What is a measurement point?
One horizontal and vertical angle at which intensity is read, chosen from the grid so the beam axis, the compliance points and the steep edges are all covered.
A measurement point is one angular position, a horizontal angle and a vertical angle measured from the beam axis, at which a fixture’s intensity is read. The photometric grid is the layout of angular points; the measurement points are the ones actually read and recorded. At each one the fixture is turned to that angle, the photometer reads once, and one candela value goes into the table.
Which points are read matters as much as how many. The beam axis is always included, because it shows where the peak is and whether the fixture is aimed correctly. Contour points sit on or inside the areas where the standard sets a minimum intensity, so they decide compliance. Extra points are added closer together where intensity changes fast, usually near the beam edge, because a coarse step can step over a drop. Outside points show how quickly the light falls away. How the readings are combined, for example as an average over an area, comes from the standard.
Frequently Asked Questions
Q1What is a measurement point?
One angular position, a horizontal angle and a vertical angle measured from the beam axis, at which the intensity of a fixture is read. The fixture is turned to that angle, the photometer reads once, and one candela value is recorded.
Q2How is a measurement point different from the photometric grid?
The grid is the layout of angular points over and around the beam. The measurement points are the points that are actually read and recorded, which may be the whole grid or the grid plus extra points added where the beam needs them.
Q3Which points are always measured?
The beam axis is always included, because it shows where the peak is and whether the fixture is aimed correctly. The standard also names the points or areas where a minimum intensity is required, and those are measured too. Which points are required, and how the readings are combined, comes from the applicable standard.
Q4Why add extra points where intensity changes quickly?
Near the edge of the beam the intensity can fall steeply over a small angle. A coarse step can step over the drop and hide it, so points are added closer together there to capture the true shape.
Q5Why measure points outside the beam?
Readings outside the main beam show how quickly the light falls away and confirm that there is no unexpected spill or second lobe. They also give a low-intensity reference for the rest of the table.
Q6What can go wrong at a measurement point?
If the beam axis is not aimed at the photometer, every angle is shifted and the whole table is wrong. A point read at the wrong angle, before the fixture has settled at its rated current, or with stray light reaching the photometer gives a value that does not belong to that point.
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