4.1 This test method represents the preferable means for calibrating both narrow-band and broad-band ultraviolet radiometers. Calibration of narrow- and broad-band ultraviolet radiometers involving direct measurement of a standard source of spectral irradiance is an alternative method for calibrating ultraviolet radiometers. This approach is valid only if corrections for the spectral response of the instrument and the spectral mismatch between the calibration spectral distribution and the target spectral distribution can be computed. See Test Method for a description of the spectral mismatch calculation.

4.2 The accuracy of this calibration technique is dependent on the condition of the light source (for example, cloudy skies, polluted skies, aged lamps, defective luminaires, etc.), and on source alignment, source to receptor distance, and source power regulation.

Note 5-It is conceivable that a radiometer might be calibrated against a light source that represents an arbitrarily chosen degree of aging for its class in order to present to both the test and reference radiometers a spectrum that is most typical for the type.

4.3 Spectroradiometric measurements performed using either an integrating sphere or a cosine receptor (such as a shaped PTFE³, or Al₂O₃ diffuser plate) provide a measurement of hemispherical spectral irradiance in the plane of the sphere's entrance port. As such, the aspect of the receptor plane relative to the reference light source must be defined (azimuth and tilt from the horizontal for solar measurements, normal incidence with respect to the beam component of sunlight, or normal incidence and the geometrical aspect with respect to an artificial light source, or array). It is important that the geometrical aspect between the plane of the spectroradiometer's source optics and that of the radiometer being calibrated be as nearly identical as possible.

Note 6-When measuring the hemispherical spectral energy distribution of an array of light sources (for lamps), normal incidence is defined by the condition obtained when the plane of the receiver aperture is parallel to the plane of the lamp, or burner, emitting area.

4.4 Calibration measurements performed using a spectroradiometer equipped with a pyrheliometer-comparison tube (a sky-occluding tube), regardless of whether affixed directly to the monochromator's entrance slit, to the end of a fiber optic bundle, or to the aperture of an integrating sphere, shall not be performed unless the radiometer being calibrated is configured as a pyrheliometer (possesses a view-limiting device having the approximate optical constants of the spectroradiometer's pyrheliometer-comparison tube).

4.5 Spectroradiometric measurements performed using source optics other than the integrating sphere or the "standard" pyrheliometer comparison tube, shall be agreed upon in advance between all involved parties.

4.6 Calibration measurements that meet the requirements of this test method are traceable to a national metrological laboratory that has participated in intercomparisons of standards of spectral irradiance, largely through the traceability of the standard lamps and associated power supplies employed to calibrate the spectroradiometer according to G138, the manufacturer's specified procedures, or CIE Publication 63.

4.7 The accuracy of calibration measurements performed employing a spectroradiometer is dependent on, among other requirements, the degree to which the temperature of the mechanical components of the monochromator are maintained during field measurements in relation to those that prevailed during calibration of the spectroradiometer. [1]

Note 7-This requirement is covered in detail in an ASTM standard under development in Subcommittee G03.09 on Radiometry.