For emergency lighting, the following additional measuring conditions apply:
measurement in battery-operated installations at the end of the nominal service life
measurement in installations with power stand-by unit or particularly secured grids at lowest voltage supply
The standard illuminance values for emergency lighting specified in EN 1838 are exclusively direct values, without reflected components. An approximate measurement of illuminance without reflected components can be executed by placing a lens tube in front of the receiver. The relation of lens tube diameter and lens tube length must be selected in a fashion that allows light from every part of the luminaire to reach every part of the receiver. A further possibility is to measure average luminaire luminance with an image-resolution measuring instrument and to determine illuminance on this basis.