A word of caution, when the bulb has gone like this it gets very hot. enough to burn the lamp holder and your fingers if you attempt to pull it out when it is on... (guess how I found that out)
To a lesser degree I have seen the same effect with dial lamps although they seem more prone to blackening than silvering .
Rich.
slidertogrid is offline  
Old 14th Jun 2019, 11:23 pm   #20
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Lamp bulbs - what causes 'mirroring' inside?

A trace of water vapour plays havoc with lamp life. It facilitates removal of tungsten from the filament and onto the glass as a result of the 'water cycle'. Basically, the White-hot tungsten decomposes water into hydrogen and oxygen. The oxygen reacts with tungsten, forming tungsten oxide. But this vaporises readily, so it does, and condenses on the comparatively cool inside of the glass bulb.

Once there, the free hydrogen re-enters the picture, and reduces the tungsten oxide to metallic tungsten, re-creating water - to continue the process.

It's possible that cheaper bulbs don't take precautions to exclude and absorb water up to the point where the bulb is sealed. So a pair of cheap bulbs, made at different times, could show quite different lifetimes under the same operating conditions.
kalee20 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:54 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.