Light sources for the digital reproduction of slides and lanterns
The Swiss DIASTOR research group in their analysis of 35mm and 16mm film scanners states: “Every scan is a reading under certain conditions” (Flueckiger et.al. 2016, p. 109). This is also very true for a photographic reproduction. To take a good picture the camera needs auxiliaries such as external lights etc. Several accessories will be commented on: lamps, flashes and light modifiers, filters, light pad and camera-stand.
- Light sources in the working space – some general reflections
There are many lamps on the market but not all are suitable to work with in photography. Different parameters determine what can be excluded and from which models the photographer should choose. The first question is: what should be photographed? It indicates the application range, e.g. “limited”: just concentrated on one location (corner in a working space) and for homogeneous use (slides), or “broad”: work in and outside the premises under flexible conditions. A second thought should go to the qualities of the objects – slides are transparent, quite reflective, sensitive to UV light, lanterns are opaque, with mat and brilliant sections thus absorbent and (to a certain degree) reflective.
Parameters for the selection of the best light can be the following ones:
Lamps differ in “(1) Spectral quality. (2) Light output. (3) Size. (4) Luminous efficiency. (5) Cost. (6) Constancy of output. (7) Convenience.” (Horder 1958, p. 12) Such lamp characteristics can influence how the slide will look when photographed. As the way the lamp illuminates an artefact follows certain rules – “quality, direction, contrast, evenness, colour and intensity” (Langford 2008, p. 117) – these points will also be discussed.
“Spectral quality” means the colour of the light that is emitted. Most lamps seem to send out “white light” but according to their nature – whether they are incandescent light bulbs (now replaced by energy saving light bulbs), neon tubes, flashbulbs etc. – they cover certain bands of the visible spectrum. Unlike a sensor which registers everything as it is, the human eye ignores the difference as brain and eye adapt to the circumstances and identify the light as “white”. But the sensor will register the specific colour temperature of the used light source precisely. Therefore it is necessary to choose the lamp carefully as its colour spectrum can be decisive for the colour balance of the reproduction.
Photographic expert Friedrich-W. Voigt (1973, p. 14) explains the correlations between light condition and object. A lamp generates light in the form of electromagnetic waves, an object without a radiating element just receives light which it reflects and/or absorbs (the eye sees the artefact as black as no light is coming back). Due to the reflected light the eye sees the colour of an object: it perceives it correctly when the conditions are neutral, or falsified when the object’s surface is under the influence of (invisibly) coloured light which interferes with its external appearance: the ratio of absorbed and reflected light has changed. If the body reflects all frequencies it looks white if the lamp is neutral, if not the object will show tints: UV-light is invisible to the human eye but on the picture it can make white objects look blueish, suntanned skin olive or wine red purple (Voigt 1973, p. 92). The same can happen with the aniline-dyes and the chromo-lithographs on the slide’s surfaces.
“Light output” signals the power of a lamp, its capacity to radiate light in a certain direction; it is defined by “luminous intensity, brightness (luminance) and luminous flux” (Horder 1958, p. 18). It concerns the quantity of light that comes out of the lamp, signalled in candela or candle (luminous intensity, also called candle power), in lumen (luminous flux or pharos), the light flow falling from a light source (of one candela) from a specific distance (one foot) onto a surface of a specific size (one square foot)) and in nit or lambert (brightness, luminance, signalling the “luminosity of illuminated surfaces and of incandescent bodies”) (Focal 1969, p. 818).
The Focal Encyclopedia of Photography (1969, p. 890-892) draws attention to the following fact: what comes out of the lamp is different from the light intensity that reaches the slide (illumination, luminous flux density or pharosage, measured in lux) which differs from the amount of rays that are reflected from the artefact (luminosity, surface brightness or helios, also measured in nit or lambert, but also in candela per given area as one candela per square meter is one nit) and can reach the objective. The latter depends of the reflecting power of the slide. The difference becomes obvious when a light-meter is directed towards the light source and then towards the slide which not only has let beams pass through the glass plate, but also diffused broadly and absorbed many of them. To better know the reflection factor of typical genres (defined by the kind of glass, the technique of colourisation, the production method, the aesthetic “layout” etc.) could help to reach a better result when standards have to be worked out for the reproduction of the collection of an institution.
The light performance should also be taken into consideration when choosing the objective to work with: a zoom will need more light (or a longer exposure time) as its physical length (the “tunnel” that the beam has to pass) is much longer and the maximum aperture of the diaphragm smaller than with a wide-angle lens.
“Size”: Whether a lamp, or better its bulb or tube, is small or big decides on “[…] the nature of the shadows cast by the source. Small lights give shadows with hard edges; large sources give soft edges.” (Horder 1958, p. 19) Of course, the production of shadows depends also on the position of the lamp. Spotlight (thus directed light) from the side creating sharp shadows is recommended when the slide’s colour texture (the way the lanternist has applied the brush, a number cut into the wooden frame) should be revealed. Large sources producing less directed light beams are welcome to illuminate the slides from right and left.
“Luminous efficiency” signals how much of the input electric power is obtained in output of light. “A normal 100 watt lamp has an efficiency of 15 lumens per watt, which means that only 7 per cent of the energy supplied is obtained as light – the remaining 93 per cent is wasted as heat!” (Focal Press 1973, p. 891) The less efficient the lamp, the higher the expenses in electricity and the bigger the ecological damage. The efficiency of (electric) lamps is indicated in lumens per watt.
“Cost”: The price for the total equipment, the life span of the bulb or tube (that is the time that they be used without difference in colour temperature and efficiency), the running costs as well as the cost of their replacement are parameter to be considered.
“Output constancy”: As is the case for scanners, the lamp’s constancy in light intensity and composition (colour temperature) is compulsory to get good slide reproductions. In areas where power supply is not so reliable, a kind of voltage control could be helpful to guarantee an image output which is uniform in quality. Also light bulbs and tubes should be controlled regularly and checked to see if their life span is near its end and the envelope is starting to darken. Also an eye should be kept on the physical condition of reflector and/or diffuser. Alan Horder (1958, p. 42) recommends that, for a “uniform ageing of bulbs, they may be moved from lamp to lamp periodically”. High attention to this aspect helps to avoid time-consuming correction in post production.
“Convenience”: Horder (1958, p. 21-22) mentions here, among others, the heat emanating from the lamp and the ease of operation when the act of photographing begins or is restarted after a break. One could also think of the easiness to handle the light sources under mass production condition or the lamps’ flexibility to fit changes in requirements.
These last parameters should be considered before buying lighting equipment as light sources are the most important “accessory” in a reproduction studio.
- Light sources in the working space – for slides
When a camera-stand is used, the bulbs in the light box determine the colour temperature of the beam which transits the slide’s glass. Side lamps fixed right and left to the construction illuminate the slide from above. To use both at the same time, it it necessary to choose a neutral light to avoid different tints irritating the camera’s automatic and falsifying the object’s colour appearance. Some photographers also use a flash as filling light. A parameter for all light sources has to be selected: it could be 5500-5600 Kelvin, considered as colourless sun light at noon, which flash lights are often equipped with.
The Magic Lantern Society mentions a special case: “polarisation slides”. With a trick “the subject of the slide, for example a flower, change[s] colour”. (http://www.magiclantern.org.uk/alphabet/alphabetop.php) On its website, the association explains how it works and recommends using polarised light when digitising the artefact: “Some slides require polarised light to show them to best effect, for example effect slides where thin sheets of crystal or mica were arranged in a pattern. By rotating the slide in a polarised light source, the colours of the crystal change. The Elbow Polariscope was the device used to produce polarised light. It uses a stack of reflecting glass plates to polarise the light, and as the light arrives and departs at 45 degrees to the stack, the device is shaped like a human elbow.” (http://www.magiclantern.org.uk/alphabet/alphabetef.php)
It is important to keep slides that have been stored in a cool vault for some time in a climatised zone so that that the slide glass can adapt to room temperature and conditions. As lamps can produce a certain amount of heat, it would give the cold glass plate a shock and put plate and dye under risk if they are exposed right away. Especially important is the fact that “lantern slides with dark areas get hotter than clear ones” (Horder 1958, p. 43) which puts the slide under a lot of additional physical stress. A good example is given by lantern expert Damer Waddington (1995, p. 11) who has conducted a number of experiments in order to learn more about the consequences of projecting slides: “Damage may result to slides if they are permitted to get hot. Generally speaking, a slide will be in the direct beam [of the lantern] for less than a minute, so will not have the time to get hot. However, in some cases, when a slide is used as a background in a dissolving sequence, for example, it could remain in the stage for several minutes.” Waddington states that only the darker parts get the “full force of the heat” as the transparent glass let pass most of the light and “absorbs only a small proportion”. Nevertheless dyes may show craquelé, little particles can fall of, also an uneven heated glass is certainly put under a lot of stress.
- Light sources in the working space – for voluminous objects
A more flexible scenario is needed when photographing objects such as optical lanterns and accessories. Several lamps in the room may be necessary to photograph a voluminous object. Lamps have different qualities (especially when it comes to their wave-compounds which determine their colour temperature) but they differ also in form and size, light power and luminous efficiency, diffusion quotient and power rectification. A selection depends on what the photographer needs or, conversely, cannot accept as suitable for her/his work results. An illuminant can massively influence the human perception. According to colour specialist Huanzhao Zeng (2011, p. 20), “cool” light from greenish and blueish lamps (colour temperature between 3600 and 5500 Kelvin) produces higher contrast which makes the result look sharper which “is “considered better for visual tasks”. Zeng stresses that “warm” light with its red and yellow tint (between 2700 and 3000 Kelvin) is “considered more flattering to skin tones and clothing” why it is preferred when photographing humans. This rule may be applied productively when reproducing lanterns as here “mise-en-scene” is generally involved. However, when it comes to reproducing a slide as faithfully as possible, a lamp’s not-neutral beam which is hardly visible to the eye always interferes and changes its look. Therefore neutral white light is important: it is considered “colourless” as the whole spectrum of visible light-waves is equally present and no light band is preponderant. Not all lamps can fulfil this condition.
3.1 High pressure lamps are the “whitest”. They should be used with gloves and face protection as they can get very hot, and as the gas filling was done under high pressure, they could explode. Therefore they should not be moved while in use. They need about twenty minutes to reach their full power and have to cool down at least for the same time.
A Xenon bulb contains the widest spectrum of visible colours as these are consistently covered. Bulbs filled with Xenon gas emit the whitest, most neutral light among all light sources. It is not flickering; its colour temperature is constant. But as the lamp gets hot, it uses more energy, and at a greater cost, proportional to its luminance. The luminous efficiency is about 40 lumens per watt.
Metal halide bulbs are filled under high pressure with a mixture of noble gas (Argon, Mercury or Xenon) and halides (Sodium iodide, Scandium iodide). Their rendition in luminance is better, but it is less white as it covers the visible light spectrum non-continuously and has an insufficiency in the deep red area. Its precise colour spectrum depends on the used gas. Some can have a tint of blue.
According to Myers (2000, p. 8-9), HMI stands for “hydrargyrum medium-arc iodide” or in short “mercury-halogen lamp”. Mercury or other gases such as iodine “are excited by electric discharge with light emitted across the spectrum”, but with a considerable percentage of long waves. The colour temperature lies between 4500K and 6500K which brings the lamp close to daylight. It is a bright light “with a high lumens per watt”, but the “high ultraviolet and infrared output makes […it] undesirable for fine art reproduction since these two areas of the spectrum can cause appreciable damage to the artwork.” Myers insists also on the “high cost of HMI lamps, fixtures and power supply [which]makes this type of lightning the least commonly encountered in photographic studios.” The lamps need to be warmed up and only give its full lightning of 60 lumens / watt after three to five minutes.
3.2 Incandescent light bulbs, also called tungsten filament lamps, were in use since Edison brought them on the market. They were replaced yet by low-energy light bulbs after the European decree was published in 2009 as a consequence of its extreme weak luminous efficiency: a maximum of 5% light and 95% heat. The luminous efficiency of an incandescent lamp is between 12 and 17 lumens per watt (https://en.wikipedia.org/wiki/Incandescent_light_bulb). Compared to other bulbs their life span is relatively short. Low-energy light bulbs produce about five times more light. Tungsten filament light emits a continuous spectrum containing little ultraviolet and blue light and a high portion of red and infra-red waves; it shines yellow-reddish. Photographic expert Robin D. Myers comments: “Due to the high amount of light at the red end of the spectrum and the sensor’s increased sensitivity to red light, digital cameras must turn down the gain of the red sensors and turn up the gain of the blue sensors to achieve a neutral color balance with tungsten light. As the sensor gain is increased the signal is increased but the noise is also increased. For tungsten lightning the noise in the blue channel for a digital sensor is more than the other channels. Also, since the red is lowered and the blue raised, the yellow and orange colors are reduced in saturation in the resulting images.” (Myers 2000, p. 7)
3.3 Halogen light bulbs still work with a tungsten filament, are filled under low pressure with a small amount of halogen gas (Iodine, Bromine) added to a noble gas (Xenon, Krypton). They get hot, produce a continuous spectrum of visual light, also emit ultraviolet (which normally should be absorbed by a glass filter around the bulb) and infra-red light rays, and they generally tend to have a blue tint. They have to be handled with gloves to avoid damage. The luminous efficiency of a halogen lamp is about 30 lumens per watt (for a bulb of 3000 Kelvin). Lantern experts recommend it for projection: “Tungsten Halogen is a form of illuminant which is used in modern slide projectors and in magic lanterns which have been converted to electricity. It is a low voltage, efficient light source which helps to reduce damage to slides.” (http://www.magiclantern.org.uk/alphabet/alphabetgh.php) The lamp emits “[…] a whiter light than the standard tungsten lamp. The colour temperature range […] from 3000 to 3400K.” (Myers 2000, p. 7)
Robin Myers mentions also the “Solux tungsten-halogen” lamps working with dichroic filters that not only keep a lot of heat away from the illuminated object, but also “converts the spectrum of the light to closely match the spectrum of daylight at a color temperature of 4700K. […] The 50 watt output […] does not make them suitable for large area illuminations but their daylight color makes them very effective for small area lightning applications such as illuminating artworks.” (Myers 2000, p. 8)
In 2000 Myers recommended tungsten-halogen and tungsten lamps for photographic colour reproductions as they can be used as directional light source, but diffusion tools turn them also into a filler light for large areas. But filament lamps are not longer welcome on the European market. Today halogen light bulbs, light-emitting diodes (LED) or compact fluorescent light bulbs are allowed in the EU.
3.4 LEDs (light-emitting diodes) are made of semi-conducting material, cover the whole range from infra-red (warmth) via the visible part of the light up to the harmful ultraviolet rays, produce little heat and have a long life span. The luminous efficiency can be up to 150 lumens per watt. As ultraviolet rays can harm the slides it is less convenient for taking photographs of slides.
3.5 Compact fluorescent light bulbs, often referred to as energy-saving light bulbs, also called fluorescent tubes, are filled under low-pressure with a small amount of Mercury vapour gas which is excited by an electric current and makes the phosphorous coating glow that covers the bulb’s inner surface. Its luminous efficiency lies between 50 and 100 lumens per watt, about 85% of the energy is turned into light composed of visible and ultraviolet rays (most of the latter is absorbed by the inside coating). The symbol “Hg” on the bulb stands for mercury and makes it hazardous waste for safe disposal. They can give “cold” blueish or warm reddish light depending on the phosphor coating. This allows “a variety of spectral distributions with typical color temperatures from 3000 to 7500K.” (Myers 2000, p. 9) This kind of lamp converts ultraviolet rays into visible light. It is therefore possible that not all of the harmful waves are converted. Nevertheless these bulbs can be found in photographic ateliers as according to Myers they have “a very high lumens per watt emission” and can illuminate large areas.
Neon light is a glass tube (or a bulb) filled at low pressure with noble gas like Neon, Argon or Helium. An electron flow crosses the tube and “excites” the gas atoms. Neon emits orange-red light, Argon (mixed with Mercury) sends blue light-waves, Helium is pinkish, other gases or gas mixtures shine in other colours.
3.6 Cold-cathode fluorescent lamps are generally filled with a noble gas such as Argon or Krypton which have to be “ignited” (ionized) by a starter. The tube needs a short starting phase, when working they use comparatively little energy but nevertheless can get relatively hot. The light is produced by electrons operating between the two ends of the tube where two electrodes (anode and cathode) with different voltage are situated and allow the building of a continuous arc. About 30% of the power is turned into light with 30-100 lumens per watt. Fluorescent lamps emit coloured light according to the gas filling (argon: mauve, krypton: white), are problematic in photography as they flicker and their colour temperature is inconsistent. They are used as back lightning in LCD monitors.
3.7 A flash-light (c. 5500-6500 Kelvin) is a strong white light which has the quality of normal day light at noon as it is supposed to imitate this “neutral” condition. It can be helpful to reproduce e.g. devices as lanterns, written indications on slide frames, or eventually be used as filling light. A flash is a short intense illumination that can carry deep into the room, so it should be used in controlled ways: indirectly by letting the flash bounce against a reflecting surface (white ceiling, cardboard, white diffuser) or so-called “flash bouncers” (cards put opposite the flash) which will diffuse and reduce its intensity or redirect its light beam. Used directly this light source produces overexposure on the parts nearest to it and throws shadows onto the rest.
Light sources are complex, and to find the “right balance between energy efficiency and aesthetics” as the research group around digitizing expert Franziska Frey (2011, p. 123) claims may not always be evident. In any case, no light source should contain ultra-violet light as it is bad for human eyes, and also for slides as the Magic Lantern Society states: “Ultra-Violet light is the form of light that can do most damage to slides. It is present in sunlight and particularly in florescent light. It is not good at penetrating glass however, and so slides where the image surface is covered by glass are relatively protected from it.” (http://www.magiclantern.org.uk/alphabet/alphabetuv.php) Slides for toy lanterns are commonly not protected by a second glass plate and therefore vulnerable. Sunlight can affect the slide’s colours and also the tint of the glass.
For a true reproduction of the colour variety of a slide the light source should preferably be white which is defined as “light which is not noticeably deficient in any particular colour” (Horder 1958, p. 13). The eye is not the best judge: a tungsten lamp (with a red glowing component) seems to shed white light, but it has a high quantity of red light-waves, not all colours of the visible spectrum are equally present (as certain light bands are missing) which is the condition for really “pure” white light. Thus when looking at a white sheet of paper under these conditions it reflects the red waves and should look reddish, but eye and brain adapt to the situation and declare the sheet “white” based on the experience that this object is normally white. To get a grip on the lamp’s real colour composition one can measure its colour temperature with a colour measuring tool and analyse the spectrum emitted by the light source (its colour temperature in Kelvin or as Mired value). This indicates which colour correction tool would counterbalance the lamp’s effect.
The photographers John Hedgecoe (2004, p. 136, 138-139) and Hugo Schöttle (1978, p. 97) show in an overview the difference in colour temperature. (Mired is an alternative scale to indicate colour temperature; Hedgecoe indicates that to change Kelvin into Mired (Micro reciprocal degree) the cypher 1.000.000 has to be divided by the Kelvin in question, e.g. 1.000.000 : 1930 = 518.)
|Light source||Colour temperature in Kelvin (Hedgecoe)||Colour of the wave-length (H. / Schöttle)||Colour temperature in Mired (H.)||Filter for artificial light by Kodak (H.)|
|Candle light||1930 K||1000-2000 K dark red||518 M||82C|
|Tungsten light 40 W||2600 K||2500-2800 K (dark) red||385 M||80D|
|Tungsten light 150 W||2800 K||3000-3300 K red / reddish||357 M||82C|
|Floodlight||3200 K||312 M||No filter necessary|
|Halogen lamp||3400 K||3300-3400 K red-orange||294 M||82A|
|Photo lamp||4000 K||3200-3800 K red – orange||250 M|
|Normal daylight at noon||5800 K||5500-5600 K colourless / yellow white||172 M||85B|
|Electronic flash||5800 K||5500-8000 K yellow whitish||172 M||85B|
|Daylight on a hazy day||8000 K||8000-10000 K greenish-white||125 M|
|Blue summer sky||12500 K and more blueish-white – blue||80-40 M|
To resume: whatever light source is used, the photographer should keep in mind when shooting slides what James Ferwerda, professor at the Rochester Institute of Technology, proclaims: “Digitization has had an enormous impact on access to fine art collections. Art that was once sequestered behind closed doors can now be accessed worldwide through digital images.” (Frey 2011, p. 111) A common standard for all those who are involved in heritage preservation should be to keep reproduction as close to the original as possible. Of course, no one can have any illusions about the transformation in the “look” of a reproduction that occur involuntarily or on purpose during the workflow inside and outside the institution. One also knows that “various light sources (e.g. conservation laboratory with daylight or exhibition area with low light levels)” change the impression of an original object, not to speak of images taken under certain conditions and inspected under others: “images [that] are looked at onscreen dramatically influences the perceived image quality”. (Frey 2011, both quotations p. 124) But this should not be used as an excuse for not giving the artefact a chance to be depicted in its essence. Once a slide is damaged or even gone, there may be nothing to go back to. As a slide is mainly photographed under even light conditions, and as the image has to serve more needs and should also be sustainable (usable for other present and even future activities), it should be photographed with neutral illumination to leave all options open.
|Practical hints given by researchersThe group responsible for the Benchmarking Art Images Interchange Cycles. Final Report which has examined the influence of light conditions on the digital reproductions of art work in US-American institutions suggest to involve more than one person in the photographic process: “Since the aesthetic value of the images taken was of high importance, it was obvious that the photographers, often in conjunction with the curators, made decisions on lighting an object.” (Berns, Frey 2005, p. 57)|
As we have seen, the characteristics of light that falls into the objective determines how the object will look when photographed. If the light in the room has a distinct tint, the light bulb or the whole lamp should be changed, or a filter could be used. Filters to protect the objective of unwanted light bands from a lamp can be quite practical. The overview of light sources and colour temperature (see “The potential of different external light sources for photographing voluminous objects” in the photographic section) shows some of the potential filters which are on the market and could prevent that tints in the illumination influence the digitisation.
According to photographer Julia King (2017, p. 47) some autofocus (AF-)systems have difficulties with highly reflective objects. To prevent the disturbing effect of shiny surfaces when a glass plate is only half covered with paint this kind of filter may be useful: “A polarizing filter mounts onto a lens and reduces the amount of light coming in from a specific direction. For example, if you’re getting glare from the sun or if your photo is being ruined by glinting reflections on water, a polarizing filter will mitigate those issues. A must-buy if you do a lot of nature photography.” (https://www.makeuseof.com/tag/essential-accessories-photography/)
In Benchmarking Art Images Interchange Cycles. Final Report 2011 one institute recommends to use filters on the lens to protect against ultraviolet light, and also against infra-red beams as it is necessary to “remove components of the light that the camera is sensitive but the human eye is not. Without such a filter, over-exposure in the red channel in the presence of IR can occur.” (Frey 2011, p. 105). Another museum states according to the report: “Some of the common techniques that institutions use to light the object include double polarization (to reduce specular reflections) and adding a UV block filter over lights to prevent fluorescence of the object (which can erroneously elevate the blue response in the camera).” (Frey 2011, p. 101)
- Accessories for lamps (light modifiers)
Several tools can be adapted to lamps to produce effects, intensify the lamp’s output or diminish the light intensity, and to send the beam towards the place where it is most needed. Light can be directed with the help of barn doors, a cone reflector (“snoot”) concentrates the light waves to perform a spot effect, the light beam is (re)directed or spread by (collapsible) reflectors, reflective umbrellas or diffusers, softboxes and translucent (“shoot-through”) umbrellas soften it, gels and filters change its colour temperature etc. (https://www.popphoto.com/features/photography-lighting-modifiers/) There are many instruments to help playing with light. These are accessories for a mise-en-scene of an voluminous artefact such as a lantern when it has to be presented under its “best light”, as is an illumination form above, side, or below to create shadows and intensify the impression of volume. However a faithful reproduction is to be achieve without effects.
As to the slide on the camera-stand, the influence of other lamps in the room should not be forgotten. Tools such as reflectors, soft-boxes, light tents and other diffusers can be put around the copy stand to make the lamps shine through white material (textile, paper), attenuate their beam’s intensity to the degree needed and shield the glass plate from unwanted interferences.
- The camera-stand and the mise-en-scene of a light setting
A flat slide needs another illumination than a voluminous object. Therefore the two situations of reproduction ask for a different scenario. (For practical advise on how to use the camera-stand see section “Practical examples of accomplished digitising projects”.)
6.1 Slide reproduction
The main objective is to cover the whole surface of the slide homogeneously with neutral white light in order to reproduce everything, even the smallest dark detail nearest to the frame, and to avoid any what so ever small modification of dye traces which may occur by a not-neutral light sources. Two main dispositives are currently in use: a ground glass screen mounted on a box with lamps inside for the transmitting of light, and reflected incident light from at least two lamps, opposite each other and fixed left and right near the top of the camera-stand, which shed light from above.
The ground glass screen of the light box should work with a light source producing pure white light (thus without a specific colour temperature) which has to be sufficiently intense (without getting hot) as a certain quantity of the light will be diffused and lost inside the box, to a certain degree reflected and absorbed by the light box’s frosted glass plate and also diffused by the slide’s glass plus absorbed by the dyes. The light pad has to illuminate the glass plate evenly from below to avoid a centralisation of light in the centre and a creation of darker parts at the edges. The plate should be free of spots and scratches (which otherwise will be integrated in the reproduction) and should not become warm or even hot.
Most light pads on the market have LED light as they are produced to check transparent objects such as diapositives. Older boxes emit a reddish light due to the degradation of its (plastic) surface or the oldness of the incandescent filament of the bulb. Therefore old light pads should be used with caution. Special ones were made for the printing industry to compare the original and the reproduction before printing process would start. These are equipped with the best white light.
The still-camera is fixed to a extension which is attached to the central pole and can be brought up or down the stand by a crank. The camera stays always in parallel to the object. Next to it two lamps are fixed at a certain distance left and right on bars and illuminate the slide on the worktop from an angle of 45°. They illuminate the object from above and from two sides. They follow a rule: as a slide is made of glass which is normally flat and brilliant, the glass produces a “mirror effect”: a certain portion of light is lost due to diffusion. As the lamps are arranged left and right from the slide numerous rays are missing the objective of the camera due to the optical law “arrival angle = emergent angle”. The applied dyes on the glass are normally mat and not shiny. For mat surfaces it does not really matter from which side the light is coming, mat objects diffused light regularly in all directions, and a big portion will always be send towards the objective (see Langford 2008, p. 28).
6.2 Lantern reproduction
When photographing lanterns and accessories, one can be more flexible with the illumination. A voluminous object always has several sides, it can be approached from different viewpoints, thereby producing multiple faithful reproductions. If the apparatus does not have to be reproduced in true colours, one could think of “ambient” light with a certain tint: “warm” light has a big share of long light-waves (red) and is darker, “cold” light a lot of short light-waves (blue) and shines brighter. With ambient light the photographer achieves nice effects to promote the collection or the exhibition. For photographer Michael Longford (2008, p. 117) the way lamps are used is more important as light changes an object by its “quality, direction, contrast, evenness, colour and intensity”. “Quality” is the kind of shadows a lamp creates: hard ones with directed small (spot) light and high luminescence, soft ones with big lamps, diffusing tools or dimmed lightning. “Direction” means from where the light touches the object (from above, the side, below etc.). Longford suggests creating clear and shadowy parts on the object to stress certain of its qualities. “Contrast” indicates the ratio between the darkest (shadowy) and the clearest (most illuminated) parts created by strong (hard) light from above or the side which may cause the loss of details which the object contains in those parts; he suggests diffusing the lamp’s rays to keep the details. “Evenness” concerns an uneven exposure by a strong spot light or a flash which can be avoided by removing the light source from the object or diffuse its beam. The “colour” of the lamp has to be controlled and corrected if needed by tracking the white balance and eventually by using filters. “Light intensity” is to be regulated by aperture opening (“f-stop”) and exposure time as it can influence the depth of field.