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The Nature of Light

Light is a form of electromagnetic energy which is radiated from a source. Natural light travels to earth from the sun at approximately 300,000,000 metres per second, having the same velocity as all forms of electromagnetic energy, which include microwaves, radio waves, x-rays, gamma rays, infra red and ultra violet waves. These differ from each other only in their frequency, or rate of oscillation. Each is designated by its wavelength, which is determined by dividing its velocity in metres per second, by its frequency in cycles per second, or hertz. The resultant is in metres.

The illustration, above, depicts the electromagnetic spectrum and illustrates the position of visible light in relation to other forms of radiation. The visible spectrum is enlarged to show its individual components which, together, form "white light". The frequency of light is in the order of 600 billion hertz and accordingly, its wavelength is so short that it is expressed in nanometres, one nanometre being one thousand, millionth part of one metre (1nm = 10-9 metres).

The varied colours of nature owe their origin to the composite character of white light and the fact that, if some of the components are subtracted from it, that which remains appears coloured. A red rose absorbs most of the white light falling upon it, except the red component, which it reflects to the eye. The same rose appears black in yellow light from a low pressure sodium lamp however, since it absorbs yellow light and the source produces no red to be reflected. Hence, the colour of an object depends upon the nature of the object and also the nature of the illumination.

Measurement

Light intensity is generally measured by the "photometric" system, which is designed to gauge in keeping with the sensation of brightness in the human eye. An alternative mode of measurement, based on the watt, a unit of power and known as the " radiometric" system is less commonly used outside the laboratory.

The following units of measurement will be found useful when designing a lighting system or reading a lamp or luminaire manufacturer's catalogue.

Lumen: A unit of luminous flux ( light energy) emitted by a source.
Lux: A unit of illuminance, the measure of density of lumens falling on a surface. One Lumen per sq. metre = One Lux of illuminance.
Candela: The apparent luminous intensity of a light source, expressed in lumens per solid angle (cone). One Candela = One Lumen per steradian.
Candelas per Unit Area: The observed brightness of an illuminated surface is termed luminance.

The formulation of the photometric units and the calibration of measuring equipment has a most interesting history but the modern standard, based on the candela, was established, internationally in 1939 and relates to the luminance of a small hole in a hollow enclosure, maintained at the temperature of solidification of platinum.

The Inverse Square Law: The intensity of light falling on a surface varies inversely with the square of the distance between source and surface. If a surface receiving illuminance of one lux, at one metre from the source, is removed to two metres from the source, it will receive 1/4 lux, and at three metres, 1/9 lux.

Colour: Colour is expressed by a temperature on the thermodynamic, or Kelvin scale. In this, the colour content of a light source is related to that of a black body raised to a specific temperature. The Kelvin scale employs the same temperature interval as the Celsius scale but relates to absolute zero temperature, which is taken to be minus 273°C, instead of the freezing point of water, zero°C.
Accordingly, Kelvin temperature (°K) = Celsius + 273.

Despite a broad variation between locations and seasons, the colour temperature of natural daylight at mid-day, is taken to be 5,000°K. An ordinary tungsten filament lamp produces an output of around 2,360°K and a tungsten halogen capsule in the region of 3,000°K.

Efficacy: The efficacy, or efficiency of a light source is measured in lumens per watt, the watt being a unit of electrical power. The efficacies of lamps in general use are compared in Fig.5.

The Transmission of Light Through Water

Natural daylight travels the distance between the sun and the earth's atmosphere with little or no interference along the way. Our atmosphere has a filtering effect on the sun's rays however, due to the presence of certain gasses and suspended matter. Suspended matter within the atmosphere both absorbs and diffuses the components of white light, the effects varying with the types of matter and their density in the air.

Dry, dusty countries enjoy glorious sunsets due entirely to the selective absorption and diffusion of certain wavelengths by dust particles in the air. Sunsets in Britain have become less spectacular with the establishment of smokeless zones and the considerable reduction in heavy industry since the 1950s. But the effects of the earth’s atmosphere on daylight are small compared to those of water.

Suspended matter in water absorbs and diffuses in a similar fashion but clear water absorbs the components of white light over a relatively short distance and it does so unlinearly.

The above table depicts the effect of clear oceanic water on a beam of sunlight. The red component is largely absorbed at only 10 metres from the surface, orange is down to 20%, and so on. At 20 metres, only the green, blue and violet components are present in significant proportions. The effect of this subtraction of three components from white light and the considerable reduction of the remainder will be apparent from the foregoing observations. Below 20 metres, the diver's world is green, blue, violet and black, notwithstanding that much of the terrain, flora and fauna may be potentially very colourful.

The depletion of the components which remain after filtration, together with the loss of red, orange and yellow, also gives rise to a general reduction in visual contrast despite that the scene may appear quite bright. This apparent brightness is due to the adjustment of the eyes to the lower light level and the phenomenon of the relatively long transmission of green and blue components is known as "the green window". In coastal and estuarine waters, underwater vision may be further impaired by the presence of heavier dispersions of suspended matter.

Since humans evolved in the sunlight, our vision works best for in white light. Accordingly, it must be assumed that any artificial light we may produce as a substitute, should simulate daylight as closely as possible if we is to make full use of our visual powers.

As may be expected, artificial light, with similar components to those of natural daylight, suffers similar absorption and diffusion when projected through water. If sufficient red, orange and yellow remain after 5 metres of travel from the source, to give rise to a colourful image, the observer may experience this at a distance of 2.5 metres from the subject, assuming that he is carrying the luminaire and the light must travel from himself to the subject and back to him. If the light source is placed close to the subject, at say 1 metre distant, the observer will enjoy the same sensation of colour at 4 metres from it. It sounds quite simple, but we often forget the light must travel not only from the luminaire to the subject, but then from the subject to the observers eye.

Particles of matter suspended in water absorb and reflect the components of white light in the same way as larger objects and whilst the absorbed components are lost, those which are randomly reflected cause a milky overlay on the scene, its density varying with the nature, size and density of the particles. Diffusion occurs in both the outgoing beam from a luminaire and in the image bearing light travelling from the object to the eye. Its greatest effect on vision is produced when the outgoing and returning paths are coincident. See Fig. A. And it may be minimised by the placement of the luminaire close to the object and projecting its beam obliquely, as shown in Fig. B.

Artificial Light Sources

The following diagram compares the spectral power distribution histograms of five different types of light source with natural daylight. Fluorescent tubes are excluded since they are difficult and expensive to package for deep immersion and the low output relative to its bulk makes them uneconomical.

The incandescent, or filament lamp, which includes the tungsten halogen series, offers an output which is particularly rich in the red and orange components, compared with natural daylight, rendering it very 'warm'. The early absorption of red and orange components by water has a 'cooling' effect however, and at a source, subject, eye distance of 3 metres, in clear water, the light from a tungsten halogen source exhibits a spectral content which approximates to that of daylight at the surface. The incandescent source is compact and requires no control gear. It is available for operation on AC or DC over a wide range of voltages, and is easily packaged for submerged operation, which explains why it has been used almost exclusively in the diving industry over the past 25 years.

On the minus side, the filament in the incandescent source is fragile and has a relatively short life. Further, the tungsten halogen capsule is inefficient when compared with discharge lamps, in terms of light output against power consumed and whilst this is of less import in underwater lighting equipment which is served with power from the surface, it becomes a major consideration in the design of independent, submarine vessels operating entirely from batteries carried on board.

The remaining four histograms relate to the outputs of gas discharge lamps. Having no filament, the discharge lamp is more robust than the incandescent source and for the same reason, it has a considerably extended life. Tungsten halogen capsules offer between 25 hours and 2,000 hours life, depending upon type, against between 5,000 and 10,000 hours for a discharge lamp. The main disadvantage presented by most discharge lamps is the 're-start time'. If a lamp is extinguished, by accident or design, it cannot be re-started for between two and ten minutes, depending upon the rate of pressure reduction within the discharge tube. This is accelerated in a fully immersed luminaire however, and the re-start time is reduced to between two and four minutes, depending upon water temperature. Vehicles fitted with discharge lamps should be provided with auxiliary, tungsten halogen luminaires for navigation, immediately following a temporary power failure.

The output from the low pressure sodium lamp is almost exclusively in the yellow, rendering it unsuitable for most submerged applications. High pressure sodium offers a more balanced output but definitely 'warm', being particularly rich in yellow and having a high orange and red content in relation to natural daylight.

Mercury vapour offers a rather spiky, cool emission with a number of component deficiencies. It is rich in ultra violet radiation, however and accordingly, this source is particularly valuable in specialist applications in which the visible light output is absorbed in a filter and the u.v. is used to cause luminescence in suitable substances. These emit visible light when bombarded with u.v. radiation and appear to embody a light source. And since the transmission distance of u.v. in water is long, the visibility of such materials extends beyond the periphery of vision aided by projected visible light alone.

The output from the metal halide source approximates more closely to natural daylight than any of the foregoing and it would seem an obvious choice amongst the discharge lamps to provide illumination for both direct vision and television, more especially since it too, emits strongly in the ultra-violet portion of the spectrum.

In addition to offering a suitable spectral output, with a colour temperature in the region of 4,500°K, giving good colour rendering, the metal halide lamp delivers 70 lumens per watt against 19 from the tungsten halogen capsule, an improvement of 368%. Hence, in theory, it needs only 27% of the power required by the tungsten lamp to produce the equivalent level of illumination. In practice however, the approximate circuit power of a 250 watt metal halide lamp is 275 watts, which reduces its comparative efficacy with that of the tungsten halogen lamp to 335%. The relative efficacy of light sources is shown below.

Corrosion

The corrosion of luminaires constantly exposed to inclement weather and extremes of temperature presents an expensive problem, and should be designed out of any system at the outset. A basic understanding of how and why corrosion occurs will help the designer to prevent it.

When two different metals are joined together in a solution which will conduct electricity, a simple cell is set up. In this cell, the metal which is highest in the electrochemical series (Fig. 6 ) is the anode and undergoes a decomposition reaction. This change of solid metal into dissolved metal is 'corrosion'.

From the foregoing and the table: If the main constituent of a submarine structure or vessel is iron or steel, any metal which is above iron in the electrochemical series and in direct contact with it, will undergo corrosion unless it is prevented from contact with the sea, which is a conducting solution. This phenomenon is used in the protection of ships hulls, oil rigs and similar structures, to which are fitted 'sacrificial anodes' of zinc, which are allowed to corrode freely in preference to the steel hull or submerged steel framework. Similarly, metals falling below iron and steel in the series will not suffer corrosion but cause the main structure to corrode.

Aluminium occurs above ferric material and therefore, aluminium luminaires and thruster housings will corrode if affixed directly to a steel hull and, as with most chemical reactions, the rate increases with temperature, hence, unprotected luminaires corrode more rapidly in the tropics than in the North Sea.

Aluminium and its alloys may be protected by hard anodising of the surface and coating with a non-porous paint or plastic material but the use of aluminium for any item which is to be permanently immersed in sea water, is unwise, since the smallest pore in the coating will allow the commencement of corrosion, more especially if the item is connected to a steel structure. The use of aluminium luminaires should be confined to applications in which they are regularly brought to the surface and washed in fresh water.

Chromium and nickel occur below ferrous materials in the series. These are the main constituents of stainless steel 316 and, accordingly, this material will not usually corrode when attached to a ferrous metal. It will, in fact, cause the corrosion of the steel if the latter is unprotected. If, however, a stainless steel (316) luminaire is welded directly to a steel structure, rather than clamped or bolted, it may suffer some corrosion since 316 does contain ferric material.

Copper and tin are the lowest practical materials, in the periodic table, for the construction of luminaires. An amalgam of these two elements presents bronze, which is easily worked and virtually corrosion proof under the conditions described above, requiring no surface protection.

Having selected materials, in keeping with the periodic table and suitably protected the surface of any aluminium units, further protection may be afforded to equipment mounted on steel structures by the introduction of non-conducting material at the interfaces between equipment and structure. For example, a plastic block may be secured to a steel structure by a set of fasteners and a separate set used to secure the equipment to the block in such a manner that no electrical circuit is made between equipment and structure.

If the equipment is a luminaire, or other unit powered by electricity, the earth or ground conductor must remain unconnected since connection would break the total isolation of equipment from structure. The safety of personnel may be assured by the use of a Residual Current Device (R.C.D.) within the power feed circuit.


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