Because it is what makes a reading defensible. Photometric sensors drift with age, exposure and handling, and the drift does not announce itself: the meter continues to display plausible figures. Where a measurement will support a compliance claim, a payment, a dispute or a safety case, a current traceable calibration certificate is part of the evidence. Treat recalibration as a recurring cost of owning the instrument.

Typically annually, with the interval shortened for instruments in heavy use, exposed to high light levels or UV, or subject to knocks and site conditions. Recalibrate after any suspected damage, and before work that must be demonstrably accurate. The certificate should identify the instrument and its sensor head together, since calibration belongs to that combination rather than to the display unit alone.

It separates where the light is measured from where the operator stands. The sensor can sit on the desk, the floor, the working plane or inside equipment while the display is read from a position that does not shadow it. Shadowing by the operator's own body is a frequent and invisible cause of under-reading, and a detached head on a lead eliminates it while making awkward positions accessible.

For repeatability. Measurements are specified at a defined height on a defined plane, and holding a meter by hand introduces variation in height, level and orientation between readings and between operators. A tripod fixes all three, which matters when averaging a grid across a room or comparing a space before and after a lighting change. It also keeps the sensor level, which cosine response depends on.

Beyond the standard photopic head, common options include quantum or photosynthetic sensors for horticultural measurement, heads matched to specific ultraviolet bands, and heads with different measuring ranges for very high or very low light levels. Each has its own calibration. Check that the instrument supports the head electrically and that the pair has been calibrated together before relying on readings from a newly fitted head.

Generally not. Sensor heads, leads and mounts use manufacturer-specific connectors and, more importantly, sensor-specific calibration data held by the instrument. Fitting a third-party head, even where it physically connects, invalidates the calibration relationship between head and display. Buy accessories for the specific model in use, and record which head belongs with which instrument where a site holds several.

A case that protects the sensor above all, with the diffuser capped, since a scratched or contaminated diffuser alters cosine response and cannot be corrected in software. Keep the calibration certificate with the instrument, along with a note of the next due date. Remove batteries for long storage to prevent leakage, and avoid leaving instruments in vehicles, where heat and cold accelerate sensor drift.