Dimmers - I don't get it

[Josh Bass]

Okay, so dimmers change the color temp of your lights the more you dim them, making the color progessively more orange. What good are they, then, if you don't use a dimmer on all your lights in unison, at the same level of "dimitude." You couldn't just dim one, then you'd have one noticably orange source and the rest would like white, right? Do you put a CTB gel on it? Then you have to strike the balance between the level of "blueaciousness" and the level of dimitude so that one source matches all your others.

[Andrew Petrie]

I'm just having a hard time finding a high quality dimmer. Last year I read up on DIY lighting kits, and authors touted high quality $30 dimmer switches to be essential.... All I can find at Home Depot or the Building Box are $4 flimsy things.

[Rob Wilson]

Andrew - Take a look here - http://www.westsidewholesale.com/product.asp?pid=204

[Mike Rehmus]

Josh,

The 'good' dimmers don't just reduce the voltage to the light, they run the light at a fixed amplitude and higer-frequency (than the power line) with 'square' waves and then start eliminating some of the waves as you dial in dimming.

What this does is allow the lamp filament to heat up almost to normal operating temperature with each pulse (square wave) and keep the color temperature more constant. Color temperature does dip as you dial in the dimmer but it reduces at a much lower rate that that of conventional dimmers.

The limits are that you cannot reduce the light to zero, only around 60% IIRC.

NRG's on-camera light, the VariLight Pro has such a control.

NRG is coming out with a stand-alone control for AC-powered lights that does the same as the on-camera light controller.

This better control of color temperature is why my portable kit includes two of the VariLight Pros and a battery belt.

[Andre De Clercq]

Mike, I strongly doubt that the filaments of low voltage incandescent lights have thermal time constants which would allow color temp changes even within a frame period and even less at higher drive frequency. Where did you find this information?

[Mike Rehmus]

That's how the NRG VariLight Pro works if I understand them correctly. They are getting ready to unveil a stand-alone dimmer setup for studio lights as they have for non-NRG on-camera lights.

As I pointed out, this technique does not allow for complete dimming of the lights.


There is no need to dim withing a frame period, one is looking for a longer-term effect. Indeed, short term effects would be bad as they would be visible, Andre.

[Andre De Clercq]

..."one is looking for a longer-term effect" What do you mean by this? How does this relate to the higher frequency you mentioned? I still like to find the original info about this concept.

[Mike Rehmus]

By longer-term I mean relative to the video frame rate. The system uses the thermal mass of the halogen bulb to smooth out the power (as I understand it)

Try www.nrgresearch.com, that has/had the explanation I remember.

Here's what they say about the VariLight Pro:

"The varalux Professional accepts DC lamps from 20 to 100W and allows them to be instantly adjusted from 10-100% of the lamp's power. This makes it the ideal light for applications where varying distances and intensities are required because the perfect light level can be chosen without removing hot lamps or fussing with diffusion filters. The Varalux Professional uses NRG's LightGate technology which virtually eliminates color shift and conserves precious battery power. "

If I recall correctly, I asked an NRG tech exactly how the system worked and that's what he answered. That it used a high-frequency pulse population control (cannot remember the tech term for this but it's not pulse width modulation, that's what the cheapest modulators use).

My personal take is that the color temp does finally get effected but not until somewhere around 60% of the lamp's full output.

[Andre De Clercq]

Mike, thanks for your response, but I don't find anything about the color shift issue...I still don't believe that story...would be intersting to compaire side by with a standard dimmer concept. On the power supply they maybe use a resonant power supply (RPS) concept which result in high efficiency and lower radiation levels as compared to standard SMPS.

[Mike Rehmus]

Andre, I can't help your disbelief. Maybe you'll have to call their 800 number to get more info. They did adress the color shift issue in the advert blurb I provided.

But lots of people, myself included, use the VariLux Pro exactly because it has minimal color change when dimming.

I think we've exhausted the topic.

[Rob Lohman]

Doesn't "Lowel" have such a dimmer?

[Mike Rehmus]

1.

One diference between cheap and the more expensive home light dimmer controls used to be that the cheap ones just hacked the power on and off any ole time. The more expensive units turned the power on and off only when the voltage was crossing the 0 volts line. This avoided RFI problems which the cheaper dimmers caused.

2.

Here is a response I received from NRG with regard to their Studio Light Controller. It does support the high frequency statement I made in an earlier post:

"The current units only turn the light on or off therefore dimming is not applicable. When the 2010 is available it functions at high frequency like a Varalight.

Paul True
NRG Research, Inc.
575 S.E. Ashley Pl.
Grants Pass, OR 97526
paultr@nrgresearch.com
www.nrgresearch.com


-----Original Message-----
From: Chris Gesner
Sent: Monday, July 07, 2003 9:32 AM
To: Paul True
Subject: FW: Lightmaster control question



-----Original Message-----
From: Mike Rehmus [mailto:mike@byvideo.com]
Sent: Saturday, July 05, 2003 10:26 AM
To: sales@nrgresearch.com
Subject: Lightmaster control question


Hi,

Do the Lightmaster products maintain color temperature like the
Varilight Pro or do they just reduce 60 Hz power?"

[Andre De Clercq]

Mike,
Apparently you didn't get an answer to yr question..."it functions at high frequency like a Varilight" is not the answer to your question about the color temp. Still looking for somebody explaining this mystery (or NRG lie). The reason why I am interested is because I have been working in the past on color temp stabilized light dimming for medical and other applics. There are several solution available, but never with a single non moving incandescent lamp, whatever the the drive strategy is.

[Mike Rehmus]

Well, were I you, I'd email NRG directly and ask them.

Note that they never said the color temperature did not change, just that it doesn't change very much over the top of the range using their product.

I think that implicit in their reply is the point that you disputed that they used a high frequency power control to run the VarilLight Pro and thus keep the color temperature up.

Andre, you don't believe me. Fine. But go to the source of the statement and fight it out with them if you have a problem with their claim.

Many years ago, when I needed to dim a light in a sinusoid pattern for use in an psyco-optics lab at U.C. Berkeley, I built a mechanical diaphram and inserted it in the optical path of a lamp/lens combo. IIRC, it was an old slide projector. The diaphram, placed in the correct spot, evenly controls light levels uniformly across the entire field and does not change the color temperature at all.

[Andre De Clercq]

I never disputed their use of high freq controllers. I only said that using high freq, or rheostat, or AC dimmers (with or without zero crossing)...all result in exactly the same color temp changes when dimmed.