Dimming mercury lamps...


From the 'not recommended but we're going to do it anyway' files...

I'm lighting a show in a couple of weeks, its set in a scaffold company.
I'm using 6 big hanging lamps to give an industrial feel to the piece.
I want to utilise the start up and colour temperature shift of a hid (sodium or metal halide) lamp when it turns on. Im using this as an effect to start the show
I have some self ballasted mercury lamps. Unfortunately these lamps are instant-on, so yesterday i was experimenting with putting one of these babies on a dimmer, and playing with the intensities. I found out they actually dim surprisingly well! and when the fader is at zero i can get the flicker on and colour effect of a lamp starting up.
So i am thinking that i will switch the lamps on at zero, then fade them up to give the effect of a sodium or metal halide starting up

undoubtably this is bad for either the dimmer or the lamp (or both)
is there anyway i can minimise the damage i will do to either?
im thinking maybe using an equivalent theatre fixture in parallel with the mercury lamp as a ballast?

what damage will i do to the lamps life? the season is only a week long will i be fine?

thanks for any advice
What's the lamp and ballast you intend to use? Especially if electroinc or magnetic and the Ballast's ANSI code.

Lamp's wattage and description?

I’m not an expert on the subject of arc source fixtures nor dimming them but if of help I have looked into it some and have some starting points to consider. Not enough time in the day for me to start tinkering with the subject, nor is arc source gear my specilty, but some things that can be interesting to look into on a subject that comes up at times at work when some designer has a wacky idea.
I do know a very specific amount of caution I would follow in experimentation with this. Primary with this is that should the lamp or any part of the fixture while dimming receive a jump in current something very much could explode in a very dangerous way. The higher the pressure the lamp capsule operates under (AU), the easier it should be to control dimming it without losing the arc, on the other hand this also means the more explosive the lamp will be should something go wrong. Heard my first arc source lamp go bye bye today. Also saw some of the glass that came out of an enclosed housing. Even if cold and more normal in size, I once broke a 400w mercury vapor lamp by accident that was on my desk. Glass did go everywhere including just missing my eyes.

For fixtures in how they dim safely or operate, one might look into the Martin Mac 550. I believe it’s dimmable. What types of control chokes etc. are in us could be of interest. There is also a lot of dimmable florescents on the market.

Most HR - Hot Restrike lamps stage and studio are fully dimmable while maintaining a stable constant color temperature. This as opposed to the color temperature of a incandescent lamp which drops while dimmed with the luminous output. This color temperature not dropping in an arc source lamp when dimmed should be the same with any arc source lamp.

On the other hand, the above lamps are designed to be dimmed. Other lamps while they might be observed to dim well enough could receive some damage from dimming which would effect it’s life, ability to maintain an arc and receive pinch seal problems. Some more modern lamps such as the CMH line from GE advertise a smaller lamp size which helps in hot restrike, faster warm ups and in theory given these common traits it should be fairly dimmable. It’s ceramic capsule/arc tube on the newer style lamps also help to prevent against sodium seeping thru the quartz glass which might become a problem while operating at less than peak voltage or operating conditions. Also the tolerances and advanced materials on the newer lamps should offer a better lamp to dim. On the other hand, a larger globe should offer more stabilization to the inner capsule of the lamp while dimming in making the effects of heat on the lamp take place over a longer period of time and is more stable under things like a voltage spike, or lack of forced cooling.
So it’s up to question and study perhaps on what would be more efficient in a lamp not designed to dim to be dimmed.

(I forget the source but it’s a detail that can be important:)THD = Total Harmonic Distortion - A measure of the distortion of the sine wave on alternating current (ac) systems caused by higher order waves superimposed on the fundamental (usually 60 Hz.) frequency of the system. THD is expressed in percent and may refer to individual electrical loads (such as a ballast) or total electrical circuit or system in a building. The ANSI recommendation is for THD to be no greater than 32% although some electrical utilities may require lower THD’s on some systems. Excessive THD’s on electrical systems can cause efficiency losses as well as overheating and deterioration of system components.”

Maintaining internal lamp pressure and the arc are going to be two large problems which ever type of dimming is done and dependant upon the transformer type which might in combination hurt or help this process. Most of the above hot restrike lamps can be dimmed with either a magnetic or electronic ballast but the magnetic ballast should be easier to work with at very least in dealing with the added power consumption as the luminous output goes down.

Also a note that while not specific to this, something that could be of interest out of the free Osram PDF: Technology and Application, Metal Halide Lamps Photo Optics.
“Operating Conditions: Metal Halide lamps are operated on ac voltage which is supplied by chokes or high-reactance transformers or by electronic control gear. Electronic control gear operates with a constant output and ensure flicker-free light. In the clod state, metal halide lamps, like all discharge lamps, are excellent insulators and have to be made conductive with a high-voltage discharge (ignition). Whereas only a few kilovolts are needed to ignite a cold lamp, pulse voltages ten times higher are required for hot restarting.”

Also a small detail that might be important:
“What currently does adversely affect the lamp life is switching the lamp off while it is still in start-up phase because the filler components are deposited on the internal wall of the bulb and on the electrodes. This impairs both restart behavior and life expectancy.”
Letting the lamp warm up sufficiently before you start dimming it is probably going to be a very good idea. Also a side note from Philips on hot restrike lamps, “Operation of the lamp at voltages greater than 106% of nominal value could damage the lamp.”

Another interesting free PDF article might be: from Osram. Guidelines for control gear and igniters Xenon Short Arc Lamps Photo Optics, Technology and applications XBO theater lamps, Manufacturers of control gear and igniters for special discharge lamps.

And finally some notes directly on the subject:
“Dimming of HMI Metal Halide Lamps: Dimming = operation of the lamp at less than rated power with reduced light output. In this age of flexibility, there is an increasing demand for light which can be individually dosed according to the particular application. The ideal solution would be the “rubber lamp” which could “stretch” across a wide range of wattages with no loss of photometric quality. It is this loss of quality which is the prime concern when we consider dimming metal halide lamps. You may recall the rule of thumb from tungsten-halogen lamps that a 5% drop in voltage will double the life and reduce the color temper as power decreases: discharge lamps behave in a similar way, initially at least.
As you would expect, dimming causes a drop in luminous flux - as is the case with tungsten halogen lamps. The color temperature however, increases (i.e. the lamp appears more “blueish”), while color rendering (CRI) deteriorates as power input decreases. The metals, which are responsible for the red component in the spectrum, are the last of the filter components to vaporise during startup and the first to condense out again when the lamp is dimmed. They are therfore no longer available for generating light. The result is that the light appears more “blueish.” The loss of the red component also means poorer color rendering. The reason why the filter components start to condense again is the drop of the bulb temperature at lower wattages.
These effects can be avoided by regulating the amount of light which grey scale filters or mechanical shutters. The lamp continues to operate at full load, so its photometric properties remain more or less unaffected at every stage. If the lamp is dimmed by electric means it will not reach its optimum operating state and, unlike tungsten-halogen lamps, will not last longer. The best possible operating mode for a metal halide lamp is when it is operated at rated wattage.
Dimming is certainly useful for mobile news reporting teams who are reliant on batteries and will want to operate the lamps at full load only for actual shoots and otherwise stay in standby mode to save energy and reduce the startup time to a minimum.
The temperature of the bulb wall falls more rapidly on a lamp without an outer bulb than on a lamp with an outer in which the discharge tube can only be influenced by the temperature surrounding the lamp indirectly or at least with a long lag time. In terms of dimming, outer bulb lamps are therefore not as sensitive and react more favourably to reductions in wattage with respect to changes in their color quality.
Forced cooling can attenuate temperature-related problems but it cannot eliminate them.
(Osram Photo-Optic Lighting Products Catalog - 1999)”

“Boosting Power to HMP lamps: Boosting = operation of the lamp at more than the rated power with increased light output. We strongly advise you do not consider boosting metal halide lamps (i.e. operating them at overloads) unless the lamps are expressly approved for this purpose. From the photometric point of view, the effects of boosting are virtually the opposite of the effects of dimming: color rendering is improved and the color temperature drops. The increased load on the electrodes and the higher temperatures at the molybdenum foils and on the bulb walls will most probably lead to premature failure of the lamp.
With regard to boosting, the HMP range of lamps is currently an exception. They have been developed and approved specifically for “boosting operation”. Depending on the particular model, the lamps can be operated up to 1.5 times their rated wattage. At these high wattages, a reduction of up to 50% in lamp life can be expected. What we said about dimming also applies to boosting: metal halide lamps operate best at rated wattage. Users in the (overhead) projection sector readily accept this shorter lamp life in view of overwhelming advantages of boosted operation. At presentations which make use of LCD panels and traditional transparencies the ability to regulate the output of HMP lamps is of considerable value: reduced (dimmed) output (down to 50%) for the transparencies with reduced glare for the presenter and increased (boosted) output (up to 150%) for the LCD panels which because of their poor efficiency require as much light as possible.
Since the two forms are often used alternately during actual presentations, the problem of shorter lamp life is unlikely toarise, particularly considering the fact that HMP lamps naturally have a service life which is around 33% longer than comparable HMI models. With the introduction of HMP technology. Osram has succeeded in presenting a range of lamps which has been optimised for projection applications.
(Osram Photo-Optic Lighting Products Catalog - 1999)”

No. 352

1) Atomic 3000 Can Cause Voltage Drops

We have seen a number of cases of fixtures resetting intermittently
when connected to the same AC circuit as Atomic 3000's.

The Atomic 3000 can draw up to 33 A. The heavy current consumption can
cause a short voltage drop that is large enough to disrupt the 5V power
supply in other products like the MAC 2000. If this happens, the
fixture's main processor drops out and then reboots when the voltage
comes back up, causing the fixture to reset.

To avoid this situation, we recommend powering Atomic 3000s on their
own separate AC feed. If this is not practical, we we recommend using
the Atomic 3000 in low power mode, with DIPP-switch 6 "ON".

__________________________GE Lighting Institute

High Intensity Discharge Lamp Dimming
There is an increasing demand to maximize energy savings of lighting sources. While HID lamps are inherently very efficient, many users would like to further increase the energy savings of HID lamps through dimming.
There are two general classes of HID dimming systems. In bi-level dimming, HID lamps are run in a low mode of reduced lamp power when less light is required. Lamps are then switched to 100% lamp power (high mode) when full illumination is needed. The other common class of dimming systems is called “continuous” and allows for user settings from 0% to 100% wattage, and thus, complete light control.
This dimming statement is valid for any kind of dimming system that meets the stated criteria. In general, most bi-level dimming systems meet the criteria, while many of the continuous dimming systems do not.
GE Lighting will warrant its mercury vapor lamps, Multi-Vapor®, PulseArc™, stayBright® metal halide lamps and Lucalox® high pressure sodium lamps on bi-level or continuous dimming systems provided the following operational guidelines, in addition to those provided on the lamp packaging and in the GE 9200 lamp catalog, are met:

Multi-Vapor®, PulseArc™, and Watt-Miser®, stayBright® metal halide Lamps :
ChromaFit™ HPS retrofit Lamps:
Vertical base up (+/-15°) operation only for all types, except for the MVR1000/U which can be operated in vertical base up (+/-15°) to horizontal (+/-15̊) positions when dimmed per the approved guidelines indicated within this document.
Open or enclosed fixtures for 360, 400, 750 and 1000-watt standard and high-output lamps; Enclosed fixtures only for 175, 320, 250, 1500 watt and compact lamps. For other lamp wattages, see the “General Comments.”
Lamp must be started in full-power mode and must be operated in that mode for a minimum of fifteen minutes prior to reduced-power operation.
Minimum open circuit voltage (OCV) of dimming system must meet ANSI requirements in both high or low modes of operation (see appropriate ANSI C78.xxxx documents for specific metal halide lamp minimum OCV requirements.)

Minimum lamp operating wattage as indicated in the following table. If operated below these wattages, the bimetal switch that normally shorts the main and starting electrodes in standard type lamps may not function properly, and this could result in rupture of the arc tube. PulseArc™ lamps do not utilize a bimetal switch and may also be dimmed as specified below.

Lamp Power Rating Minimum Lamp Operating Power Watts)
(watts) (see “General Comments,” item 2)
150 watts (Watt-Miser®) 97 watts
175 watts 97 watts
320 watts 138 watts
250 watts (including ChromaFit™) 138 watts
360 watts (Watt-Miser®) 200 watts
400 watts (including ChromaFit™) 200 watts
750 watts 375 watts
1000 watts 500 watts
1500 watts 750 watts

Mercury Vapor Lamps
All guidelines listed for metal halide lamps in the previous section also apply for mercury vapor lamps.

Lucalox® High Pressure Sodium Lamps:
Any burning position is allowed.
Open and enclosed fixtures are allowable.

Minimum open circuit voltage (OCV) of dimming system must meet ANSI requirements in both high and low modes of operation (see appropriate ANSI C78.xxxx documents for specific high pressure sodium lamp minimum OCV requirements.)Lucalox® High Pressure Sodium Lamps:

Lamps must be operated in the full-power mode for at least fifteen minutes prior to operation in the reduced-power mode.
For dimming systems which reduce line voltage (which can affect the ANSI open circuit voltage value), it is important to reduce the line voltage slowly to avoid premature lamp cycling, especially with older lamps that are already high in voltage and close to the normal drop-out point. In changing from the full-power mode to the reduced-power mode, the time between full power and reduced power must be no less than ninety seconds, and the rate of change of power at any power level between full power and reduced power must be no greater than that corresponding to a linear (uniform) reduction between those extremes in a ninety-second time interval.
For dimming systems that instantaneously switch capacitors into the system, but retain the ANSI ballast OCV value at all times, normal lamp performance can be expected.
Minimum lamp operating wattage for all standard and deluxe high pressure sodium lamps are indicated in the following table.

Lamp Power Rating Minimum Lamp Operating Power (Watts)
(watts) (see “General Comments,” item 2)

35 watts 13 watts
50 watts 18 watts
70 watts 25 watts
95 watts 34 watts
100 watts 35 watts
110 watts 39 watts
125 watts 44 watts
150 watts 53 watts
200 watts 70 watts
215 watts 76 watts
250 watts 88 watts
310 watts 109 watts
360 watts 126 watts
400 watts 140 watts
750 watts 263 watts
1000 watts 350 watts

General Comments
Saf-T-Gard® lamps must not be used in dimming systems.

The effects of line voltage fluctuation, ballast wattage control and lamp operating voltage variation within the range of the ANSI specifications must be considered so that no combination of factors causes the lamp power to go below the specified limits.
HID lamps should not be held in the dimmed mode for extended periods of time. Periodic cycling to full power is required.
Rated life of HID lamps is the total burning time in dimmed and full-power modes.
GE Lighting warrants life, lumens and lumen maintenance will meet published rating performance on bi-level or continuous dimming systems provided the foregoing guidelines are met. Performance criteria such as CCX, CCY, CCT and CRI may vary from specification when lamps are used at lower than full-wattage mode. Lamp efficacy will be lower in dimmed modes than in full-power mode.
The dimming device should maintain the lamp current crest factor, current off time and line dip tolerance, as well as the open circuit voltage, within ANSI and IEC specifications at all operating levels of the lamp.
Lamps that were not specifically mentioned include the 325-watt metal halide lamp, low-wattage metal lamps (<175 watts) and all metal halide lamps burned outside the vertical base up +/- 15̊ position, except for the MVR1000/U which is approved for dimming when operated in vertical base up +/- 15̊ to horizontal +/- 15̊ burning position as stated within. Guidelines for using these lamps on dimming systems may be published at a later date.
GE Lighting Institute
Rev. 10/9/01

Users who are viewing this thread