Video Light Filters

[peterbkk 110]

I mentioned in an earlier thread that I had been experimenting with some video light filters with the objective of finding a better balance between underwater ambient light and the broader (red) spectrum of artificial video lights. Here is the problem, obvious in the screenshot below: the area covered by the video light has a lot of warm colour (reds and yellows) but the background is all greenish-blue. Colour-correcting this scene is either a compromise or a complex exercise of selective secondary colour-corrections.

 

 

I think that I've found a cheap DIY system that can solve this problem and can be tuned to most underwater ambient light circumstances.

 

The main component is a 4x4 filter holder made by Lee Filters. It's all plastic and rubber so nothing to rust. $20USD each at BH Photo: http://www.bhphotovideo.com/c/product/162544-REG/LEE_Filters_GS_Gel_Snap_4x4_Filter.html

 

Here is the holder with an aqua filter (the same filter that I used in the shot further below)

 

 

The slip-on rubber-band mounting system enables it to fit on most underwater video lights. I use them on a pair of iTorch 7s but they also fit the Keldan Luma4s.

 

Here is folder mounted on the iTorch7. It can be "wriggled on" easy enough and will not easily fall off - although I do take them off when getting into and out of the water so that they don't get bumped off.

 

 

The filters are pieces of plastic that I cut from document folders that I bought in stationery shops. I wandered around a few shops until I had found a mix of light blue, dark blue, aqua (green-blue) etc. The firmer document folder covers work best. I also cut some uncoloured plastic squares that are a neutral colour but make a nice diffuser for softer macro lighting for night dives.

 

You can take a few different shades of the colour filters on each dive but I find that I can usually judge from the surface what I will need. The technique is simply to look at the colour of the water and select a filter that is the closest match. If the water looks tropical blue, use a blue filter; if it looks greenish, use an aqua coloured filter.

 

So, here is the result. This is the same scene as above, shot with the aqua filter on both lights then quickly colour-corrected in FCPX to reduce the green and boost the reds.

 

 

To my eyes, this image is very close to what it looked like underwater last week in Lembeh.

(the coconut octopus inside the drink bottle is guarding eggs and in the last few days of her life).

 

I have also used the same system in the deep blue waters of North Komodo, where a light blue filter worked best.

 

So for $40, some time wandering around stationery shops and a little bit of scissor work, you can make a video light filter system that will enable you to balance ambient and artificial light in most situations.

 

Regards

Peter

Edited December 21, 2014 by peterbkk

[thetrickster 328]

That's really impressive Peter.

 

 

Sent from my iPhone using Tapatalk

[Nick Hope 151]

Nice DIY project Peter.

 

On daylight dives I used to snap blue gels into the front of my Light & Motion halogen heads. That worked well because they had a slight recess that was just right.

 

I'm thinking of making some cylindrical cups to go over my Keldan Luna 4X lights to do this. I might have to get them 3D printed unless I can find pipe that has the right diameter. Failing that, I may copy your idea.

 

Kind of ironic. Get lights with a nice high CRI and then completely knacker it with a bit of coloured plastic over the front

 

The other strategy is just to throw so much artificial light on the subject that the ambient daylight is overcome, but that's not always conducive to keeping the subject happy.

[thetrickster 328]

Keldan sell cyan units for their Luna 8s / Video 8M. If you don't want to lower your CRI. :-). Not quite as cheap gels mind!!!

 

 

Sent from my iPhone using Tapatalk

[peterbkk 110]

 

The other strategy is just to throw so much artificial light on the subject that the ambient daylight is overcome, but that's not always conducive to keeping the subject happy.

 

Did you ever see one of the old Jacques Cousteau underwater movies where they included an establishing shot of the dive team. There was the camera guy with a monster tube housing and, behind him, some poor sucker trying to maintain buoyancy, swimming with a huge, (2 to 3 sqm) panel of lights, connected by an umbilical cord to a generator on the boat. Now that's throwing artificial light on the subject! Although, in total, they probably weren't much stronger than the little LED video lights we use today...

 

Regards

Peter

Edited December 23, 2014 by peterbkk

[Nick Hope 151]

Yes Peter, I saw that on their Britannic film. Swimming around down at 100m with a huge bank of lights. In the 70s! On open circuit!

Edited December 23, 2014 by Nick Hope

[kc_moses 142]

Just curious, would the heat of the light melt/warp the plastic over time?

 

I'm still struggle with the theory of the light filter a bit. Say the light is 5000K, without filter, the camera white balance would be set to Auto, or 5000K, but if you put a blue filter on, that will make the light become cooler, what white balance setting would you set on the camera?

[peterbkk 110]

Just curious, would the heat of the light melt/warp the plastic over time?

 

They won't melt underwater. They sit about 1cm in front of the light so there is water on both sides of the plastic filter. The heat is dissipated and they don't even get warm. Out of the water, yes the thinner plastic filters will buckle quickly and melt in under a minute - maybe even burn...

 

I'm still struggle with the theory of the light filter a bit. Say the light is 5000K, without filter, the camera white balance would be set to Auto, or 5000K, but if you put a blue filter on, that will make the light become cooler, what white balance setting would you set on the camera?

 

I'm not sure we can easily describe a "unifying theory" using colour temperature numbers. It's not just about the colour temperature in Kelvins. The colour cast is also a major factor. The sunlight hits the surface of the sea already influenced by the atmosphere (e.g. angle of the sun; clouds, etc.). Then sunlight passes through the seawater, with the double effect of spectrum absorption, and the filter effect of the particles suspending in the seawater. Lots of conditions in play.

 

So let's talk logic rather than theory. The logic behind the video light filters:

• putting some artificial video light on the subject, in some circumstances, will help punch up the contrast, improve background separation and help DoF and focus - and may slightly increase the red end of the spectrum on the subject (see caveat below)
• during daylight hours, the sunlight will always dominate, especially in the further distance where the video light falls off
• at depth, sunlight becomes much more blue or green, depending on the water
• if the colour temperature of the subject (artificial light) is not similar to the ambient light (water-filtered sunlight) it looks artificial
• any color correction (WB) done in camera applies to the whole image so can not fix this problem
• fixing the colour temperature imbalance in an editor is possible but not easy - keyed, secondary colour correction masks
• so the best approach is to change the colour temperature and colour cast of the video lights to approximately the same as the ambient light
• if the video light emits from the LEDs at the same colour temperature and cast as sunlight, then a coloured plastic filter can do the same to the video light as the water does to the sunlight - cool it down and apply a colour cast
• then we can use the standard WB features in camera
• and the primary colour correction features in the editor can easily bring the whole scene to the preferred look

Caveat: this approach is not going to put the red spectrum back into the whole scene - nothing can do that (unless we dive with humungous overhead video lights like a movie set). The video light filters match the ambient sunlight so they are essentially reducing the red. Depending on the colour of the video light filter that you select, you can add a little more red spectrum to the subject - but add too much and you are back to the imbalance problem that we are trying to solve.

 

The five filters that I made (from the covers of plastic document holders):

1. Light blue for shallow, clear tropical water
2. Deep blue for deep, clear tropical water
3. Light blue-green for shallow tropical water with some suspended plankton
4. Deep blue-green-green for deeper tropical water and/or lots of suspended plankton
5. Neutral milky diffuser for night dives to produce a soft light with gentle shadow edges

How does it work in practice:

1. Select a video light filter that visibly matches the colour of the underwater ambient light, changing the filter depending on the conditions - e.g. a deeper blue filter might be required when deeper (does not need to be exact - close enough is good enough)
2. White balance the camera for the whole scene (I use the presets - see below)
3. Edit the footage, adjusting the colour correction to the preferred look.

Approach for using the camera presets for WB:

 

Rather that stuff-around with MWB while diving, I want to be able to quickly get to a WB that approximates the conditions. The typical conditions are:

1. Light blue colour cast - shallow, clear tropical water
2. Deep blue colour cast - deep, clear tropical water
3. Light blue-green colour cast - shallow tropical water with some suspended plankton
4. Deep blue-green-green colour cast - deeper tropical water and/or lots of suspended plankton
5. Video lights dominate (night, cave, etc.) - use AWB

So I made 4 A4 pages in PhotoShop that approximate each of those colour casts and printed them. Then I put the pages in sunlight and took an MWB off each one, saving one into each of the 4 WB presets of the GH4 - then saving all the GH4 settings into the 3 Custom Sets (C1, C2, C3) that I had already created for different exposure / focusing custom sets.

 

I have attached a PDF of each of the 4 A4 pages that I use to create the WB presets. I find that these presets work well in the tropical waters around Southeast Asia (e.g. the Light Green and Mid Green work in Lembeh). The plastic filters are a similar range of colours but they are stronger colours with more blue.

 

Underwater, I simply assess the conditions, pop on the appropriate video light filter to the light heads, then flip through the WB presets to the preset that matches the ambient conditions (and the video light filter) and shoot.

 

Gets close enough to a good result that everything else can be easily fixed in FCPX.

 

Regards

Peter

Light Green MWB Preset.pdf

Light Blue MWB Preset.pdf

Mid Green MWB Preset.pdf

Mid Blue MWB Preset.pdf

Edited December 25, 2014 by peterbkk

[CheungyDiver 53]

Peter you could write a little guide. I am sure this should be shared. In the spirit of what Wetpixel stands for.

 

 

One other way is have a low light video camera like the three chip type and shoot ambient. Find the correct white balance card.

Your are stuck if you do close up or dim light so it's back to using video light.

 

 

Until of course selective colour balancing is possible during live shoot. It is not impossible. UW mode already exist in some cameras albeit still very crude.

 

Cheers

 

David

[fxvideo 7]

Thanks Peter for the clear information and I like to try this setup you discribed. I use 2 x 3000Lm LED lights and like to add the blue filters as discussed. With some DIY I found some covers I can use to fit over my lightheads and the plan was to use blue gel filters normally used for lighting purposes. Is it possible to post a photo of the blue filters you using to see what tint of blue they are. I planning a trip to Bonaire this year and like the have a good start.

 

Thanks

[Davide DB 583]

Keldan sell cyan units for their Luna 8s / Video 8M. If you don't want to lower your CRI. :-). Not quite as cheap gels mind!!!

 

I nave a couple of them unused to sell

 

davide@mobile

[John Doe II 16]

Now that's reviving an old thread ! Bravo!