Myths or facts

[David A]

There is a lot of opinion in this website and I would like to address some of these issues and you may also have some thoughts.My favourite myth is the lamp pre-heating myth, I come across university graduates who have learnt elaborate rituals to pre-heat the lamps, involving gradually increasing voltage over 15 minutes.I have never preheated my lamps and my lamp life is well within normal.I have never seen any research that would support this idea yet it has become so ingrained that some dimmer manufacturers even build a preheat control into their dimmers.Logically running the dimmers on low would not allow the quartz ion cycle to kick in which would actually reduce the life.Also all the dimmers I use have 5 -10 volts of leakage anyway which will warm the filaments.The amount of electricity wasted on these rituals would probably cost considerably more than any lamplife saving even if it was a real effect. My 2nd favourite myth involves parcans of all sizes of the standard variety and so many people who put wide globes into them only to be surprised at the low output and the fact that the beam still isn't any wider.The amount of light which is wasted lighting up the inside of parcan snouts is probably more than gets out and when I supply them with short cans which efficiently light all the stage they complain that the gel doesn't last as long.Do you have a favourite myth?

[Tomo]

Preheating - yes, this does help, in two ways:

1) The preheat reduces the stress on the envelope when snapped to full.

A 15-min preheat is probably excessive though - you only really need a few seconds.

 

2) A preheated lamp will snap to full much faster than a cold lamp.

So if you have a parcan rig doing loads of flashy effects, it's a good idea to preheat them because they will flash faster.

 

The parcan lamp issue is an old one - EXG lamps simply need to be in short-nose cans.

Anyone who doesn't realise this wasn't really thinking about it - it's not a myth, it's simple geometry, and belongs in pet hates.

[David A]

on what evidence are these assertions made, what is the research to prove these assertions and what is the logic to support them?

[Tomo]

1) Thermal expansion and thermal shock are real phenomena. It's the primary mechanism whereby semiconductor chips fail, as the gold leads become disconnected from the silicon die.

(The other failure mechanism for ICs is ion transport changing the doping, but that's not going to affect something as big as a lamp)

 

For something like a parcan lamp, it's not going to be hugely significant. For linear lamps (K-series etc), it becomes much more significant.

 

2) Try it. That's how I found out!

[Ynot]

on what evidence are these assertions made, what is the research to prove these assertions and what is the logic to support them?

I would say practical experience!!

[Guest lightnix]

I don't think any actual research has been done to produce the figures, but there's a wealth of experience which, at least, very strongly suggests that preheating is not only desirable, but indeed necessary in some lamps (e.g. ACLs), in order to prolong life.

 

It's true, though, that long preheats are wasteful of electricity and that 30 secs max will usually do the trick. I prefer to program preheat when it's needed, although it's not always possible / practical to do so.

 

On a related note, certain moving lights need to be well warmed up to get the stiffness out of their colour, gobo, etc. bulkheads. This is due to the lubricant in the motors being more viscous when cold, which can impede movement. When powering up movers, I'll point them at the sky (if they're heads) and put their wheels into a slow spin, with everything else (shutter, iris, intensity, etc.) "in". Then I'll go and have a cuppa and then I'll test them; anything that faults after warm up and can't be "shaken out" or reset, is probably going to need fixing ;) :)

[David A]

I 've had 40 years of practical experience as a lighting contractor and if I could see a way of saving money I would use it.I use hundreds of lamps a year but cannot remember losing one through thermal shock.The other question is how long does the pre-heat supposedly work i.e. how quickly does the lamp cool down.As to movers one doesn't have much choice but to preheat.I do have the advantage of living in a warm climate.

[Jamtastic3]

Yup, pre-heating does work, especially for 1K parcans or fresnels. I've blown a few parcans from flashing them (rock stylee) without pre-heat. Now I do it all the time (15% for around 5 mins is enough).

[tokm]

I usually include a pre-heat as part of the rig check.. OK usually a bit higher than 15%, maybe 25% or so, just bright enough for me to check if any lamps have gone during a previous show or just then and to see if any gels are getting frosty/gobos not melted.

 

Ive come across the ritual of a pre-heat of some form being done at nearly everywhere Ive worked & studied.. Maybe its just one of the things that gets done here in the UK but hasnt been around for as long over in Australia?

 

Tom

[paulears]

I don't tend to worry too much about pre-heat on par cans - but I have some 2K old strands in the rig, and doing a flashy flashy on these kills them pretty quickly, they also lag behind the pars in the time it takes to go form off to on. A smidge of pre-heat does seem to make the bubbles last longer, and reach on quicker. I didn't lose a single lamp this summer, compared to the par cans, which is unusual. Not scientific, but I'm happy with the result.

[Nat_Keiller]

We often preheat lamps for "light switch on scenes". Preheating some of the larger lamps 2ks etc when they are cold (if they;ve been on in the previous scene, then usually not too bad) can help reduce the lag between the 100w GLS lighting up instanting and the 2k slowing fading on...

 

As to evidence for this: it comes from techs when the LD questions why the 2k loaded 1960's STMs take so long to turn on from cold..!

[lxdad]

Pre-Heating lamps in larger sources (or multiple small lamps loaded onto 1 dimmer) is essential.

The the in-rush on a 5k will take the breaker out straight away if you press the flash button from cold!

 

If you are desparate to save some money, simply limit all of your dimmers to 90%... the life saving effect is quite stunning. I have not done the research myself, but various people tell me that a 10% reduction in voltage can result in a 300% increase in life.

[Brian]

10% reduction in voltage can result in a 300% increase in life.

And a 30% reduction in light output, and a 12% reduction in power and a 8% reduction in current and a 5% reduction in colour temperature.

[GridGirl]

I don't pre-heat for the purpose of pre-heating as such, but it tends to happen anyway, as I usually put the whole rig to about 20% then go down onto the stage to check if anything has blown that I didn't notice the show before. I find it kills two birds with one stone - I can check the rig without being blinded, and it effectively warms the lanterns, as by the time I've put the rig to 20%, run down the stairs, checked all the lanterns, run back up the stairs and switched them off, it takes three or four minutes.

[tonythetechie]

I can check the rig without being blinded, and it effectively warms the lanterns, as by the time I've put the rig to 20%, run down the stairs, checked all the lanterns, run back up the stairs and switched them off, it takes three or four minutes.

 

Got to say I do a similar thing but to prevent the run back up stairs I tend to put the rig check as a cue fading up over about 30 seconds and then put another cue as a follow on after about 4 minutes fading the rig out over 30 seconds.

 

It gives the rig an adequate preheat and saves my legs.

 

Tony