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Infrared detectors

Turboprops maintain a steady RPM and vary their prop blade pitch to
adjust thrust, so I could see a turboprop having a distinct frequency.
Less so for regular piston aircraft.

Prior to LED lighting being used outdoors, I think it would be safe to
assume any flickering light faster than a few hundred Hz would be prop
reflection. Nowadays it may be more complicated. Even street lights
these days are LED based.

Most LEDs are flashed, though the arguments seem endless if DC or
flashed LEDs are better. Supposedly the eye detects the peak intensity,
so you can PWM the LED for better effective illumination. But I have
seen arguments that DC is better.

I tested a number of LEDs to answer that. Basically, pulsing mostly
just increases i^2*r losses, with few exceptions.
 
M

miso

Jan 1, 1970
0
So, you've never camped out at -40°?

I have camped out at negative degrees..well in C. I use a lighter and
have matches as a backup. Contrary to what people say, you can use
butane lighters over 10kft. I use a trick I got off the internet. You
get the all season Primus stove gas and a Brunton fuel tool to fill the
lighter. People call it my crack lighter. It makes quite a stream with
the stove gas.
 
M

miso

Jan 1, 1970
0
The long tail of blackbody radiation is on the long-wavelength side of
the curve. The reason you can sometimes see a soldering in the near
infrared is because it's so hot, it's almost visible. Some are just
visible in the dark.

But the tail does go in both directions.

When you say visible, if you aren't talking the tail, then you are
talking the peak of the black body radiation. That is mighty hot.

I will admit I don't compute this stuff on a regular basis, but I found
a black body calculator.

Plug in 700nm (edge of the visible spectrum). Way more than you need to
melt solder.
 
M

Martin Brown

Jan 1, 1970
0
the brain detects peak intensity, the eye average intensity, if it's
flashing too fast to see you don't see the peak intensity.

Isn't it more along the lines that for a given power dissipation in the
device you can get something that is perceptually brighter with the
right combination of frequency and mark space ratio for given a power.
Most of them seem to use PWM brightness control now.

The eye registers movement or transitions locally as well as intensity.
This is particularly noticeable when an LED traffic light is in your
extreme peripheral vision if you rapidly shift your gaze towards it. You
see a series of images of the flashing light source sometimes.

Walls filled with TVs also in peripheral vision can be annoying in the
UK where cheap ones flash at 50Hz. 60Hz is better.
 
F

Fred Abse

Jan 1, 1970
0
Then again, it might be
difficult to persuade the cop plane to take off into heavy overcast.

Persuading the plane is easy. Persuading the pilot, not so easy;-)
 
J

josephkk

Jan 1, 1970
0
The long tail of blackbody radiation is on the long-wavelength side of
the curve. The reason you can sometimes see a soldering in the near
infrared is because it's so hot, it's almost visible. Some are just
visible in the dark.

Seeing a soldering in serious dark ain't bad. Seeing a flat iron some 200
degrees cooler is much more impressive. I could do that as a teen.

?-)
 
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