Choosing a video light for your Underwater Video Rig

I thought of writing a brief article about video lights.

In the last years LED lights have become incredibly cost effective and there are many products on sale that look promising and cheap.

A special mention goes to the GoPro a very cost effective option for underwater video plagued by poor performance in low light. The GoPro market seem to have injected new life into the video light marketplace and of course calls for products that are not expensive in line with the typical profile of the GoPro user.

There are many examples of footage with what you would consider powerful lights that actually do not look that bright or even nice, why is that?

Lights are now measured in Lumens which is the SI unit of measure of luminous flux a light is said to emit one lumen if it emits one candela over a solid angle of one steradian. This sounds all very complicated and not very helpful so let’s try to make some sense of this.

Usually when we go an buy a light we are given three key pieces of information:

1. Lumens – this in a way is considered the power of the light*
2. Beam angle – this is the maximum horizontal beam that the light can cover – note this does not relate with solid angles but is merely a 2D view
3. Color – this represent how white is the light, a good light should not exceed 6500K which is what is considered a day light white

*we will see that this is actually incorrect

Let’s have a look at a typical best buy the Archon Video Light W38VR, this light is declared to have the following:

1. 1400 Lumens
2. 120 degrees beam angle
3. 5000K temperature

Looking at the specifications this looks like a very warm (5000K) wide beam light and very powerful so how comes that with a subject at one meter or 3 feet distance this light seems to have no effect on a bright day?

The reason is that lumens are not a real indication of the amount of light that hits the subject but just the total light emitted which includes the light that illuminates the water between us and the subject that we actually are not interest at all in illuminating.

If you had to do a video production in studio you would be looking at intensity of 1000 lux where a lux is measured in lumens/m2. The lux is the density of light that hits a flat surface at a given distance.

With a few calculations we see that the Archon light because of the very wide beam is effective at two feet and produces around 1200 Lux which is great however at 3 feet this drops to 550 Lux which is not that great.

Many times when comparing the new cheaper lights to expensive lights like Sola or Fix  despite the less nominal lumens declared the sola or fix seem brighter at the same distance.

So let’s evaluate the lux produced by a Sola 1200 a light much more expensive than the Archon with only a 60 degrees beam.

At two feet the sola produces around 4000 Lux which would nearly tan a fish, at 3 feet it still produces nearly 1800 Lux dropping to the studio value of 1000 Lux at a distance of 4 feet or 1.2 meters. This is because the illumination is in a smaller and narrower beam and is like the light was more dense.

Of course this has the disbenefit of a narrower beam so we need to step away from the subject in order to cover it properly, this diagram gives an idea

Typical Wide Angle Light Set up

The two set of lines contain the video lights beams, we can see that those beams cross and them keep as combined angle of coverage the same beam of the lights themselves, that is the ideal spot for our lights as there are no shadows there.

Another consideration is the angle of coverage that we need for our camera lens to follow the lights, two 60 degrees beam lights actually cover a 100 degrees diagonal lens or 90 degrees horizontal lens typical of a 18mm lens. The GoPro and all compact cameras with a 100 degree lens fall in this group.

Another consideration to be done is that due to the light angle of coverage there is a minimum distance to be kept if we want to avoid our subject to be in the shade, this minimum distance depends on the light beam and the distance between them.

Once you get closer than one foot it is not necessary anymore to point the lights forward as the amount of water is limited and the lights can be pointed directly at the subject like in this example

Monopod front Macro Light Position

So assuming we want to be at a distance of one foot or 30cm how far do the lights need to be?

For a 60 degrees beam the lights need to be 36 cm away or around 12 inches. For a light like the Archon with 120 degrees beam the distance between the arms needs to be 1 meters or 40″. Considering a tray is around 30 cm or 12″ this means you need arms at least 13″ in order to cover something at one foot and to cover a subject at 2 feet you need two meters distance that means again 33″ worth of arms.

What happens if you only have the tray with the lights on and your subject is at 3 feet with two very wide video lights of 120 degrees beam?

• You illuminate already after 3″ from the camera
• The two lights emit 550 Lux that combined are not such a small output

The net result is that a lot of the water in front of the subject is illuminated and this creates backscatter where all the silt in the water gets reflection and ultimately deteriorates the quality of the footage.

If you ever wonder why underwater photographers that shoot super wide have two 16″ arm segments this is why the want to avoid backscatter with their 110 degrees strobes and still be around one foot distance with their fisheye lens.

So what are the conclusions that we can draw:

1. A wide beam light is only good if you have long arms, the wider the beam the longer the arms otherwise the beams just light up the particles in front of the subject producing backscatter
2. Lumens are not a real indication of the effectiveness of the light per se but need to be taken into account with the beam angle and a lux calculation needs to be performed you want to aim for 1000 lux at the distance of choice
3. Light beams need to be evaluated together with the camera lens horizontal field of coverage 60-80 degree beam angles are adequate for most of the video rigs out there and more is only justified for fisheye type of lens not commonly used in video

I hope you find this information useful and get in touch if you want to discuss, the subject is not that simple!