As smart books write - "With an optimal arrangement of lighting devices with an increase in their luminous intensity by 10 times, the visibility range increases by only 15%." Practice confirms this statement. (Rogov A.A. 'Photography under water' - Moscow: Nauka, 1964).
Let's see why. To do this, you need to have a general idea of the work of the human visual apparatus and such concepts as reflection, absorption and scattering of light. Also, there is no getting away from understanding light as a packet of electromagnetic waves of different intensities in a certain range.
You can look at this in more detail in the first and second parts, here it will be enough for us to repeat some basic concepts we need:
- if we do not look at the light source itself, then the light that we see is a packet of waves, of different frequencies and different intensities, which is reflected from objects;
- the color of an object is determined by what part of the spectrum it reflects;
Let's go from simple to more complex - first let's see what happens to the light in transparent water and then we will add turbidity, suspension and see what happens already there. When light passes through water, it is absorbed, but not uniformly absorbed.
Because of this, depending on the distance traveled by light in water, its spectral composition changes. Therefore, even in very transparent water with increasing depth, the picture becomes poor in color. Fish and corals are very bright and beautiful in sunlight, with increasing depth they become not at all bright. We see them, but only part of the light spectrum gets into the eye, the rest is absorbed by water. You can correct the picture if you compensate for the resulting skew with an artificial light source. At the same time, its spectral composition should be formed in such a way that, taking into account absorption in water and the presence of some light background, ensure that a light packet similar to the light on the surface hits the eye.
It is clear that in this case the characteristics of this source should differ from light good for air. The practical takeaway from the above - striving for high CRI LEDs in underwater flashlights is a waste of money. Even in very transparent water and at night, after passing the distance from the flashlight to the target and from the target to the eye, it will inevitably be partially absorbed by the water and change its spectral composition and the resulting picture will be far from ideal. We will come back to the choice of a lantern for transparent water - while we are interested in the physics of the process.
Now let's complicate the conditions and make the water muddier. Now the light will not only be absorbed but also scattered - reflecting from the particles in the water. The bulk of the particles of turbidity are much larger than the wavelengths of light and the scattering does not depend in any way on the wavelength or, in other words, on the color. This refutes a common misconception, like the color of the fog lights is yellow, because it is not scattered by fog particles. The fog particles are too large and the physics of the process is completely different.
Let's try to compare it all with underwater light. Light collides with particles and is reflected from them scattered. The part that has slipped past them reaches the target, is partially reflected and through again this turbidity gets into our eyes. In principle, there can be a lot of total light entering the eye with a powerful lamp, but most of this will be stupid light reflected from particles of turbidity. This light will reduce the photosensitivity of the camera, even though the eyes and useful light will move beyond this limit of photosensitivity.
How to deal with this? The first thing that suggests itself is to separate the line of sight and the position of the illuminator in space - the light is reflected as much as possible in the direction of the light source. The second point is to make the beam narrower. In this case, there will be relatively few illuminated particles, the field of illumination will be smaller and this will allow the eye to more effectively adjust its light sensitivity.
Further - which means - the light is reflected from the particle - this means the particle does not absorb it. For example, a particle of brilliant green is green - it does not absorb the green part of the spectrum. If you illuminate water with brilliant green with light with a green spectrum, then only brilliant green will be clearly visible, and if you remove the green component from the light, then it will not be visible at all - the particles will be black. In other words - by changing the spectral composition of light - we can reduce the light reflected by the haze.
With green haze everything is relatively simple. The rest of the turbidity is more complicated - the trouble is that the particles in the water have a different color, and often have the same color as the objects of interest to us. Therefore, for the effectiveness of this method, it is necessary to be able to change the spectral composition in a wide range and smoothly - and this is quite difficult technically, but we are working in this direction.
Be that as it may, there is so little light in the muddy water that there can be no talk of any daytime color vision. Everything - now we get the basic information for creating images through night vision. And night vision is about contrast. That is, we are no longer interested in color - we are interested in the brightness of the illuminated objects. This is an important point to understand - all fish, driftwood, grass - have their own color on the surface of the water - that is, they all reflect some part of the spectrum. Different parts – conditionally: grass - green, driftwood - brown, carp - yellow.
If we illuminate objects with full-spectrum light, then we will perceive their color as shades of gray. And how will the picture change - if you remove some part of the spectrum from the light - for example, the green one? The grass will turn black - its brightness will decrease and accordingly the contrast between the grass and the fish standing in it will grow. More contrast - it is easier for the brain to form an image. That is why foglights have a depleted spectral composition - they are yellow. In principle, a similar effect can be obtained with green light, for example.
That is - again, it is desirable to be able to smoothly and in a wide range to change the spectral composition of our light and select one at which the contrast will be maximum.
Now about the shape of the beam. There are many opinions on this, but I would like to get an answer to this question from the point of view of logic.
In order for visual work to be carried out, the required amount of light must enter the eye. Since the hunter is looking not at a flat wall, but at objects separated in space, in order to obtain the necessary light response, these objects must be illuminated in different ways. Let's imagine a spatial scheme - a hunter - a lantern - a reservoir. The light from the flashlight will travel different distances in the water falling and reflecting from the grass and from the fish located further away. Therefore, the light illuminating the grass should be weaker, and the light illuminating the fish should be stronger. Or, in other words, the center is brighter and the halo is weaker. What should be this ratio? And this depends on the transparency of the water - it will be different. Therefore, it is very important to be able to adjust this ratio.
The diver first turns on the halo, adjusts its intensity so that nearby objects can be seen, and then increasing the intensity of the central part gets the depth of the picture. Having this opportunity, you can optimize your flashlight to the maximum for specific conditions, while receiving the necessary and sufficient amount of light for comfortable hunting.
Now I can formulate my requirements for a conventionally ideal underwater flashlight:
- beam shape - cone with an angle of 120-130 degrees, with the ability to adjust the intensity of the central part from 1: 1 to 1:30;
- the spectral composition of the central and peripheral part of the luminous flux should be adjusted smoothly and in the widest possible range.
Unfortunately, such flashlights do not yet exist in nature. We have coped with the first part of the task - independent adjustment of the center and the halo and implemented such an opportunity in the flashlights Dnepr”, on the second part we are still working.
Good luck with your choice.