4
UNITS OF MEASUREMENT
Ultra-Violet (UV)
Traditionally UV has been measured in museums as the proportion of ultraviolet
present. This result is useful for checking a particular lamp or window because the
proportion of UV does not change rapidly with the distance from the light source.
Using a simple rule the amount of UV on an object can be limited. It is usual to
arrange that the proportion of UV should not exceed 75µW/lumen in museums and
galleries, though some organisations try to keep UV levels below 25µW/lumen
The damage is done by the total amount of UV falling on the object so it is useful to
be able to measure this directly, especially if non standard amounts of illumination are
required. The amount of UV should be as little as possible but in general should not
exceed 20mW/M
2
, again some organisations keep the level below 6mW/M
2
.
Both the above units are displayed when the UV button is pressed, one in large
characters, the other smaller at the bottom of the screen. Which is displayed where can
be swapped by pressing Set\Units\µW/Lumn-mW/M².
When measuring low levels of UV the 765 can take up to 5 seconds to take the
reading. So, to get a reasonably quick response, when initially turned on, the unit only
measures for one second and if UV levels are low this will show as zero. If the "UV"
button is pushed again the full 5 seconds is allowed for the measurement and if a low
level of UV is present it will be detected and displayed.
Visible Light
This can be displayed either in Lux or Foot-candles. To change the units press
Set\Units\Lux-Footcandl.
A visible light readout is provided to control illumination and limit damage done by
visible light. Normal museum light levels should be limited to 150-250 Lux.
Once measurements have been made the light level can be altered if necessary and UV
filters can be fitted on windows, fluorescent tubes or other UV producing light sources
as required. These filters often deteriorate over a period of years so it is essential to re-
check them periodically.
Suggested light levels for various purposes are given below:
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