Because a UV radiometer responds only to the band its detector and filter were built for. Pointing a UVA meter at a predominantly UVC source, or the reverse, produces a reading that is not merely inaccurate but unrelated to the quantity of interest, and nothing on the display indicates a problem. Always confirm the meter's band against the source's emission and against whatever standard or specification the measurement must satisfy.

Irradiance is the rate at which UV energy arrives at a surface, in watts or milliwatts per square metre or square centimetre. Dose, sometimes called radiant exposure, is irradiance accumulated over time, in joules per square centimetre, and it is what determines whether a chemical or biological effect has been achieved. A short exposure to a strong source and a long exposure to a weak one can deliver the same dose.

Measure at the position the workpiece occupies, not at a convenient distance, since irradiance falls off sharply with distance from the lamp. Use a detector matched to the lamp's band, and capture dose as well as peak irradiance, because cure depends on total energy delivered. On conveyor systems a radiometer that travels through the tunnel with the product gives the profile the product actually receives, which is what a fixed reading cannot.

Because germicidal lamp output declines substantially over life, and the decline is invisible: a UVC lamp continues to glow long after its output has fallen below the level needed for the specified inactivation. Quartz sleeves also foul, absorbing UV before it reaches the target. Periodic measurement with a UVC radiometer is the only way to confirm that a disinfection system is still delivering the dose it was designed around.

Yes. Fluorescent penetrant and magnetic particle inspection require a specified minimum UVA irradiance at the surface being examined, together with a limit on visible light in the inspection area, and both are verified with a meter. UV lamp output falls with age and the measurement is normally part of routine inspection-cell checks, so the radiometer is a compliance instrument in that context rather than a convenience.

Some instruments accept interchangeable detector heads for different bands, which is the practical solution where a site has both curing and germicidal applications. A single fixed-band meter cannot be adapted by calculation. When using interchangeable heads, confirm that the head and the instrument are calibrated together as a pair, since the calibration belongs to the combination and not to the display unit alone.

Traceable calibration at the stated interval, and more frequently where the meter itself is heavily exposed, because UV degrades the detector and its filter over time so the instrument drifts in the direction of under-reading. Keep the detector capped when not in use, avoid heat from the lamps being measured, and let the head reach a stable temperature before recording, since response varies with temperature.