Underwater Lighting
One of the challenges in underwater photography and video is light. And more so, bringing down all the required light. An understanding of how light propagates through water is essential to make informed and correct lighting choices.
Light in water behaves rather differently than above water. And this is a result of the interaction of atomic scale to the macroscopic life that fills the oceans. Light no longer follows a straight line from the source to the subject, and the inverse square law becomes a poor predictor of luminous intensity.
To understand this, it is useful to frame the discussion in terms of total illumination of a subject at a certain distance from a light source. Total illumination is the sum of the direct and diffuse illumination that reaches the subject. Direct illumination is light hitting in direct line-of-sight from the source; whereas, diffuse illumination is light travelling in an indirect path through multiple scattering processes to arrive at the subject.
Characteristics of direct and diffuse illumination are governed by physical and practical limits of temperature, salinity, concentrations of dissolved solids to the atomic structure of water molecules themselves. They all contribute to the attenuation of the light by absorption and scattering phenomena.
Direct illumination losses are primarily caused by the absorption of light in the water volume, between the source and subject. Absorption of light in water is generally dominated by the interaction of photons with water molecules - as a photon contacts a water molecule, it is absorbed and converted to heat energy. This interaction can be easily observed in the visible colour spectrum, light in the red and violet ends is strongly attenuated while the blue and green region of the spectrum has minimal attenuation, this is what gives water its blue-green appearance. Because of the higher concentration of organic matter, that is generally higher in coastal and estuarial waters, the level of light absorption increases and additional absorption of the blue spectrum, typically causing a greener appearance.
Light scattering relates to refracting or reflecting part if the light away from the original direction of light travel. These variations are caused by temperature transitions, suspended particles, dissolved solids, and biological suspended particles.
The colour of an underwater white light source will desaturate due to absorption and scattering - it will resemble the colour of a monochromatic source. Due to the spectral attenuation through the water, colour information transmitted through water is constricted.
The important thing to take into consideration is that while the increase of illumination to the subject may help, the generated scatter will reduce the contrast between the subject and the background, the organic matter along the illumination path will also disperse the light. This imposes practical limits on the distance at which an object can be detected.
The scattering behaviour of light underwater is proportional to the beam angle of the source. Having more tightly parallel light source helps reduce scatter at longer distances.
In sum, the closer the camera is to the subject and the more parallel positioned and powerful lighting we have, the better results are achieved in underwater photography and videography.
Sources:
http://www.deepsea.com/knowledgebase/technical-resources/understanding-the-basics-of-underwater-lighting/
https://en.wikipedia.org/wiki/Snell's_window
https://en.wikipedia.org/wiki/Snell%27s_law
Light in water behaves rather differently than above water. And this is a result of the interaction of atomic scale to the macroscopic life that fills the oceans. Light no longer follows a straight line from the source to the subject, and the inverse square law becomes a poor predictor of luminous intensity.
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| Snell's Window (also known as Snell–Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air. |
To understand this, it is useful to frame the discussion in terms of total illumination of a subject at a certain distance from a light source. Total illumination is the sum of the direct and diffuse illumination that reaches the subject. Direct illumination is light hitting in direct line-of-sight from the source; whereas, diffuse illumination is light travelling in an indirect path through multiple scattering processes to arrive at the subject.
Characteristics of direct and diffuse illumination are governed by physical and practical limits of temperature, salinity, concentrations of dissolved solids to the atomic structure of water molecules themselves. They all contribute to the attenuation of the light by absorption and scattering phenomena.
Direct illumination losses are primarily caused by the absorption of light in the water volume, between the source and subject. Absorption of light in water is generally dominated by the interaction of photons with water molecules - as a photon contacts a water molecule, it is absorbed and converted to heat energy. This interaction can be easily observed in the visible colour spectrum, light in the red and violet ends is strongly attenuated while the blue and green region of the spectrum has minimal attenuation, this is what gives water its blue-green appearance. Because of the higher concentration of organic matter, that is generally higher in coastal and estuarial waters, the level of light absorption increases and additional absorption of the blue spectrum, typically causing a greener appearance.
Light scattering relates to refracting or reflecting part if the light away from the original direction of light travel. These variations are caused by temperature transitions, suspended particles, dissolved solids, and biological suspended particles.
The colour of an underwater white light source will desaturate due to absorption and scattering - it will resemble the colour of a monochromatic source. Due to the spectral attenuation through the water, colour information transmitted through water is constricted.
The important thing to take into consideration is that while the increase of illumination to the subject may help, the generated scatter will reduce the contrast between the subject and the background, the organic matter along the illumination path will also disperse the light. This imposes practical limits on the distance at which an object can be detected.
The scattering behaviour of light underwater is proportional to the beam angle of the source. Having more tightly parallel light source helps reduce scatter at longer distances.
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| Underwater Lighting Scheme |
In sum, the closer the camera is to the subject and the more parallel positioned and powerful lighting we have, the better results are achieved in underwater photography and videography.
Sources:
http://www.deepsea.com/knowledgebase/technical-resources/understanding-the-basics-of-underwater-lighting/
https://en.wikipedia.org/wiki/Snell's_window
https://en.wikipedia.org/wiki/Snell%27s_law
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