IELTS Academic Reading Practice Test 167

For much of the nineteenth century the most feared hazard in a coal mine was neither flooding nor collapse but firedamp: the mixture of gases, chiefly methane, that seeps out of coal seams and gathers in the roof of a working. A candle flame was enough to ignite it, and an explosion underground could bring down galleries, burn everyone in them and release suffocating gas. On 25 May 1812 an explosion at Felling colliery, near Gateshead in the north of England, killed 92 men and boys. The disaster prompted local clergymen and mine owners to establish, the following year, the Sunderland Society for the Prevention of Accidents in Mines, which set out to find a safer form of underground light.

The society turned to Humphry Davy, the most celebrated chemist of the age, who had made his name at the Royal Institution in London. Davy had never been inside a coal mine, but in 1815 he travelled to the north-east, went underground and took samples of firedamp back to his laboratory.

He established that the gas would burn only when mixed with air in certain proportions, and that a flame would not travel along a narrow tube, because the metal of the tube drew heat out of the burning gas faster than the gas could supply it. From this he built a lamp whose flame was enclosed in wire gauze — a fine mesh of metal wires that behaved like thousands of very short tubes. Air passed in through the mesh to feed the flame; the flame could not pass out to ignite the gas beyond.

Unknown to Davy, a similar answer had been found a few miles away at Killingworth colliery. George Stephenson, the colliery's enginewright, was a self-taught mechanic who had learned to read and write only in his late teens. From his own experiments he produced a lamp in which the flame burned inside a cylinder of glass, with air admitted through a small hole in the metal base. Because its flame was shielded by glass, the 'Geordie lamp' was less easily disturbed by draughts, and collieries in Northumberland and Durham adopted it in large numbers.

Neither man was the first to attempt a solution. In 1813 William Reid Clanny, a Sunderland physician, had demonstrated a lamp in which the flame was isolated from the mine air by a seal of water, bellows forcing air down through the water to feed it. The design was genuinely safe, but the lamp was heavy, needed constant attention and produced so little light that few collieries would use it.

Davy's lamp was tried underground early in 1816 and declared a success. He deliberately did not apply for a patent, so that the lamp could be made and sold cheaply, and the coal owners of the north-east thanked him with a dinner and a service of silver plate. Stephenson, whose claim to priority was disputed by Davy's supporters, had to defend his reputation for years, and a public subscription raised a smaller sum for him. The wider consequence was that seams previously abandoned as too gassy could be worked again, and mines were driven deeper and further from the shaft, into ground where a candle could never have been carried.

The lamps had serious weaknesses. Their light was dim and orange, and the gauze soon became clogged with coal dust, so that men worked in near-darkness. More seriously, the protection depended on the mesh staying intact and comparatively cool; in a strong current of air the flame could be driven through the gauze, and if the wire became red hot the gas outside might still be ignited. Miners complained that they could not see their work, and some removed the gauze, or turned up the wick to obtain a better light, thereby destroying the very protection the lamp was meant to provide. Nor did the new lamps end explosions. Mines where they were used continued to suffer disasters, and some historians argue that the lamp encouraged owners to send men into places that should have been left alone.

Designers responded with a series of refinements. The Belgian engineer Mathieu Mueseler added an internal metal chimney, which improved the draught and the brightness of the flame while keeping the products of combustion away from the gauze. Later lamps used two layers of gauze, and later in the century many were fitted with magnetic locks so that the lamp could not be opened underground. Mining regulations gradually made the lamp compulsory in gassy seams, and the flame acquired a second use: because it lengthened and showed a pale blue cap when firedamp was present, an experienced worker could judge the proportion of gas in the air from the appearance of the flame.

From the early twentieth century, electric lamps with rechargeable batteries took over the everyday job of lighting a mine, being far brighter and cleaner than any oil lamp. The flame safety lamp survived, however, as a detector, since a flame gives an immediate visual warning that no electronic instrument quite matches, and it is still used for that purpose in some mines today. Its image has proved equally durable: the safety lamp became a symbol of the industry, and the small, sturdy lamps carried by generations of miners are now more often found in museums than underground.

A hologram is not a photograph of an object but a record of the way light from that object interferes with a second beam. When two beams are coherent — that is, their waves rise and fall in step — they combine to produce a pattern of fine bright and dark bands, which can be captured on a photographic plate or a digital sensor. On its own the record looks like meaningless stripes. Shine the reference beam through it, however, and the stripes diffract the light so that the wavefront which once came from the object is recreated. Anyone looking into the plate then sees an image that appears to hang in space, in front of the surface or behind it.

The idea was conceived by Dennis Gabor, a Hungarian-born engineer then working in Britain, who published it in 1947 in the hope of improving the electron microscope. The lamps available to him produced light of too many different wavelengths, and the technique remained a curiosity. It became practical only with the laser, a source whose light is highly coherent, and in the early 1960s Emmett Leith and Juris Upatnieks, at the University of Michigan, used one to make the first recognisable holograms of three-dimensional objects. Gabor was awarded the Nobel Prize in Physics in 1971.

A display that could reproduce wavefronts electronically would have properties no ordinary screen possesses. A stereoscopic screen, such as that of a headset, shows slightly different pictures to the two eyes, and the brain reads the difference as depth, yet every part of the image is emitted from the screen's surface. The eyes therefore focus at the screen while converging on a point that seems nearer or further away, and this mismatch, known as vergence–accommodation conflict, is thought to explain much of the eyestrain and nausea reported by some users of virtual-reality headsets. A holographic display removes the conflict, because the light reaching the eye genuinely comes from an apparent object at a definite distance. Because the wavefront fills the space in front of the screen, moving the head also reveals a slightly different view.

Making such a display requires a device that can control light pixel by pixel. The component usually chosen is a spatial light modulator, a thin layer of liquid crystal on a silicon chip. Each pixel can delay the light passing through it by a chosen fraction of a wavelength, so the array imprints a pattern of phase delays on the beam, which then diffracts into an image. Because nothing is being recorded, the pattern has to be calculated: a computer takes a three-dimensional model, works out the wavefront it would produce and converts that into pixel values. A pattern of this kind is called a computer-generated hologram.

The optics impose a stubborn limit. The angle through which a pixel array can bend light is roughly proportional to the wavelength divided by the spacing between pixels, so a wide viewing angle demands pixels much smaller than a micrometre, far finer than those of any commercial display. A related constraint is the sheer quantity of data: the number of pixels, multiplied by the number of viewing angles that must be represented, has to exceed what engineers call the space–bandwidth product of the scene. Experimental displays therefore tend to be small and to be viewed through a narrow window.

Calculation is the second obstacle. A full hologram of a moving scene must be recomputed many times a second, and straightforward methods are far too slow. Researchers have responded with approximations, look-up tables and dedicated chips, and progress has been steady. Laser light brings a further nuisance, speckle: because the light is coherent, tiny irregularities in the optics produce a grainy, glittering texture across the image. Averaging several slightly different patterns in quick succession, or using light that is only partly coherent, reduces the effect.

Colour adds complexity, since red, green and blue must be modulated separately and aligned to within a fraction of a wavelength, or the viewer sees coloured fringes. The smallness of the human pupil is, oddly, an advantage. Because the pupil is only a few millimetres across, the display needs to deliver the correct wavefront to that small opening alone, and a camera that tracks the eye can steer the image to follow the pupil as the viewer looks around, relaxing the demands on the rest of the screen.

Some of these ideas are already in use outside the laboratory. Holographic microscopes, which record the wavefront passing through a sample and reconstruct it numerically, allow cells to be examined without staining and in three dimensions. Holographic data storage, in which pages of information are written throughout the volume of a disc rather than on its surface, has been demonstrated but has not displaced magnetic media. The shimmering images on banknotes and passports are not true holograms at all but embossed surface patterns.

Genuine holographic video remains an aim rather than a product. Metasurfaces, arrays of tiny pillars that shape light on a scale smaller than a wavelength, may eventually replace liquid crystals, though the wide-angle demonstrations reported so far are only a few millimetres across.

When Landsat 1 began returning images of the Earth's surface in 1972, each picture element covered about 80 metres of ground: a village appeared as a smudge, a field as a single tone. Anyone with a credit card can now buy a photograph of a suburban garden in which cars are distinguishable. Commercial satellites return images whose pixels are 30 centimetres across, and the best of them are ordered not by governments but by insurance companies, farmers, journalists and, occasionally, curious neighbours. The question is not whether the technology is remarkable but how far the right to photograph the ground from above should extend.

The gain in sharpness has been relentless. The French SPOT 1, launched in 1986, offered 10-metre images; IKONOS, the first high-resolution commercial satellite, reached one metre when it opened for business in 2000; later spacecraft have gone three times finer. Revisit times have shortened at the same pace: a single satellite passes over a point every few days, but a constellation of a hundred can see it several times a day. Falling launch costs have turned a state monopoly into a crowded market.

The benefits are not hypothetical. When an earthquake flattens a city or a river bursts its banks, the first reliable picture of what has happened usually comes from orbit. The International Charter on Space and Major Disasters, a voluntary arrangement set up in 2000, lets a relief agency request images from member operators within hours of an emergency; the pictures are used to route convoys and decide which villages need shelter. Aid agencies plan food distribution with the same data, and human-rights groups use it to document the destruction of hospitals.

Agriculture has been changed in quieter ways. Because healthy plants reflect near-infrared light strongly and stressed ones weakly, a satellite's measurements can be turned into maps of crop vigour and those maps into instructions: this field needs nitrogen, that one is short of water. Precision farming of this kind cuts the fertiliser that runs into rivers, and seasonal forecasts built on satellite data help governments decide how much grain to import. In parts of India and East Africa, smallholders who own no machinery receive the same information by telephone.

Forests are watched in much the same way. Brazil's space agency runs a system that flags new clearings almost as they appear, and the alerts are published rather than kept for officials; independent platforms now combine imagery from several sources so that anyone can follow a logging frontier. Journalists have become skilled users of this material, counting ships that have switched off their transponders, identifying prison camps and measuring the damage done to towns, using pictures that no government chose to release.

Against these benefits lies a genuine unease. An image that shows a field also shows a garden, a swimming pool, a boundary hedge and the car in the drive, and the sharpest pictures are sold to whoever can pay. Repeated passes add a further dimension: a single photograph is a snapshot, but a sequence reveals when a house is empty. In 2001 the Supreme Court of the United States held, in Kyllo v. United States, that using a thermal camera to detect heat escaping from a home was a search under the constitution. The reasoning was narrow, and courts almost everywhere still treat a photograph taken from public airspace as no intrusion at all.

That gap has been filled, unevenly, by licensing. In the United States, commercial operators need a licence from the civilian agency that oversees remote sensing, and successive licences have set ceilings on the sharpness of images that may be sold. Other exporting countries impose their own terms, and the sharpest imagery of certain territories is restricted for reasons of national security rather than privacy. The United Nations adopted principles on remote sensing in 1986, but they were written for a world of states: they speak of the rights of the country observed, not of the farmer whose land appears in the picture.

A blanket prohibition would be the wrong response. The harm done by satellite photography comes from the use to which a picture is put, not from the fineness of its pixels: the image that exposes a swimming pool also exposes an illegal dam or forest clearance that a government has denied. What is needed is a distinction of the kind aviation law already makes. Many countries regulate drones closely when they hover over private land, because a machine that can loiter above a garden is more intrusive than an aircraft passing overhead. Satellites cannot loiter, but a constellation can return to the same address every day.

Who, then, should set the rules? National licensing bodies are the obvious candidates, since they already attach conditions to a licence, and they could require operators to refuse orders whose purpose is harassment. A purely national approach will always leak, however, because a buyer can order from an operator in another country and sharp civilian imagery is now sold by several. An international baseline, agreed by the handful of states that build and launch these satellites, would be more effective than a patchwork of national rules.

The principle to adopt is straightforward. The law should not ask how small a pixel may be; it should ask what the picture is for. Satellites should be free to photograph the ground in as much detail as the optics allow, and constrained only in what may be done with the result.