What does before/after photometry show?
How measuring the same fixture before and after an action shows whether cleaning, re-aiming or replacing a part really worked.
Before/after photometry means measuring the same fixture twice: once as found, and once after an action such as cleaning the lens, re-aiming, or replacing the source, optic or filter. The same required area is drawn over both isocandela results. The contour for the minimum required intensity must enclose that area, so the effect of the action can be seen directly rather than inferred.
The comparison is only fair if the set-up is identical: the same distance, grid, aiming reference, lamp current, stabilised output and a calibrated photometer. If any of these change, the difference may come from the test rather than the fixture. An improvement is also not a pass by itself; acceptance is judged against the standard and the laboratory report, and both results are kept with the maintenance record.
Different actions leave different signatures. Cleaning or replacing a lamp or LED module mainly changes the intensity level, so the contours grow around the same centre, as animated above. Re-aiming moves the pattern toward the required area instead. In service, ICAO Annex 14 treats a light as unserviceable below 50 per cent of its specified main-beam average intensity, so the after result must clear that level as well.
Frequently Asked Questions
Q1What is before/after photometry?
It is measuring the same fixture twice, once as found and once after an action such as cleaning the lens, re-aiming, or replacing the source, optic or filter. The two results are compared against the same required area, so you can see whether the action actually improved the light output.
Q2Why must the set-up stay the same?
A different distance, grid, aiming reference or current can change the result on its own. If the set-up differs between the two measurements, you cannot tell whether the fixture changed or the test did. Keep the set-up, the lamp current and the calibrated meter the same, and record them.
Q3When is a before/after comparison useful?
It is useful after cleaning, re-alignment, replacing a source, filter or optic, and after refurbishment. It shows which action produced the improvement, supports the maintenance record, and helps decide whether a fixture can stay in service or needs replacing.
Q4What if the fixture still fails after the action?
Look at the shape of the remaining gap. A beam that is still too small, off target, too narrow or out of colour points to a different cause than the one just treated. Check the alignment and the test set-up again, then trace the cause before condemning the fixture.
Q5Does an improvement mean the fixture passes?
Not by itself. A better result is only an improvement. Whether the fixture meets the requirement is decided against the required areas and limits in the applicable standard and the test laboratory's report, so compare the after result with those, not just with the before result. For lights in service, ICAO Annex 14 (Volume I, Chapter 10, maintenance) treats a light as unserviceable when its main-beam average intensity is below 50 per cent of the specified value, or of its design value where that is higher. A fixture still below that level after the action cannot be counted as serviceable.
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