Light Hum and Noise Detector
Électronique / Electronic

Did you know that the light you are using might not be a constant light. Fluorescent lights actually go ON an OFF 120 times per second. Even incandescent lights have some variation in light level at this same rate. Do you know what causes this, or why we don't see it with our eyes?

We all have something called "persistence of vision". That is, what we see tends to stay with us for a fraction of a second. Therefore, if a light is turned completely OFF for a fraction of a second, we can't see the difference. To us, the light would appear to be dimmer, but not to be going ON and OFF.

Persistence of vision allows us to use alternating current (AC) directly on our fluorescent and incandescent lights without trouble. There are some places where this cannot be tolerated. One such place is in the exciter lamp of a movie projector. This lamp must be powered by either DC or an AC current with a frequency above the audio range (ultrasonic).

An ultrasonic AC current is usually used in movie projectors. Can you hear the noise from the sun?