Videolight desgin - PWM dimming?

[OS74]

A typical cheapo dimming light is PWM modulated with some frequency, and the on-part of the cycle can be quite low.

I think these will cause some side effects when interfering with the shutterspeed (actually frequency) of the camera.

Anyone here with experience how to control the power on light intended for:

1 Videolighting
2 Photo lighting


Is it enough with just a gigh enough requency of the PWM, and what is high enough in that case?

Cheers
/O

 

[Steve K]

just a thought..... seems like it would depend on the shortest time that the shutter is open. The pwm period would have to be at least shorter than that shortest shutter time.

of course, even in that situation, the pwm period and shutter wouldn't be sync'ed, so while the shutter is open, you might get the "on" time of two pwm periods, resulting in the wrong exposure. Based on that, I'd say that you would want to have 3 or more pwm periods occur while the shutter is open.

As an example, let's say that the shutter might be open for 0.001 seconds. Using a rule of thumb that says there should be 3 pwm periods during this time, the pwm period would be 0.001/3 seconds, or 333 us. This is a pwm frequency of 3kHz, which is pretty easy to do, generally speaking.

 

[DIWdiver]

That's fine if you are using film. But who uses film today?

Digital images are captured differently. Some are scanned, much like the image on an old CRT television is scanned. If you have a scanning camera with a max 'shutter speed' of 1/1000th of a second, chances are it completes a scan in 1/1000th of a second, or somewhat less. If your light turns on and off three times during a scan, you will have three light and three dark probably horizontal bars in your image.

Imagers can only react at a certain speed to changes in light, so to eliminate artifacts you have to run a PWM period substantially less than the reaction time of the imager. This may be many times shorter than the shutter speed, and may well be beyond the ability of a driver to drive an LED. Or not.

On the other hand, I believe there is a digital imaging technology (maybe more than one) that does not use scanning, and is not as susceptible to these bars as a scanning imager is, and a PWM'd light source would be less likely to cause artifacts in the image.

Bottom line is you have to know your camera and your light before you will know if a PWM light source will cause problems, and if you want a light that you can depend on to create good images with a variety of cameras, you want one that does not have pulsating light output (not necessarily the same as not using PWM). And there won't be a general 'rule of thumb' that correlates PWM frequency (or period) to problems at certain 'shutter speeds', at least not reliably across various camera technologies.

 

[lucca brassi]

small camera comparation http://www.gizmag.com/2013-actioncam-comparison-guide/30064/

shutter vs fps http://www.mediacollege.com/video/camera/shutter/shutter-vs-fps.html , http://www.red.com/learn/red-101/flicker-free-video-tutorial, http://provideocoalition.com/aadams/story/rolling_shutter_and_flickering_hmis/

fps http://www.steves-digicams.com/knowledge-center/video-frame-rates-24p-25p-30p-60i.html

led flickering http://www.sonycreativesoftware.com/Forums/ShowMessage.asp?Forum=4&MessageID=628740

 

[arek98]

I'm using HBFlex in one of my lights. It goes to PWM 400Hz below 500mA output and flickering is visible when filming with both my cameras: Canon S95 and Sony NEX 5N.

IMO for video stay away from PWM.

 

[OS74]

I could imagine that serious commercial video lights would use proportional current limitation instead of PWM at least for low powers.

Then there would be no issue with flickering, but the sacrifice is reduced efficency and more heat (ok at low power)

Is this correct?

/O

 

[joachimk]

hello

i build videolights from 7000L up to 44.000L
all used professional ( national geographic) and tested with canon 5Dmk3 up to red epic cameras

and for this i use pwm dimming ( the best you can get)

but with 400hz or 2Khz it isnt possible

small test we did
https://www.youtube.com/watch?v=BKyaFGOZtaM

 

[DIWdiver]

hello

i build videolights from 7000L up to 44.000L
all used professional ( national geographic) and tested with canon 5Dmk3 up to red epic cameras

and for this i use pwm dimming ( the best you can get)

but with 400hz or 2Khz it isnt possible

small test we did
https://www.youtube.com/watch?v=BKyaFGOZtaM

What PWM frequency do you use?

 

[joachimk]

that i won't say
it has taking me a lot of building testing and money to get it.
what i will say is that it not only your pwm generator, but also your driver that has to support the pwm rated.

and most drivers i now about that you can buy on the www are:
500hz
2000Hz

 

[H20doctor]

hello

i build videolights from 7000L up to 44.000L
all used professional ( national geographic) and tested with canon 5Dmk3 up to red epic cameras

and for this i use pwm dimming ( the best you can get)

but with 400hz or 2Khz it isnt possible

small test we did
https://www.youtube.com/watch?v=BKyaFGOZtaM

Edge Dive tech makes the brightest Video light on the market FYI

 

[Packhorse]

Edge Dive tech makes the brightest Video light on the market FYI

No they dont.

 

[joachimk]

Edge Dive tech makes the brightest Video light on the market FYI

I've never said i make the brightest video lights . :wave:
if i put 4 of the 44.000 l in one housing than thats a video light
But what battery will you need to power that thing for 60'

 

[H20doctor]

No they dont.

? the Zeus is 90,000 Lumens thats 8ea 100 watt Led chips... I cant find a Larger Light system than that on the current video market.? Even the Big Blue VL15000P cant compete with these Monsters...

 

[joachimk]

? the Zeus is 90,000 Lumens thats 8ea 100 watt Led chips... I cant find a Larger Light system than that on the current video market.? Even the Big Blue VL15000P cant compete with these Monsters...

i have 44.000L with 1 cob led. :candle:

i have seen one big *** rig it was with surfuce support ....
i don't know exact how maby lumen but is was a lot , putting the sun on 70m depth

 

[OS74]

I am pretty sure the project you mention is this one:

The incredibly well preserved Swedish Mars wreck of 1564

This is A 4,5kW ligthing rig. Does anyone have pictures of the actual rig?

http://www.oceandiscovery.org/?q=node/68

 

[joachimk]

​

I am pretty sure the project you mention is this one:

The incredibly well preserved Swedish Mars wreck of 1564

This is A 4,5kW ligthing rig. Does anyone have pictures of the actual rig?

http://www.oceandiscovery.org/?q=node/68

Yes thats it.

 

[Illum]

400,000 lumen you'll run into issues cooling with air, in water oughta be fine, hard part is the enclosure, and power, and not running electrolytic corrosion via electrolysis with that power.

in general, unless you know the shutter speed of your camera and are designing a video light specifically for that camera, I would not consider PWM dimming at all.

 

[H20doctor]

I am pretty sure the project you mention is this one:

The incredibly well preserved Swedish Mars wreck of 1564

This is A 4,5kW ligthing rig. Does anyone have pictures of the actual rig?

http://www.oceandiscovery.org/?q=node/68

very nice , now thats a Light Rig !!! I too would love to see the case and how its feed voltage ...

 

[LEDPunisher]

? the Zeus is 90,000 Lumens thats 8ea 100 watt Led chips... I cant find a Larger Light system than that on the current video market.? Even the Big Blue VL15000P cant compete with these Monsters...

There are 100,000 lux 1,000w COBs. Have fun with the heat sinking.

 

[Packhorse]

There are 100,000 lux 1,000w COBs. Have fun with the heat sinking.

The ocean is a pretty big heat sink. I dont think its an issue.