Agents of Deterioration:

A. What is Light?

B. Damage Caused by Light

C. Preventing damage and Controlling Light.

- Rini Hazel Templeton

Ph.D Scholar Conservation

NMI

A. What is Light?

Electromagnetic waves are described by any of the following three physical

properties: the frequency f, wavelength λ, or photon energy E.

A.1. Light- Meaning

• Light is a form of energy known as electromagnetic radiation.

• It is expressed in wavelengths and the unit of measurement is nanometer .

• It ranges from very short wavelength to long wavelength radiation

• We can divide the spectrum of electromagnetic radiation into different parts

based on their wavelength.

• Visible (normal/sunlight) light is the band of radiation to which our eye is

sensitive and helps us see.

A.2. The light spectrum

1. UltraViolet Radiation(UV):

• Wavelengths shorter than 300nm-400nm are called ultraviolet radiation.

• They are invisible to the human eye.

• They shortest wavelengths and therefore the most damaging.

• Common sources of UV are natural daylight and fluorescent lamps, but tungsten halogen lamps and

high density discharge (HID) lamps also give off significant levels of UV radiation

2. Visible Light-: The band between 400nm and 760nm is called visible light. We see it as the colours

of the rainbow. (VIBGYOR)

3. InfraRed Radiation (IR): Wavelengths longer than 760nm are called infrared radiation.

• This radiation is also invisible, but it is felt as heat.

• Wavelengths in daylight cause most damage to

interior fittings and finishes which include visible

light and ultra violet light.

• This is because the energy content of light varies

according to its wavelength: the shorter the

wavelength the higher the energy content.

• Therefore, UV radiation has more energy than blue

visible light, which in turn has more energy than

red light and so on.

• Wavelengths shorter than UV is not penetrative,

and is absorbed by ozone in the earth's atmosphere.

• By the time sunlight reaches the surface of the

earth the sun's radiation consists of approximately

50% visible light and just 3% UV radiation.

• Window glass prevents the passage of wavelengths

up to 325nm, and the most harmful effects UV and

visible light are confined to the surface of an object

for the same reason.

A.3. Measurement of Visible Light

• The technical term for the amount of light falling on a surface is "illuminance," but

informal phrases such as "light intensity" or "lux level" are used in the museum

literature.

• The unit of measurement is lux. In simple words visible light is measured in LUX.

• We measure illuminance because we are concerned with the light energy that falls

on our objects, not how much light energy comes from the source.

Lux Meter

Summary

• Only visible light helps you see.

• IR and UV only cause damage.

• Light energy is either absorbed or reflected.

• Only absorbed energy causes damage.

• Minimize absorbed energy to minimize

damage.

B. Damage Caused by Light

• Damage by light is permanent and irreversible.

• Natural organic materials are more sensitive to light

damage than inorganic materials.

• The amount of damage that light causes depend on :

the type of light,

the materials from which the objects are made

the time exposed to light.

• Exposure to light in all forms causes a chemical reaction to happen within the molecular

level of an artifact.

• Decreasing the intensity of light will decrease the rate of photochemical damage but

will not completely prevent damage.

• The best preservation practice would be to house all artifacts in complete darkness but

some amount of light is needed to see an object.

B.1. Damage by UV, Natural light and IR

All three are harmful to artifacts and the damage caused by all light is cumulative and

irreversible.

A. UV causes :

yellowing, chalking, weakening, and/or disintegration of materials.

B. Natural Light fades (or "bleaches" colours).

Those colours that fade can disappear within as little as a few hours of direct sunshine,

or just a few years at low museum lighting (e.g. some felt tip pen inks, some colour

photographs).

Those which do not fade may last centuries in direct sunshine (e.g. ceramics, Minoan

frescoes).

All coloured objects fall somewhere between these two extremes.

C. IR :

heats the surface of objects, and thus becomes a form of incorrect temperature (too

high).

B.2. Damage to Inorganic Materials

(metals, stone, ceramics, glass)

o Exposure to light has less effect on inorganic materials.

o However it can lead to Cracking, chalking of modern plastics, rubbers, paints that

contain UV stabilizers, designed for outdoor exposure.

o Ultraviolet light has been shown to oxidize metals over a long period of time.

o Light is not harmful to ceramics as such, but pigments used in surface decoration

could be damaged by over exposure

B.3. Damage to Organic Materials

o Textiles are very sensitive to light, which can quickly fade dye pigments. Exposure to

light will also break down the chemical structure of textile fibres, making them weak and

brittle.

o Weakening and eventual fragmentation of most wool, cotton, silk, paper.

o Yellowing of some low quality papers such as newspapers.

o Wood turns grey, erodes.

o Yellowing of pale woods.

o Cracking of most plastics, resins, varnishes, rubber.

o Chalking of most indoor and artists' paints, ivory, bone.

o Too much light falling on a drawing will fade watercolour and pastel pigments, and will

start breaking down the cellulose of which paper is made, resulting in a sheet so

discoloured and brittle .

o Chalking of oil paints with photosensitizing pigments (zinc white, early titanium white).

***

C. Controlling Light Proponents of daylight in museums often use the terminological trick of "natural"

light to mean daylight, and "artificial" light to mean electric sources, but all light

sources are natural, whether glowing stars, glowing filaments, or glowing

phosphors.

Categories of Objects

Visible Light

Sensitive collections

Including textiles, photographs, works on paper,

watercolours on any medium, feathers, etc.

Maximum: 50 lux

Less sensitive collections

Including oil paintings, wood, leather, acrylic

paint surfaces, polychrome, panels, furniture,

Maximum:

150 lux

Least sensitive collections

Including most metal, ceramics, stones and glass

Maximum:

300 lux

C.1. Controlling- Ultraviolet light

UV should be completely eliminated.

All of the techniques used to limit visible light will also cut down on UV light.

To block the remaining UV light:

i) Install filtering material. Types of filters include:

UV filtering film for windows or glass on framed objects

UV filtering plexiglass instead of glass

Filter sleeves for fluorescent tubes

UV filtered fluorescent tubes

ii) The plastic material that carries the UV filtering coating often breaks down faster

than the filtering chemical.

iii) Should replace filters whenever they begin to turn yellow or crack.

iv) Monitor UV radiation at least every five years to be sure the filtering material is

still effective.

C.2. Controlling Infrared radiation

• IR (heat) generated by natural or artificial lighting should also be controlled to

prevent rapid changes in relative humidity.

• Window coverings and filters and good air circulation systems (for example, fans

and air conditioners) help control heat buildup.

• Control the heat produced by artificial lighting fixtures by using filters and good air

circulation systems, as well as keeping lights outside exhibit cases.

C.3. Stages of Control

Avoid

• Avoid direct sunlight.

• Establish rules for light levels, UV levels, and light sources.

• Switch off electric lights whenever no viewer is present. Use proximity switches

whenever possible.

• In historic houses, select locations in the house, and within the room, that are low in light

intensity throughout the day. If there are no UV filters on the windows, place objects

where no direct light from the window can reach them.

Block

• Use UV filters on light sources that are high in UV

• Outdoors, use shading devices such as simple roofs or take advantage of the north side of

a building.

• Indoors, use curtains, use screens, blinds, solar screen, paint, etc. to block windows.

• Separate bright public access areas from display areas and provide adaptation paths

between the two.

• Close curtains, blinds, shutters, etc. when the museum is closed. Cover cases when no

viewers are present.

Detect

• Look for signs of light and UV damage in the museum.

• Use light meters.

• Use museum UV meters.

• Use a simple thermometer, if an IR heating problem is suspected.

Respond

• When faded objects are noted, determine causes and possible solutions.

• When light meters and UV meters show unexpectedly high values in a location, determine

causes and solutions.

Recover

• There is no true recovery possible from faded colours or disintegrated surfaces. Restoring

such losses requires replacement by new material.

***

C.4. Precautions

• A reliable roof that covers all organic artifacts (and preferably most inorganisc artifacts).

Historic vehicles or historic machine cannot be expected to survive many years if

exposed to sun and weather.

• Reliable walls, windows, and doors that block local weather, sunlight, local pests,

amateur thieves, and vandals.

• Avoid areas of direct sunlight and intense spot lamps at close distances on all organic

artifacts

• Minimize exposure to light, especially UV.

• Keep levels as low as possible.

• Use storage rooms that are windowless.

• Rotate the artifacts for ‘resting period’ in the storage.

• To help limit the duration of exposure of materials, timed light switches can be used in

storage rooms.

Photography:

• Floodlights used for professional and motion picture photography and

photocopy machines can cause excessive heat buildup.

• Discourage photography in museum storage areas.

• When photography is allowed in museum areas request heat absorbing light

filters and be sure the area is well-ventilated with fans or air conditioners.

• Lights should be turned off whenever filming is not taking place. If lighted

rehearsals are necessary, use dummy objects until the final filming will take

place.

Effects of Different Lighting Techniques f

Fibre-optic lighting system for display cabinets: the light guides are inside curved tubes. An optical connector at the end of the fibre/tube distributes the light.

Miniature LED installed here in the ceiling of a display cabinet for showcase lighting