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2.3 Metrological parameters
Satisfactory values for the specified metrological properties of lux metres are listed in Table 1.
Table 1 – Defined values for specified metrological properties

Characteristics	Symbol	Satisfactory value
Deviation of correction coefficient kA from value 1 ascertained for measurements with standard source A.	│kA-1│	 8 %
Deviation of correction coefficient kj from value 1 for other sources of light.	│kj-1│	 12 %
Integral characteristic^*)	f1'	 9 %
Sensitivity in UV range^*)	fUV	 4 %
Sensitivity in infrared range*)	fIR	 4 %
Index non-linearity	f3	 5 %
Temperature dependence index^*)	f6	 20 %
Directional dependence index^*)	f2	 6 %
^*)For type approvalonly.

2.4 Relative spectral responsivity
2.4.1 The relative spectral responsivity curve s_rel (λ) must be adjusted to the function V (λ) as well as possible. The spectral responsivity of the lux metre must be known in order to characterise the quality of the lux metre as compared to sources with varying spectral density. This responsivity must be specified in the entire spectral range of the specified spectral weighting function (from 360 nm to 830 nm), which should ideally be depicted in tabular form with increments of 5 nm. If the spectral increment is greater, it will be necessary to select a suitable mathematical form of interpolation. A lux metre’s responsivity to radiation outside of the visible spectrum must be suppressed (see Articles 1.10 and 1.11).
2.4.2 Several different parameters can be used to describe the quality of a lux metre’s spectral adaptation. The inadaptability of a lux metre’s relative spectral responsivity s_rel (λ) to the weighting function V (λ) is generally quantified in the form of the integral characteristic of spectral inadaptability f₁'. Measuring relative spectral responsivity within a range between 380 nm and 780 nm is sufficient for assessing the integral characteristic f₁'.
2.5 Directional dependence
The directional dependence for lux metres and their deviation from the required curve is assessed in the range of angle of incidence from 0° to 85°.
The imperfect nature of the optical-mechanical design of the lux metre’s sensor results in an orientation error if the light incidence on the sensor’s surface is below the angle β.
The manufacturer must determine the maximum permissible orientation error as the difference between the lux metre’s readout at an angle of radiation incidence below β and the value measured at a perpendicular angle of incidence multiplied by the value cos β. Perpendicular incidence is always used when verifying photometric graduation, since the influence of the orientation error does not apply in this case. In practice, however, light may fall on the lux metre’s sensor at various angles, which means this influence must be quantified.
A lux metre which is intended for measuring illuminance on terrain must be equipped with a corrective optical element to correct the orientation error known as a cosine adapter.