Film-Tech Forum ARCHIVE: Auditorium Dimmer with LEDs

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Posted by Justin Hamaker (Member # 2165) on 03-05-2017, 10:16 PM:

We are interested in converting our auditorium dimmers from halogen at one theatre, and incandescent at the other. Both theatres currently have a Xetron XD-3KS panel - single channel at one theatre, dual channel at the other.

With this dimmer, would we be able to simply convert the bulbs to LED? Or is there some kind of change needed to handle the change in power requirements. My theatre uses the halogen bulbs, with 8 100w bulbs per channel. The bulbs have an E11 (miniature candelabra) base. I have tried a couple different LED alternatives and they don't work. Either they won't dim at all, or they are bright or off with no in between.

There is not problem with converting the fixtures to a different base - A19, or something else.

If this dimmer won't work with LEDs, are they any suggestions for one which will work with our automation system. We have the Christie ACT.

Posted by Lyle Romer (Member # 1266) on 03-05-2017, 10:55 PM:

Have you used LED bulbs that are specifically marked "dimmable?"

I don't know the specifics of the dimmer you are using but I do know that not all LED bulbs are dimmable.

Posted by Leo Enticknap (Member # 534) on 03-05-2017, 11:52 PM:

We have Kelmar dimmers in two of our three houses, and in one of them I've put 20w (80w equivalent) LED PAR 38s, marked as dimmable on the packets. Works without any problems, except that they will only dim to a certain point (which is still quite bright) and then go out. The dimming isn't as smooth and as elegant as with old school bulbs, but it is saving us a lot of power, and staff time replacing blown bulbs. I installed those bulbs in January 2015: since then, one went flickery and had to be replaced, but that's it. Previously, we were having to do the ladder and grappling stick ritual about once a month.

However, I've noticed this with dimmable LED bulbs at home, too. When you switch them on, they don't light up at all until the slider is about half way up, and the same thing happens in reverse on the way down. They are sorta dimmable, but not as smoothly as a filament bulb is, and with a smaller range of brighness.

Posted by Frank Cox (Member # 6258) on 03-06-2017, 12:59 AM:

I don't know why you would have to change the bulbs so frequently. I have a Kelmar dimmer and the bulbs that I use (standard 75w floodlights) last roughly forever. I think it's because the lights always come up and go down so slowly; the only time they get "shocked" and full-on immediately is after a power failure. I have six lights in my auditorium and I think I"ve changed them all once, maybe, in over twenty years of daily use.

I did try a "dimmable CFL" once to see how it worked, and it was a hopeless case. After dimming to a certain point that wasn't very dim, it started flashing on and off. So I bought about 20 of the 75w floodlight bulbs when they were on sale once upon a time, and figure that they will last me for as long as I need them here.

Posted by Steve Guttag (Member # 268) on 03-06-2017, 07:11 AM:

I'm amazed that the lighting and dimming industry hasn't gotten together on all of this. I guess that the market for people that need lamps to dim to zero is so small as to not warrant the change, yet.

LEDs don't dim like incandescents. It is kind of like a linear versus logarithmic taper faders. With a standard dimmer, you'll spend 70% of its range to dim the lamp about 10% and then compress the rest of the dimming in that last 30% so you need to change your upper limit and throw away that top 10% of its light. The other thing about "dimmable" lamps is that typically they are only rated to dim until 10% output and then it is all bets off from there. And even if you find a lamp that has the characteristics you like, there is no guarantee that it will remain that way on future purchases.

Of the ones that have worked, I've noted that I do need to see the lower limit a bit lower to ensure that none of the ones that want to flicker/sputter cause grief when they should be off. You don't want to dim to zero or when your up cue hits, there will be a delay. You probably have to slow down the dim/raise rates since you are now compressing everything in that last 30%, if you want it to feel traditional.

What I wish would happen is to abandon conventional dimmers and LEDs that screw into 120V sockets and just go to straight up LEDs and variable output low-voltage DC supplies so that you could dim the LED over its entire range. Make the dimmer such that it knows that characteristics of LEDs so that 50% level is really 50% lumen output too and so forth.

Has anyone tried using dimmable LEDs with Variac dimmers like "Luxtrol" from Superior Electric? I wonder if they would do better with a variac than a chopped up waveform. Or would their internal circuits just wig out over the lowering amplitude.

I'd think there would be enough of a market out there for people that want a zero output light that someone would start making/advertising them for theatrical use. Heck, even home-theatres might want such a thing (I know I do).

Posted by Gordon McLeod (Member # 33) on 03-06-2017, 09:27 AM:

Idealy a LED would like a smooth dc feed and they do dim very well with a variac
that said the issue with the screw in led lamps is that they have a switch mode supply in the base a triac dimmer (leading edge)switches on at various points of the cycle and as such the switch mode supply has issues with the steep turn on
A trailing edge dimmer will dim far better but they are a very expensive device
Also if you put a 100watt incandescent lamp on the dimmer in the booth it will keep the current through the triac high enough to keep it latched

Posted by Scott Norwood (Member # 30) on 03-06-2017, 12:45 PM:

How do the stage lighting companies do this? ETC has some excellent PARs and lekos with LED light sources that are fully dimmable. I am sure that other companies sell similar fixtures. As mentioned above, the curve is different from tungsten lamps (and the color temperature does not become warmer when they are dimmed), but they look fine and dim properly down to 0%.

I have yet to see LED house lights that look acceptable to me when dimmed. Is this a function of cheaper LEDs and/or power supply circuits being used for house lights, or is something else the reason for this disparity?

Posted by Carsten Kurz (Member # 5396) on 03-06-2017, 01:12 PM:

quote: Scott Norwood
How do the stage lighting companies do this?

Once you agree on a dedicated control line (in this case, DMX), it is no problem to dim LEDs with PWM. The problem with replacing incandescents on dimmers with LEDs is that there is no such control line. With DMX controlled LEDs, you have constant line voltage to the fixture, and tell the light to dim up or down through the control line.

I have found some very few of the latest filament LEDs to even work smoothly from 0 to 100% and back on our old variable voltage rheostat dimmers. But I had to test three dozen different bulbs to actually find them.

Some newer electronic dimmers (Kelmar) work nicely with dimmable standard LED bulbs.

Yes, the curve is always slightly different from incandescent, but people get used to that.

- Carsten

Posted by Paul Mayer (Member # 355) on 03-06-2017, 01:31 PM:

The local art house here uses strips of tiny LEDs laid out in the ceiling in the shape of a chevron. They dim very smoothly and completely to full off. I'll ask the owner, when I next see him, what they are using for their house lights and dimmers.

Guild Cinema

Posted by Martin McCaffery (Member # 37) on 03-06-2017, 02:43 PM:

We put in LED's last year to replace our fluorescent house lights (original 1941 installs).
Better than fluorescents, no where near as good as incandescents. The do not fade down/up to zero. If you search, you can find some that will get close. When I have some time and enthusiasm, I'm going to work on the timing with the curtain spots to see if I can get them to "mask" the dimming of the house lights.

FWIW: We replaced all of the lights in the building with LED's and I'm fine with those. Just never got the 100-0 dimming I was hoping for.

Posted by Manny Montes (Member # 5639) on 03-07-2017, 05:29 AM:

quote: Gordon McLeod
Also if you put a 100watt incandescent lamp on the dimmer in the booth it will keep the current through the triac high enough to keep it latched

This is exactly what we did and it works amazingly.

Posted by Lyle Romer (Member # 1266) on 03-07-2017, 05:35 AM:

I have Philips BR-30 LEDs in my house that dim very close to 0% in a smooth manner. I don't know if they make them anymore but they have a skinny "neck" and a flat disk as the front. The model I have has a 90+ CRI.

I've recently seen at Home Depot, LED bulbs with "warm dimming" where they change the color temperature as they dim. There was a Cree candelabra bulb on display that simulated incandescent pretty well.

If dimming performance and light quality are more important than electric savings, just go with halogen bulbs instead of LED. How much electric do LEDs save in the auditoriums? They are off most of the time. The lobby is a different story but you don't have to worry about dimming in the lobby.

Posted by Justin Hamaker (Member # 2165) on 03-07-2017, 06:31 AM:

Lyle, it may wind up that's what we stick with. However, we are exploring the LED solution to see if it's feasible. For one thing, when you're talking about 40 bulbs, using 1/6 the power is not insignificant. We're also looking into whether the LED solution could also be powered by a battery backup for our emergency lights.

Posted by Steve Guttag (Member # 268) on 03-07-2017, 07:30 AM:

Though they are not cheap, MIT does have their ALF series of LED light fixtures. They are indeed low voltage (36-volt) LED fixtures that work with 1-10-volt dimmers that should dim in a uniform fashion. Even those, however, don't dim to zero. They dim to .1% and require the power to be interrupted to completely quash them.

Posted by Leo Enticknap (Member # 534) on 03-07-2017, 09:47 AM:

quote: Frank Cox
I don't know why you would have to change the bulbs so frequently. I have a Kelmar dimmer and the bulbs that I use (standard 75w floodlights) last roughly forever.

The Aero's auditorium has 36 PAR 38 recessed fittings. When they had filament bulbs in them, I'm guessing that they probably lasted around a year, but that given that they were all blowing at different times, this resulted in about a monthly ladder ritual. The LED replacements have already saved us, at a guess, around 20-30 hours in staff time, and so they've probably paid for themselves in that alone.

quote: Steve Guttag
LEDs don't dim like incandescents. It is kind of like a linear versus logarithmic taper faders. With a standard dimmer, you'll spend 70% of its range to dim the lamp about 10% and then compress the rest of the dimming in that last 30% so you need to change your upper limit and throw away that top 10% of its light. The other thing about "dimmable" lamps is that typically they are only rated to dim until 10% output and then it is all bets off from there. And even if you find a lamp that has the characteristics you like, there is no guarantee that it will remain that way on future purchases.

Thanks for the explanation. I was guessing that this had something to do with the current rectification that takes place on the board in the bulb itself.

quote: Carsten Kurz
Once you agree on a dedicated control line (in this case, DMX), it is no problem to dim LEDs with PWM. The problem with replacing incandescents on dimmers with LEDs is that there is no such control line. With DMX controlled LEDs, you have constant line voltage to the fixture, and tell the light to dim up or down through the control line.

We were given that advice when contemplating LED replacements for the Egyptian: that the only way to make the dimming look really nice is a system that does not require you to increase and decrease the AC line voltage going to the fixture. DMX was mentioned. It's a lot more expensive, and as Tarantino might put it, "pretty f***ing far from plug and play," given the need to install all that low voltage control wiring, though.

quote: Martin McCaffery
If you search, you can find some that will get close.

Agreed - not all "dimmable" E27, straight swapout LED bulbs are made equal. After experimenting with a few different ones, we decided to use these Feit Electric PAR 38 bulbs. They're not the cheapest, but they have the nicest light quality, the closest color temperature to the incandescents they replaced, and the smoothest dimming of any we've tried. The seriously chunky heatsinks mean that they shouldn't overheat and crap out, either; though so far, one has. Incidentally, we have two of them lighting the stairwell, too, which have been running 24/7 since January 2015 and are still going strong.

Posted by Gordon McLeod (Member # 33) on 03-07-2017, 10:28 AM:

Cree makes reliable dimming LED replacements
Juno has a line of dimming LED potlights that get warmer as they dim and they are controlled by a 0-10v ddc control signal (max of 4 per control circuit and they follow sq law curve
But they are not cheap

Posted by Justin Hamaker (Member # 2165) on 03-07-2017, 04:45 PM:

For everyone: Out of curiosity, do your auditoriums dim to 100%? We dim ours down to <5%,but want some light for safety purposes.

Posted by Carsten Kurz (Member # 5396) on 03-07-2017, 05:35 PM:

0-100%

Only during end credits we allow a little light. Of course we do have separate emergency lights, but they are very dim.

- Carsten

Posted by Frank Cox (Member # 6258) on 03-07-2017, 05:48 PM:

I dim my lights down to really dim, but not completely off. The fire inspector would not allow me to have the lights off completely.

Posted by Frank Angel (Member # 248) on 03-07-2017, 06:17 PM:

In all our theatres we have three settings --

House level (about 85% -- we found it makes incandescent last MUCH longer than if we burn them full light output.

Attraction Reel level (down to about 40%)

Show level (as low as we can go but keeping the filaments still at a glow before they would go out)

We have 6 rows of house lights each with between 60 to 100 as the they get progressively wider; from what I have seen of LEDs that are "dimmable," they do go out as noted, down to a low percentage, but that point is not fixed. If you got 10 of them together and dimmed them, they wouldn't all go out at the same time, so what I am batting around is the idea that perhaps the random nature of 500-600 of them jumping off randomly and not at the same time, that might actually give the effect of a smooth dim. Problem is, you would need to buy 600 LEDs test the theory. And then to complicate it more, we don't want them out -- we need there to be a glow which would probably be harder to get accurate and repeatable with that cut off point being so arbitrary from one LED to the next.

Posted by Scott Norwood (Member # 30) on 03-07-2017, 06:48 PM:

I have never really thought much about auditorium lighting, but, thinking about it now, the typical cinema has at least five different lighting circuits in the auditorium:

- "stage lights" or "screen lights" (or "curtain warmers")--light up the blank screen or curtain; these are often colored lamps; these are always at 0% during screenings

- "ceiling lights"--typically PAR-type lamps in recessed fixtures or sometimes chandeliers; these are usually at 0% or just barely glowing during a screening and often come up to mid-level during trailers and credits

- "wall sconces"--sometimes have two circuits, one of which dims to 0% during screenings and one of which either has very small lamps or otherwise is always on, but very dim--these are often at mid-level during trailers and credits

- "aisle lights"--these are usually part of the seats or LED- or Christmas-light bulbs embedded in the floor along the aisles--these sometimes dim and sometimes do not, but are never very bright

- "exit signs"--these always stay lit and, depending upon the placement, can cast an annoying red/orange glow on the edge(s) of the screen

Posted by Steve Guttag (Member # 268) on 03-07-2017, 08:17 PM:

The only reason to have overhead lights "ghosting" is if you don't have your aisle areas sufficiently lit via other means (lamps in the seat standards, runway lights...etc.). There needs to be sufficient light such that people may enter/leave with what lighting is left during the show.

I'd say that if one needs to leave the overheads on for any of the reason above, that it would almost be better to have separate lamps that are permanently on at the desired level (size the lamp, LED, preferably such that whatever requirements are met). Then let the overheads dim fully out. It is doubtful that anyone is going to get a completely uniform dimming action such that when those lamps are at 5-10% they will appear uniform. Not until such time that the industries abandon using incandescent dimmers to run lamps that are going into incandescent sockets. It makes no sense to me to not move away from traditional dimming/fixtures in favor of a purpose built LED system, particularly for plain old overheads (PARs). I think MIT has the right idea here with their ALFs...they just need to make it a dim to zero or smart enough that if the voltage drops below a threshold to automatically drop the lamp out.

Posted by Stephan Shelley (Member # 8587) on 03-08-2017, 06:10 PM:

First off there are minimum required light levels and normally aisle lights and exit signs do not make these requirements. If someone gets injured while exiting in the dark the theatre can be liable.

The Grand Lake has three color circuits in its lighting setup. Was originally for red, blue and yellow. My boss does not like blue so we have doubled up on the yellow. The reds are used as running lights and stay at a fixed level. The yellows go up and fully down with luxtrol dimmers. We have put in the cree bulbs with a colored rubber sock over them to get the color. Going down if smooth. but coming up from off they do tend to blink up and then come up the rest of the way. The red running we had to dim them down as they were much brighter at the preset we were using for incandescent. We have a couple that started blinking on and off. If you have incandescent on the same circuit the will end up being very low.

Posted by Manny Montes (Member # 5639) on 03-09-2017, 01:02 AM:

We're required by city ordinance to leave a certain amount of light on, I forgot exactly what it is but its a certain measurement in lux measured from the floor. I explained to the fire inspector that its silly because the light reflecting off the screen along with aisle lighting would easily meet the level but it is measured with all lights off besides the overheads in a dark auditorium.

Posted by Frank Cox (Member # 6258) on 03-09-2017, 11:51 AM:

I can see the point of not including the light reflected from the screen, since that light could go out during a show. And if the movie quits, that's exactly when a bunch of people will start getting up and moving around. "What's going on?" "Might as well get some more popcorn", etc.

Posted by Justin Hamaker (Member # 2165) on 03-09-2017, 06:43 PM:

The other issue with using reflected light from the screen is this can vary widely depending on the lighting during certain scenes. If you have a movie with dark lighting, it will be very difficult for people coming in from outside to see, and it will also be difficult for many older people who don't have sharp eyesight.

Posted by Frank Angel (Member # 248) on 03-11-2017, 03:23 PM:

In NYC theatres are required to have lighting that allows patrons to clearly see all paths to the exits at all times and of course the exits must have exit lights as well. To my knowledge, the ordinance does not stipulated how you light the isle -- runner strips along the isles, isle down lights in the seat stanchion, overheads at a low level.

I had seen the ordinance years ago and I don't think they gave a measurement in lumens or any other light measurement as to what that level is supposed to be; that certainly may have have gotten more specific over the years. In our big theatre we use both seat stanchion isle lights and house lights at a glow level. The small theatres just use seat stanchion lights.

In cinemas, as opposed to other type of theatres or concert halls, the trick is to adequately meet requirements designed for safety yet not have that light degrade the screen image. I also found that there needs to be a way for that isle and foot step lights not to degrade the experience of the patrons well. I have now been in more than one cinema where the isle runner lights are placed in such a way that they will easily catch the eye of many patrons, especially those seated close to the isle. The effect of these little pinpoints of light constantly in the the corner of ones eye is about a thousand times more distracting than someone with an open cellphone screen. Now fiber optic isle and step lighting is available that is much less obtrusive than the multiple LED light strips. But even those should be of a brightness just enough to fulfill safety requirements and always with consideration of screen intrusion as well as patron distraction potential.