Introduction
Throughout history people have been exposed to high levels of infrared radiation, in the making of steel and glass, simply being out in strong sunlight, or anywhere close to a hot object. Infrared gives a heating effect, which we sometimes use to keep us warm, but too much radiation on the skin can cause blistering.
Bright natural light, which contains infrared, causes the eye to blink, the iris to contract and the viewer to look away. These normal defense mechanisms are not triggered when the visible part of the light spectrum is removed. Exposure to Infrared radiation over long periods may give rise to retinal damage and cataracts. As shown in Figure 1, filtered Infrared is approximately 100 times less hazardous than visible light.
Figure 1
Relative Hazard Verus Wavelength
There are two scenarios which have to be considered with respect to security systems; safety in use, (which is long term), and safety during installation, testing and maintenance, (which are shorter term).
Safety in Use
If possible the lamp and camera should always be installed where they cannot be touched by passes-by, otherwise the system is not secure. The lamp should also be positioned such that passers-by cannot stare at the lamp at a distance closer than the Minimum Safe Working Distance (MSWD) in Table 1. The MSWD refers to the central part of the beam which is the most intense, and under conditions of continuous viewing for up to 8 hours.
Table 1 - Minimum Safe Working Distance for up to 8 Hours Viewing
Beam Angle
Uniflood 500 Series
Uniflood 300 Series
m
ft
m
ft
60
1.64
5'6"
1.5
5'0"
100
1.4
4'6"
1.23
4'0"
300
0.9
3'0"
0.81
2'9"
600
0.89
3'0"
0.8
2'9"
The Derwent range of LED illuminators have a narrow band output and can be viewed continuously at even shorter distances with an MSWD of 0.3m (12').
Safety During Installation, Testing and Maintenance
Derwent lamps have a centre beam irradiance of 10 mWcm-2 at the MSWD which is the minimum recommended viewing distance under normal conditions. In circumstances where closer viewing has to take place then Figure 2 gives the time/distance relationship needed to maintain safe irradiance limits.
A second observer at a distance greater than MSWD should be used to check that the lamp is functioning and that the filter is acceptable for either overt or covert operation. During final adjustments a monitor linked to the camera is recommended, as described in the installation instructions supplied with the lamp. For regular maintenance at the camera head, the lamp should be extinguished. Multiple lamp systems have designed configurations so that
overlapping intensities do not exceed the centre beam intensity of a single lamp at the MSWDs.
Figure 2
Minimum Safe Working Distance Versus Time
