reaching the threshold limit value (TLV) proposed by the ACGIH
[1]. If desired, the instantaneous effective irradiance can be displayed in µW/cm2. The radiation can be integrated over time and the dose in
mJ/cm2 can be shown on the LCD.
The TLV exposure dose has been chosen by the ACGIH as one
that can be
tolerated, even if repeated every work day, without causing any long term effect. The TLV
for occupational exposure takes into consideration the risks of acute and chronic injury
to both an eye and skin.
The eye is the most radiation sensitive organ, therefore the dose which
will cause no eye damage is the maximum allowable radiation dose. Actinic ultraviolet
radiation (UV-B and UV-C) is strongly absorbed by cornea and conjunctiva. Overexposure of
these tissues cause keratoconjunctivitis, commonly referred to as welder’s flash or
arc-eye.
Measurements of eye damaging radiation in rabbits and in monkeys show a peak
sensitivity at 270 nm. and a minimal dose for damage of
about 4-5 millijoules per square centimeter. Ultraviolet radiation
damage is gradually repaired depending on degree of damage. Damage from a minimal dose
should be repaired within 24 hours.
The meter indicates how long a worker may remain at the measured
position before reaching the threshold limit value (TLV) proposed by the ACGIH The TLV of
3 millijoules/cm2 in an 8 hour period has been chosen by the ACGIH
as one which can be tolerated, even if repeated every work day, without causing any long
term effect.
The meter reading should be of the highest intensity at a site. The
worker’s natural avoidance of the usually bright UVR light source, and the
contraction of the pupil provide additional safety from the
measured UVR when it emanates from a visually bright source. Even without these normally
occurring safety enhancements not exceeding the TLV will provide adequate protection.
The TLV for UV radiation specified by ACGIH apply to sources which
subtend an angle less than 80°. Sources which subtend a greater angle need to be measured
only over an angle of 80°.
The instantaneous value indicates how long an exposure at the intensity being measured
is required to produce a dose of 3 mJ/cm2. The PMA can also be set
to continuously monitor UV and sound an alarm when the preset dose or irradiance level is
reached.
