AN ULTRAVIOLET MONITOR FOR MUSEUMS* BY GARRY THOMAS? and E. T. HALL**

t Research Labarutory, The National Gallery, London. ** Research Laboratory for Archaeology and the History of Art, Oxford University.

It is now generally recognized that UV (ultraviolet) radiation causes a sizeable proportion of the fading and surface deterioration suffered by museum objects on display. The proportion varies widely. Colourless materials such as varnishes, paint media and raw textiles are degraded predominantly by UV radiation, but most dyes and fugitive pigments are faded also by visible light, especially at the blue end of the spectrum. Not enough data has yet been accumulated for a reliable estimate of the overall benefit to be obtained from the use of UV-absorbing filters. However, using figures recently published by Padfield and Landi (1966), we have estimated that the rate of colour change of natural dyes under average daylight might be reduced to about a third or a quarter by removing the UV (i.e. lifetime under UV-absorbing filters three times as long). The Damage Factor published by Harrison (1953) gives a rather more optimistic figure for museum materials in general.

The benefit from placing UV-absorbing filters over fluorescent lamps is smaller, though still very worthwhile, except for those lamps of particularly low UV emis- sion, since it is such an easy thing to do. Installation of UV-absorbing filters is comparatively cheap and involves no visible disturbance in the lighting.

PURPOSE OF A UV MONITOR A UV Monitor, being an instrument for detecting and measuring UV radiation,

has two main purposes in the museum: 1. To check that UV elimination procedures are working correctly. This use alone makes it an important instrument for the conservator. 2. To investigate the presence of UV radiation in real and experimental situations with a view to improving protection.

An alternative approach for detecting the presence of UV-without measuring it-is by using what might be called a UV Detector or Snooperscope. Such an instrument has been described by Padfield (1967). The scene is viewed through the eyepiece of this pocket-sized instrument and the suspected source of UV is centred in the field of view. The visible image is then extinguished by operating a shutter. UV radiation is revealed by a yellow-green fluorescence from a small target in the centre of the field.

One of the authors (G.T.) has constructed an even simpler, though cruder, device in which all sources of UV radiation, such as lamps and windows, appear as dim fluorescent images, the rest of the scene remaining black. Here the radiation, after passing through a Wratten 18A UV-transmitting filter, is focussed on a sheet of cellulose acetate containing a dye which fluoresces green under UV. This sheet is viewed through a magnifying eyepiece.

The Snooperscope and the Monitor have complementary uses in the museum. The Snooperscope is useful, for instance, in detecting whether an area of window

A fuller description of this Monitor was given at the London Conference on Museum Climatology sponsored by the International Institute for Conservation of Historic and Artistic Works in September 1967 (Hall 1967).

A R C H A E O M E T R Y 121

or a fluorescent tube has not been given its UV-absorbing filter, or whether the filter has become ineffective with time. It is also cheap and easy to construct. The advantages of the UV Monitor are its great sensitivity and its ability to give a scale reading of the proportion of UV in a light source.

WHAT SHOULD THE UV MONITOR MEASURE? It must be emphasized that glass is opaque to ultraviolet radiation of wavelength

shorter than about 0.311. (0.3 microns, 300 rtanometres or 3000 Angstrom units) and that radiation from sun and sky also drops to zero at about this point. Since, except in unusual cases, all the light which reaches museum exhibits has passed through glass, the conservator can confine his interest to the UV band between 0.3 and 0 . 4 ~ . This is what the Monitor measures as UV radiation.

Radiation of wavelength shorter than 0.3~ is emitted by certain light sources in quartz envelopes, such as the high-pressure mercury-vapour lamp, Such radia- tion could be highly damaging, but tbere is no reason to use it in a museum. The Monitor, whose UV-transmitting filter is made of glass, is insensitive to this radiation.

The UV radiation falling on a surface could be measured in energy intensity units, such as cals cm-* sec-', or watts cm-'. An ordinary incident light meter with a UV-transmitting filter over it would give an adequate reading under the open sky. Unfortunately there are two difficulties to be overcome in the realisation of this simple-sounding device. I . The photo-emissive photocells used in most light-meters produce a current in the microamp range under visible light. The usual UV-transmitting filter, such as the Wratten 18A, cuts out the visible light plus about half the UV. Thus in museum conditions, where the light level often falls below 50 lux, an impracticably large photocell area (perhaps 200 cmz) would be required to give a healthy reading on a microammeter. Amplification would therefore be required. 2. To measure all the radiation incident on a surface, including that at narrow angles and grazing incidence, the sensitive surface of the photocell should be flat and completely unobstructed. Alternatively a diffusing screen could be used which further reduces sensitivity.

But fortunately we need not pursue these difficulties, for an alternative approach is preferable; to measure the proportion of UV in a light source, rather than its quantity a t some particular point. (The proportional scale used is described below.)

Consider the main use of a UV Monitor-to check that the UV-elimination procedures are working correctly. or whether any UV is penetrating the defences. A meter measuring incident UV radiation would give a reading proportional to the UV falling on it. which in turn wou!d depend on the overall illumination level. Such a meter, taken closer or further from a lamp, could be made to give almost any reading. Though this might be useful for measuring conditions at a particular exhibit, it would be meaningless if we wanted to deal with the light source unless associated with an independent measurement of illumination value.

In the general lighting situation the conservator finds that he must deal with two controls independently; control of visible illumination value, and elimination of UV. The UV Monitor, measuring proportion of UV which is independent of distance from light source, therefore complements the visible light-meter each giving one of the two figures required.

122 A R C H A E O M E T R Y __

CHOICE OF DESIGN FOR A PROPORTIONAL UV METER Once it was seen that an incident UV meter was neither necessary nor

desirable the geometry of the design was immediately simplified. To transduce UV to electrical energy the choice, because of its high sensitivity, was the photo- multiplier. The instrument described has an angle of acceptance slightly narrower than the average camera or photographic light-meter.

Proportional scale used The proportion of UV energy emitted by a light-source may be defined as: the

UV energy divided by the energy emitted over some other relevant range of radia- tion. This could be the range from 0.3p/0.76p (the UV plus visible range), the visible range alone (0.4 -0.76p), or the whole spectrum emitted by the light-source (U.V.. visible and infrared). To spread our scale out we could choose the ratio which gives the greatest differences between light-sources. This would be the ratio UV/complete spectrum. But it would be a tiresome wale to use since most light- meters are relatively insensitive 10 the infrared, and the photomultiplier of our choice is completely so. Also infrared radiation is not photochemically damaging so it is of small importance to us. Indeed the published spectral energy distribu- tions of the light-sources in which we are interested generally omit it. A reasonable choice might seem to be the proportion:

UV energy/Visible energy % and this is what was at first attempted.

However, on plotting the values of UV/Visible energy obtained on the Monitor for a variety of light-sources against the theoretical values derived from spectral energy distribution data, the light-sources were found to fall into two groups: fluores- cent lamps versus the rest. This was not due to the irregularity of the UV spectra from fluorescent lamps, but to the fact that the radiation from these lamps falls off very sharply at wavelengths longer than about 0 .66~. a region to which the photomultiplier is completely insensitive. This being the case, a large slice of red energy, ranging from 0.66 to 0 . 7 6 ~ influences the theoretical figure for the UV/ Visible energy according to its presence (in daylight) or absence (in fluorescent light). but never affects the Monitor in either daylight or fluorescent light. Conse- quently fluorescent lamps fail to conform to a calibration curve that does very well for tungsten and daylight. Further examination of the problem shows that this apparent short-coming of the Monitor can be put to advantage.

What the conservator needs as a measure of the UV in a light-source is an answer ‘to the following question:

“If I light a picture or exhibition case first with tungsten lamps, then with fluorescent lamps, then with daylight. ull ut the same illumination value (for instance 100 lux, or 10 lumens per sq. ft.) how will the amount of UV radiation falling on the exhibits vary?”.

The UV Monitor can be made to yield just this comparison if it is calibrated to read, in effect, UV energy divided by illumination value. We have called this ratio the UV CONTENT, abbreviated U.V.C. Details of this calibration have not been given in these pages, but are to be found in the proceedings of the conference on Museum Climatology (1967).

A R C H A E O M E T R Y 123

FIG. 1.

OPERATION OF THE UV MONITOR

The Monitor is shown in figure ]:The photomultiplier lies at the back of the meter dial. Two filters are mounted on a slide in such a way that either one or the other is aligned to cover the field of view of the photomultiplier. One filter, the UV filter, transmits UV radiation only, in the range 0.3-0.4~. The other, the YIS fifter. transmits visible light, though very little at the blue end of the spectrum. Since the photomultiplier itself is insensitive to red, its response in combination with the VIS filter corresponds closely enough for our purpose with that of the human eye. The VIS filter also reduces the intensity of visible light so that readings on the two filters come within the same range; light-sources with much UV will give a higher reading on the UV filter than on the VIS filter, and vice versa for low-UV sources.

In its calibrated form the Monitor carries three scales. The pA scale measures the output to the microammeter directly, and is retained for laboratory use. It can be ignored for the moment.

Operation using the HIGH-UV and LOW-UV scales is as follows: 1. Point the Monitor at the light-source and find which filter gives the higher

reading. It will soon be apparent to the user that almost all kinds of daylight give a higher reading on the UV filter, whereas almost all artificial sources give a higher reading on the VIS filter.

2. With the higher-reading filter in place adjust to full-scale deflection using the coarse and fine potentiometer controls. The fine adjustment is by a knurled wheel which can be seen near the user’s thumb in figure 1.

3. Without moving the Monitor, switch filters and read the UV CONTENT (UVC) on the appropriate scale. If this reading is taken through the VIS filter (which means that the full-scale adjustment was on the W filter), the UVC is read on the HIGH-UV scale. A reading through the UV filter is read on the LOW-UV scale.

1 24 A R C H A E O M E T R Y

As pointed out above, the UVC reading is independent of distance from light- source, though it may be disturbed at very high intensities (above a few thousand lux). The Monitor is usable down to about 10-30 lux (1-3 lumens/ft*). though at these low values a slight drift may make readings less certain. However, for values lower than this, one can often contrive to get nearer the light-source. Note: To convert UVC to microwatts of UV energy per lumen, multiply the UVC

reading by 16.

THE INSTRUMENT Ddgn C O a s i d e r a t i O ~

In designing the instrument certain other considerations than those mentioned above must be made: a) Besides a high sensitivity to U.V. it should have a large dynamic range of intensity level. b) The unit should be portable and be driven by internal batteries having a reasonably long life; also the batteries should have a long "shelf" life since the instrument might not be used for considerable periods. c) The measurements must be repeatable with as little drift as possible over the temperature range likely to be found in museums i e. 5°C to 30°C.

Figure 2 shows the spectral response of the type of photomultiplier used in the Monitor (E.M.I. 9660 B). It has a SbCsO photocathode and is fitted with a

1.0

0.1 > 0 2 k! Y LL LL w 0.01 4 c z U 3 0 0*001

0*0001 2 WAVELENGTH (p)

Fio. 2.

A R C H A E O M E T R Y 125

thin UV-transmitting window. It will be noted that the response is at a maximum over the frequency range and the curve is approximately flat over the 0.3 to 0 . 4 ~ range. It should be noted that the sensitivity falls very rapidly at wavelengths in excess of 0.65~. The “squirrel cage” dynode structure OE the tube is of the semi- transparent type. This type of construction allows much higher cathode currents, before linearity is lost or fatigue of the tube occurs. than are possible with many other types of tube, This allows the use of these tubes at comparatively high illumination levels without deterioration of results. At the same time the sensitivity of the tube is about right at the low illumination level; the dynamic range of the instrument is in this way adequate.

Choice of UV filter. For ultraviolet light the simplest type of filter is a coloured glass type (Kodak

18A). The only immediately apparent disadvantage of this type may be seen in figure 3: a considerable band of the spectrum in the near infra red (from 0.68 to

Fio. 3.

0.88~) is passed by this filter. Reference, however, to figure 2 will show that the S11 type of photocathode used in the 9660 photomultiplier is not sensitive to the spectrum above 0.65~; this part of the spectrum, therefore, makes no contribution to the monitor reading.

Photomultiplier Power Supply. So that the Monitor can be conveniently portable it is necessary to generate

the high voltage across the tube from internal batteries by means of an inverter. This inverter is of the Cockcroft-Walton type and is able to generate up to 700 volts at 200pA. When lower sensitivities are required this output voltage may be lowered by reducing the low vo!tage input to the transistor oscillator feeding the

126 A R C H A E 0 hl E T R Y

FIG. 4.

inverter transformer. This low voltage is stabilised by reference to a Zener diode; the input supply is from 8.4 volts Mallory mercury cells. Overall temperature stability is achieved by the use of a positive coefficient resistor in the stabiliser network. A stability of about 0.1 %/degree C is achieved.

Mechanical Construction. Figure 4 shows an exploded view of the apparatus. The methods of mounting

the batteries and the vertical photomultiplier can be clearly seen. The generator transformer is mounted between fibre-glass discs on which the electronic compo- nents are also mounted. The batteries can be easily changed without disturbing the rest of the apparatus. The batteries will give some 100 hours continuous operation; moreover, since they are of the mercury type their shelf life is very long and their output voltage is comparatively level over their useful life.

Figure 1 shows the instrument in use. The changeable filter can be clearly seen. This filter assembly can be moved into two positions-in one operating position the photomultiplier window is covered with a Kodak 18A filter, whilst in the other a combination yellow and neutral filter is placed in position.

REFERENCES

Hall. E. T., 1967, ‘An Ultraviolet Monitor for Museums’, pp. 151-7, and Gamy Thomson, ‘Calibration and use of a UV Monitor’, London Conference on Museum Climatology, available from IIC, c /o The National Gallery, Trafalgar Square, London, W.C.2.

Harrison, L. S., 1953, Report on the Deteriorating Effect of Modern Light Sources, Metro- politan Museum of Art, New York.

Padfield, T. and Landi, S. 1966, The light-fastness of the Natural Dyes, Studies in Conser- vation, 11, No. 4, 181-96.

Padfield, T., 1967, A Simple Ultraviolet Radiation Detector for Museum Use, Studies in Conservation, 12, No.1, 1-4.