Although the ACGIH guidelines can be interpreted to suggest the Derwent lamps can be safely viewed at very close distances (fig.3) Derwent discourage direct viewing at any distance below the MSWD. If the lamp is on the heat from it can be felt by passing a hand in front of the lens. A second observer at a distance greater than MSWD should be used to checked 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.