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 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):
Light blue for shallow, clear tropical water Deep blue for deep, clear tropical water Light blue-green for shallow tropical water with some suspended plankton Deep blue-green-green for deeper tropical water and/or lots of suspended plankton Neutral milky diffuser for night dives to produce a soft light with gentle shadow edges
How does it work in practice:
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) White balance the camera for the whole scene (I use the presets - see below) 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:
Light blue colour cast - shallow, clear tropical water Deep blue colour cast - deep, clear tropical water Light blue-green colour cast - shallow tropical water with some suspended plankton Deep blue-green-green colour cast - deeper tropical water and/or lots of suspended plankton 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.
Light Green MWB Preset.pdf
Light Blue MWB Preset.pdf
Mid Green MWB Preset.pdf
Mid Blue MWB Preset.pdf