The ACGIH publish the TLV (Threshold Limit Values) for various physical agents; they recommend a TLV of 10 mWcm.^-2 for the irradiance of near infra red radiation under conditions of viewing for longer than 16mins. For shorter times and higher irradiances the following formula can be used to find the Maximum Permissible Exposure (MPE), this is based on guidance given in ref. 1.

MPE = 491 t^-0.25 Wm^-2

Where the numerical factor 491 is specific to DERWENT lamps.

Together with the inverse square law and measurements of the centre beam irradiances of Derwent lamps, this formula has been used to produce fig. 3 which gives the time/distance relationship needed to maintain safe irradiance limits. Time/distance conditions within the MSWD should be regarded as unsafe and should be avoided.

Derwent lamps have a centre beam irradiance of 10 mWcm^-2 at the MSWDs which is the minimum recommended viewing distance under normal conditions. In circumstances where closer viewing has to take place then fig. 3 gives the time/distance relationship needed to maintain safe irradiance limits.

EN 60825-1 1994-1996 gives Maximum Permissible Exposure limits of the radiance on the eyes for light of various wavelengths, these MPE values have been used to produce fig. 1 which indicates that visible blue light is around 100 times more hazardous than the filtered IR light used in Derwent security lamp systems.