🚀 go-pugleaf

   Well, it seems to glow pretty steadily, except that it does flicker
   enough to show 3600 rpm on my Royal Tach.

 Yes, they flicker. Those optical tachos are VERY sensitive to lighting
CHANGES, and they do NOT have any image processing to smooth the input
as we humans have. You probably all know some optical joke (the classic
is a fork with two, oh excuse me, three pins, no two it was...). Human
eye is not a very accurate instrument, at least not combined with human
brain, which tries to make something reasonable out of the incoming vision,
not to count flickers....


   Yes, a florescent light has noticable pulses.  Hold up your hand
   against a florescent light, looking at it through your fingers.  Wave
   your hand frantically.  Now, apologise to the person who thought you
   were being over friendly.  You may have noticed, though, before you
   embarrassed yourself, that you see a bunch of finger images.  That's
   the light pulses.

 You can do the very same test in front of your monitor, at least if it is
not one of those long-glow models, which are designed to accurate b/w
typing/typesetting for 8hrs/day. Another test you can do is to hold a spiked
wheel in front of you and then turn it. If you turn fast enough, at first
you do see through it (human brain again), then you see spinning blur which
changes direction (you may have any sort of mains lighting you like) as the
wheel slows down, then again until you can see the spikes clearly.


   |Whether they flash 60x per second or 120x per second I don't know.
   You can see 60 cycles/second as a noticeable flicker.  120 is high enough
   that your persistance of vision makes it look steady.


 They flash 100 (120 in 60Hz region) times a second. The voltage (and the
current in the same phase with a lamp, which is in fact a resistor) across
the lamp terminals varies from 0V to 310V (here in the 220VAC mains) and then
to -310V and again to 0V. This is the cycle which repeats 50 times (60 there
again) per second. You can see two voltage (and current) peaks if you draw
this on the paper in a neat sine wave. So, there are two flashes per cycle.

 BTW, thats why some monitors flicker LOT in the TV, they use the same refresh
rate as the mains cycle is, and if the phase difference gets right you`ll see
the image jumping on the screen in a way you know is unbearable to watch
for even short times.

--
Teemu Hakala     |     E-Mail to :     | 'So, there is no need to do any
Kasavuorentie 9B | teemu@krksun.krk.fi |  homework, mom, if there is no school'
02700 Kauniainen | Teemu.Hakala@krk.fi |   -- Calvin