What does a photometric failure look like?
How common failures show up on an isocandela diagram once the requirement is drawn over it, and what each shape points to.
Photometric failure visualization means drawing the required area over the measured isocandela diagram so the shortfall can be seen rather than read from a table. The shape of the gap tells you something about the cause. A beam that is the right shape and place but too small all over is too dim, often from a dirty lens, an ageing source or low current. For an incandescent lamp, output falls much faster than the series current does, so a small current error can cause a large intensity shortfall; an LED’s output is much closer to proportional to its drive current. A beam of the right size that sits off the required area is shifted, which points to alignment.
A beam stretched in one direction leaves the sides or the top and bottom of the required area uncovered, which suggests a wrong or damaged optic. A colour failure shows on the chromaticity plot, with the measured point outside its limits even when the intensity contours are fine. Colour can also drift with the source: an incandescent lamp runs redder at low current, and an LED can shift with drive current, temperature or age. Because a distorted or shifted pattern can come from the test itself, alignment and the test set-up are checked before a fixture is condemned.
Frequently Asked Questions
Q1What is photometric failure visualization?
It is drawing the requirement over the measured isocandela diagram so you can see where the beam falls short. The shape of the shortfall, such as a uniformly small beam, a shifted beam or a narrow one, points to the likely cause.
Q2What does a 'too dim' failure look like?
The contours are all smaller than they should be, so the required area is not reached even though the beam is the right shape and in the right place. Common causes are a dirty or damaged lens, an ageing or wrong source, or a supply current that is too low.
Q3What does a shifted beam mean?
The contours are the right size but sit off the required area, so the peak is outside it and one side is uncovered. This often points to alignment: the fixture was mis-aimed in service or mounted off its reference axis on the test goniometer.
Q4What does a narrow or wide beam mean?
The contours are stretched in one direction, so the required area is uncovered at its sides or top and bottom. It suggests a wrong or damaged optic or reflector, or a fixture of the wrong type for the requirement. A beam that is too wide has the opposite problem: the same light is spread over a bigger area, so the peak and the values inside the required area can fall below the minimum.
Q5Can a fixture fail on colour but pass on intensity?
Yes. Colour is checked separately against chromaticity limits, so the intensity contours can cover the required area while the colour point sits outside its limits. A faded filter or the wrong source are typical causes. Incandescent lamps also shift colour with current, and LEDs with temperature, drive current or age.
Q6Is a failure always the fixture's fault?
No. A shifted or distorted pattern can come from the test itself, such as poor alignment, an unstable supply or stray light. Before a fixture is condemned, the alignment and set-up are checked and the questionable part of the test is repeated.
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