How is light created 3

Light and light protection in the museum

Teaching unit at the University of Applied Arts, Vienna (Text partly in preparation)

introduction

Like radio waves, microwaves and X-rays, light is one of them electromagnetic waves. The only difference between the waves is their wavelength. The shorter-wave the radiation, the more energetic it is.

Visible light

Only a narrow range of this radiation is visible to us humans as light, in the range from 380 nm (violet) to 780 nm (red). The visible area is framed by the ultraviolet (UV) and the infrared (IR) radiation (see Fig. 1).

Children see a larger section, with increasing age their perception decreases. The perception in the 380-400 nm range is so low that a lack of this wave range is practically unnoticed. For this reason, sunglasses usually not only filter out UV but also the range below 400 nm (UV 400). Since the range 380 - 400 represents the most energetic and therefore most harmful part of visible radiation, it makes sense in museums to filter out not only UV light but also visible light <400 nm (see below, under UV filter). If there is also a filter in the 410 nm range, a visible yellow tinge is produced.

UV radiation

Invisible ultraviolet radiation is divided into three areas:

UV-C(100 - 280 nm)is absorbed by the atmosphere and does not appear in the museum.
UV-B(280-315 nm)reaches the surface of the earth and makes people tan in the sun. However, UV-B is almost completely retained by simple window glass, so that interiors are largely protected from it.
UV-A(315 - 380 nm)is not absorbed by window glass and can therefore penetrate the works of art in the museum.

Sunlight is relatively UV-A rich. When the sky is cloudy, the UV-A component is particularly high (up to approx. 1600 µW / lumen), because the clouds absorb more visible light than UV-A. But artificial light also contains UV. Incandescent lamp light usually contains less UV radiation than halogen light (dimmed less than undimmed). Fluorescent tubes, compact fluorescent lamps and discharge lamps (quartz lamps) emit more UV-A. Halogen, fluorescent and discharge lamps also contain a certain amount of UV-B, which, however, is largely filtered out by protective windows (Hilbert 1994, p. 28).

Infrared radiation

As a rule, light also contains Infrared radiation. A distinction is made between near IR (770 - 1400 nm) and far infrared (1.4 - 1000 µm). Infrared radiation, and to a lesser extent also the visible red radiation, leads to the heating of the irradiated surfaces. Here, too, there are major differences between the light sources. For example, direct sunlight and incandescent lamp light contain quite a lot of IR radiation (dimmed proportionally more than undimmed). Halogen, fluorescent and discharge lamps produce less IR radiation. The IR radiation leads to the heating of the irradiated surfaces and the heating of the room. Dark areas heat up more strongly than light areas. Gold plating reflects infrared particularly well. The energy transfer by radiation takes place much faster than would be the case with convection of warm air and thus leads to an abrupt increase in temperature.

The Warming of the surfaces locally leads to a drier ambient climate. This may be an advantage for metals, which are less prone to corrosion. In the case of materials made of organic materials (wood, paper, textile, layers of paint ...), on the other hand, this leads to drying out and dry tension, which, however, usually only leads to quickly visible damage in direct sunlight or extreme heating by photo lamps. A heating of the surfaces also causes an air flow that sweeps up along the surface. This encourages dust and dirt on the surface. The surface temperature can be measured without contact with unglazed surfaces using infrared thermometers.

Through the "Greenhouse effect"Showcases and glazed paintings heat up faster than unglazed ones. The greenhouse effect is based on the selective permeability of glass: While short-wave infrared radiation is allowed through, glass is largely impermeable to wavelengths above 2700 nm and from 3500 nm. The heat radiation is at surface temperatures of 20 - 30 ° C between 3,000 and 40 µm. The (long-wave) heat radiation from the showcase can no longer take place. To understand: a heated showcase is much cooler than the sun and therefore emits much longer-wave heat radiation than the sun.

Light and the human eye

The Retina The human eye has two types of receptors for light that are color-sensitive Cones and the light-dark sensitive ones rod. The rods are about 10,000 times more sensitive to light than the cones, which only have a luminance of 3 cd / m2 take action. Because of this, all cats appear gray to us at night. Full color sensitivity is above 10 cd / m2 available, best color vision starts at 50 cd / m2 and increases with higher luminance.

For the eye, it is not the illuminance that is decisive, but the luminance, i.e. how much light is returned from the exhibit. Depending on the degree of reflection of the surface, the luminance is far below the illuminance.

The full Visual acuity, i.e. the ability to perceive two neighboring points as separate, only becomes apparent above 100 cd / m2 reached. It only increases slightly at higher luminance. Visual acuity decreases sharply with age, especially visual acuity in low light. The light requirement of a sixty-year-old in weak lighting is about twice as great as that of a twenty-year-old (cf. Hilbert p. 12ff).

Spectral sensitivity of the human eye (400 - 700 nm)

The tenons contain three Visual pigments for blue, green and red. The combined stimulus signal of the three visual pigments results in our color perception - an equally strong stimulus signal in the area of ​​green and red creates the color perception yellow for us.
Screens therefore light up in the colors red, yellow and blue and thus achieve virtually all colors (additive color mixing, everything together results in white). The complementary colors cyan, green and magenta are used in printing (subtractive color mixing).
The blue and red cones are stimulated by magenta, the blue and green by cyan. If cyan and magenta are mixed, the resulting color impression is blue. Yellow is caused by simultaneous irritation of the red and green cones, etc.



 

The Clairvoyance the cone is largest in the region of 555 nm (yellow-green). In other words: even strong radiation in the blue or red range is perceived as not very bright by the eye. (The light sensation of the light-dark-sensitive rods reaches its maximum in the blue area.)

This peculiarity is taken into account when measuring brightness: Brightness meters (lux meters) are therefore equipped with special photocells that have a similar one spectral light sensitivity like the human eye, i.e. they are particularly sensitive in the area of ​​green-yellow.


from: http://spectracine.com

Lux meters therefore do not measure how energetic a radiation is or how harmful the lighting can be for the works of art, but only how bright the lighting appears to the human eye. This is a fundamental difference. Since the yellow light is overemphasized and the far more dangerous blue light is hardly counted, lux measurements are basically rather unsuitable for assessing how harmful a lighting condition is for the works of art (for more details see Padfield).

adaptation

The ability of the eye to see something at 100,000 lux, but also at a millionth of it, is largely the adaptation of the eye, the widening and closing of the pupil. This adaptation process takes time, especially in older people. The adaptation from light to dark requires considerably more time than the other way around: in the case of strong differences in brightness, the adaptation of the suppositories is only complete after 1 -10 min. There should therefore be a transition zone (light lock) between light and dark rooms, in which the eye can get used to the darker surroundings.

By the way, doubling or halving the illuminance means only a relatively small change for the eye - for a double one Impression of brightness 4.5 times the luminance is necessary. Replacing a light source with a slightly weaker one may not be noticed by anyone.

In order to be able to show the mandylion, the earliest representation of Christ in the world, at low illuminance (behind the glazing in the center of the picture), a short tunnel was made as a transition zone in which the eye can adapt. The tunnel should have been much longer for optimal adaptation.

from the Vatican booth at Expo 2000

 

 

Mandylion in a splendid metal frame

Glare

A distinction must be made between (physiological) glare that obstructs vision - as is the case with an oncoming car at night. One reason for the glare effect is the scattered light that forms in the lens, cornea and vitreous humor of the eye. Older people are therefore particularly sensitive to glare. The closer the glare source is to the viewing direction, the stronger the glare.


Stray light in the eye

In the more subtle variant, the psychological glare, the view is not perceived as obstructed, however, after a while, a feeling of discomfort or stress may arise. Visible light sources or strong points of light in the field of view divert attention and confuse visitors.

Light color

Human perception can also adapt to different light sources. We perceive daylight, incandescent lamp light and many other light sources as white, even if their light spectrum is completely different. The color cast of the individual light sources is only recognized when two different light sources illuminate a single-colored wall, for example. Colors are only ever seen in comparison to the reference of the light source - it is therefore not surprising that humans do not have an absolute memory for colors.

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