Myths or facts

[Roderick]

A QI is the Australian common name for cheap outdoor floodlights such as this one here.

They are often used as worklights or 'low-budget' cyc units.

[dwh]

Floods? Who honestly expects RGB matching with floods? Honestly?

 

(Well, to any standard)

[peternewman]

My next favourite myth is that you can do red , green ,blue colour mixing with QI lights, the sale of rgb QI lighting units is one of the best cons since pet rocks and about as useful.

Admittedly it wasn't for theatre, so we weren't worried about any possible multicolour shadows. But I've lit a wall with floods in red, green and blue along with some other architectural lighting, and with each flood dimming at the rate of a prime number (if that makes sense) gave quite a nice random colour changing effect.

 

PN

[Guest lightnix]

My next favourite myth is that you can do red , green ,blue colour mixing with QI lights, the sale of rgb QI lighting units is one of the best cons since pet rocks and about as useful.

Sorry? Pardon my ignorance, but what are "QI" lighting units when they're at home? ;)

A QI is the Australian common name for cheap outdoor floodlights such as this one here.

They are often used as worklights or 'low-budget' cyc units.

Thanks Roderick. In that case he's right, similar beasts have popped up before in other threads and whatever they're for, it's not for doing shows

[mumbles]

Ive done several outside (in marquie) gigs where the tech crew stay on site for a week. Every morning for the scheduled shows we run the entire rig (normally consisting of roughly 40 1k par cans, a loada fresnels, many many many source 4 profiles, parnels, blinders, movers and more) at 15% for 5 mins. When kit is outside, well in the outside air temperature in british weather, it does get realy cold, colder than it would inside a building - so a large scale preheat of aout 150 lanterns does do justice.

Or, quite often, the rig will be left on on a 10% preheat, inorder to stop moisture collecting in the dimmers, cables, or lanterns. Also, movers are normally left on, with the lamp off, on the same basis, as the little bit of electricity going through it to keep it warm, and stop condensation. With the lamps, a preheat in the morning is ok, but the dimmers, and intells won't like being powered up if they're going to have moisture in them.

[David A]

sorry about the colloquialism[QI] Some very good salesmen have been doing the rounds selling schools these overpriced 'linear halogen floods' in a DMX RGB package as the perfect theatre light as they can mix any colour without ever needing to buy gels again,talk about a myth, and schools are using their years budget buying this overpriced junk which is only good for the school disco or a modern dance number.

[Brian]

Ah, I'm less confused now. I guess we're talking about this kind of thing?

http://www.aux99.co.uk/blue-room/rgb1.jpg

http://www.aux99.co.uk/blue-room/rgb2.jpg

[David A]

I have enjoyed reading the reactions to the pre-heating myth... there is absolutely no scientific evidence to support it... I have yet to hear of a single paper on the subject.

So... on the one hand, David, you seem to be jeering at people for believing a "myth", which is unsupported by scientific evidence, but at the same time fail to provide any scientific evidence of your own to disprove it. The words "pot" and "kettle" spring to mind (along with a couple of others ;) ).

 

My next favourite myth is that you can do red , green ,blue colour mixing with QI lights, the sale of rgb QI lighting units is one of the best cons since pet rocks and about as useful.

Sorry? Pardon my ignorance, but what are "QI" lighting units when they're at home?

talking of pots and kettles I seem to remember lightnix denegrating one Rich Newby after he made a totally valid point about putting wide angle lamps in birdies and my point is that manufacturers build their lamps to be switched on and off, which they are in 90+% of applications so where did the preheat ritual evolve and why?

[Ynot]

.... my point is that manufacturers build their lamps to be switched on and off, which they are in 90+% of applications so where did the preheat ritual evolve and why?

Erm, David, no.

Lamp manufacturers in a theatrical environment (which is, after all, what we're discussing) make them to be versatile. They're going to be turned hard on/off a lot, (especially in a concert scenario) and they're going to be dimmed.

 

BUT the whole point is that turning a filament based lamp full on from cold WILL without a doubt put heat stress on that filament. It may have been documented, it may have not, but it makes enough logical sense to me to be true, rather than a myth, that warming up the bubbles is going to help - especially in a cold environment.

 

In my experience, I've never seen a lamp blow at the first cue when the lamps have been warmed, OR the ambient temperature hasn't been above average. However, I HAVE seen lamps 'pop' when rigging in the cold (as our theatre has no versatile A/C, it's very often VERY cold in the winter when rigging!) as soon as they're turned up full without a preheat.

 

TD

[musht]

QI means Quartz Iodine now more common in UK to call TH, Tungsten Halogen partly because the halogen used is now the cheaper bromine. Envelope is still quartz with tungsten filament.

 

Slightly other side of the pre-heat debate, can believe in a few seconds of red glow pre-heat on a lamp in a chase or a gradual rig warm in cold conditions but 15 minute preheat outside the arctic may have an unwelcome side effect.

 

The lamp blackening myth (?), QI/TH lamps need a certain operating temperature to function correctly, the quartz glass has extremely high heat tolerance, look how much smaller the bubble is on a TH lamp compared to normal lamp. The halogen`s job inside the lamp is to capture evaporated tungsten from the filament and return it there. The halogen cycle requires a minimum heat to function or the evaporated filament ends up on the inside walls of the envelope letting less light out.

 

This deposited filament should get scoured off the lamp walls by the halogen when the lamp is run at full heat for a while. Some architectural installations have a full up programmed in on lamps that are normally held on dim as a lamp clean up.

 

Not sure if its a myth or not, the nae sayers would say that a dimmed halogen isn`t running hot enough to evaporate significant amounts of the filament and lamp blackening won`t happen. YMMV

[David A]

I certainly agree with much of Ynot 's post and I can't remember losing a lamp on a first cue either nearly all mine die during focusing or transport.Is one of our forum in a facility with a thermal imaging camera as it would be really useful to have some basic data to discuss like how long does it take for a filament to cool down as this is clearly crucial to determining if preheating can be useful.I am willing to be persuaded but so far I just can't buy it.

[paulears]

While not a published engineering paper, there is an article here that does explain in quite simple terms some of the background.

[Just Some Bloke]

I seem to remember lightnix denegrating one Rich Newby after he made a totally valid point about putting wide angle lamps in birdies

Having used the :) search function ;) , I should just point out here that Rich Newby's point about birdies and beam angles was that they are too narrow. The point was made by others (myself included) that birdie lamps come in a selection of different angles from narrow to wide. The link is here.

 

Let's try not to get personal here, chaps, shall we? Ta.

[dbuckley]

Time for a little fun in the workshop.

 

500W linear halogen tube, as used in Codas and car park floods, variac, a pair of Fluke meters.

 

The resistance of the lamp at 0V is measured, otherwise is calculated from voltage and current measurements. The bump current is calculated at 240V.

 

 

V	   I(A)	W	   R	   Calc IMAX(A)
0					   7.66	31
1.43	0.126   0.2	 11.35   21
3.44	0.245   0.8	 14.04   17
5.58	0.326   1.8	 17.12   14
9.72	0.4148  4	   23.43   10
13.27   0.479   6.4	 27.7	9   (Point of first glow)
20.4	0.577   11.8	35.36   7
30.6	0.699   21.4	43.78   5
40	  0.798   31.9	50.13   5
50	  0.898   44.9	55.68   4
60.5	0.9939  60.1	60.87   4
70.6	1.081   76.3	65.31   4
81.2	1.167   94.8	69.58   3
90.5	1.238   112	 73.1	3
100.5   1.313   132	 76.54   3
151.9   1.649   250.5   92.12   3
206	 1.957   403.1   105.26  2
232	 2.09	484.9   111	 2
243	 2.153   523.2   112.87  2
250	 2.178   544.5   114.78  2
259	 2.222   575.5   116.56  2

 

So, a bit of preheat does substantially reduce the (calculated) inrush current on a bump.

 

I was going to then measure the actual inrush current, but due to a corroded battery in the high speed current clamp, this will need to wait till another day...

[David A]

I seem to remember lightnix denegrating one Rich Newby after he made a totally valid point about putting wide angle lamps in birdies

Having used the :wub: search function :wub: , I should just point out here that Rich Newby's point about birdies and beam angles was that they are too narrow. The point was made by others (myself included) that birdie lamps come in a selection of different angles from narrow to wide. The link is here.

 

Let's try not to get personal here, chaps, shall we? Ta.

The fact is that putting a wide angle lamp in a standard birdie is a waste of time and electricity as the snout of the birdie absorbs the light and not much gets out.As to getting personal my point was that lightnix seems to have a tendency to 'play the man and not the ball'There is a hugely bigger picture to this discussion, if every theatre in the world ,every day,wastes electricity on a completely unneccesary procedure the amount of wasted power and greenhouse gas emissions is enormous so it is an important issue.

 

I have just done an experiment which I suggest proves my point about pre-heating lamps before a show and present it for your consideration.I took a 500w Fresnel and measured its resistance cold at 9.1 ohms I then ran it at 25% for 10 mins.I unplugged it and plotted the resistance over time.Initially it raced down from about 40 ohms after 10 mins it was down to 10 then after 20 mins down to 9.3 and after 25 mins back to 9.1.So it could be argued that there is an effect from pre heating but that effect certainly does not last more than 30 mins.and therefore pre-heating before a show is a waste of electricity and lamplife.If there is a flaw in the experiment I would be happy to hear it.However the design of a theatre lamp would seem to require a low thermal mass as it goes from cold to white hot in fractions of a second and has to cool rapidly or it wouldn't be much use as a theatre lamp, so it has to be inherent that the filament does not retain heat.In my experiment I could not measure the filament temp directly but had to infer it from the resistance but the lampbase resistance may have increased slightly too but I don't think that is too important.Your comments please.