Measurement of light and ultra-violet (uv) radiation

There is no difficulty in measuring light intensity. Lux sensors can be attached to data loggers. They are expensive, so one should consider if a simple light sensitive resistor, which costs about a dollar, can be used instead. If the aim is to check if the last person switched off the lights, a photoresistor is perfectly adequate.

Atmospheric water content and temperature are usually fairly even over an exhibition room but the light intensity varies enormously from place to place. The pattern as well as the intensity of illumination by daylight varies with cloud cover, time of day and time of year. It is therefore difficult, or very expensive, to get meaningful data from logged light measurements.

This question is even more relevant to the deployment of uv sensors. The uv radiation is strongly absorbed by white paint, so a sensor seeing indirect daylight will give a low reading for uv, which may be much higher on a painting that is directly opposite a window. Any institution that can afford a data logger and enough uv sensors to make any sense of doing the measurements can afford to exclude uv from its exhibitions. UV absorbing foils and varnishes are the nearest thing we have to a perfect conservation product. Monitoring no uv will soon become boring, but the monitor is even of doubtful use for quality control: if one fluorescent lamp in one showcase loses its absorbing screen, a centrally placed uv sensor is unlikely to detect the rain of energetic photons on just one or two illuminated objects.

Light sensors are used in the control of daylight illumination in galleries, but this is too large a subject for this article, which is limited to describing the processes of measurement and interpretation.

Photochemical data loggers

There is one company that supplies data loggers to the museum trade which offers, in response to requests by conservators, a colour and cosine corrected lux sensor to mount next to your favourite tapestry. The sensor sends pulses winging through the aether to a program that works out the cumulative light dose.

This seems to me a good point to introduce a completely different form of data logger: the photochemical accumulator. Many years ago the Blue Wool Standards were developed. These are pieces of dyed cloth that fade at defined rates as they absorb light. A set of such woollies discretely stitched to the tapestry will allow the conservator morbidly to follow the ravages of photochemistry through the centuries. It's true that the company with the hi-tech solution offers a twelve month guarantee, but the blue wool number 7 has a five hundred year expected lifetime in a gentle museum environments. It's also washable.

Installing sensors


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