Disco Lights?!

[iainmac]

May seem like a stupid question but why do the lights on GTi's in videos seem to be flickering like a strobe?  I've seen this a few times and can't figure out what it's all about!  See the below clip from 9:14 onwards.  Even the back ligts are at it!

http://www.youtube.com/watch?v=iIuqC1y6jag&NR=1&feature=endscreen

[Bill_the_Bear]

The LEDs are pulsing and the frequency of the camera creates this effect.  You'll not see it with the naked eye (if you're not moving anyway).

On why do they pulse?  They use a lot less power so it could be to do with that.  Perhaps more likely they have calibrated the pulse width to give a specific brightness.  The pulse width could even be actively changed over the lifetime of the LED to ensure a constant brightness, you just have to measure the half-life and do some simple maths.

[Sootchucker]

As Bill said, its just a modulation that the human eye doesn't pick up on but a digital camera shooting at 24/30frames per second can.

The same actually happens with florescent lights in a typical office which not only pulse, but actually change colour many times a second. We can't see it but a camera can.

Nothing to worry about.

[Bill_the_Bear]

Used to be common to see this with computer monitors also.  For example, news programs would often interview people in an office or in front of some displays and you'd see dark bands moving up or down the screens, or a strobe like flashing in the background.  If you imagine the pixels in the display are being rapidly turned on and off then what would happen is that periodically the pixels would be dark during one or more of the frames the camera is taking.

There are actually some weirder effects you can get.  A DLP can give an effect where a moving object (say a football) appears to break apart in flight due to delays between the projection of different colours.  Similarly low refresh rate LCD displays will often appear to blur moving objects making them terrible for watching most sports.

These kinds of artefacts are due to limitations in the generation of the image your eye is observing.  A real object is constantly reflecting light but display technology is essentially constantly redrawing an image very fast, fast enough that it should be interpreted by your eye as constant.  This works quite well for CRT displays because the CRT draw method is basically the exact reverse of the camera capture method, so the reproduction is quite good.  Things got tricky when people decided they wanted large and high resolution displays in a small form factor.  LCDs and projectors use a huge amount of software trickery to convince the eye to interpret the images correctly and understanding the operation of the eye is key for display designers.  I find it really fascinating (my previous job was developing new display technology).

[pauliem]

If you look at an old CRT with a low refresh rate at a 90 degree angle, so your ear is facing the screen, you can see it flickering in your peripheral vision.

Your eyes are least sensitive to the flicker at the centre of your viewing point.  At the edges it is most sensitive.

[Bill_the_Bear]

On a philosophical note this leads to the question "what is reality?"  Since what you see with your eyes and is subsequently processed by your brain is nothing like "reality" as it really is, so which is real?  Is the world before observation more real or is the world as exists in your brain more real?   For example, now the camera has revealed that in "reality" all those cars you drive past and all those displays you use are really pulsing and flickering is this real, or is the illusion we've created in our brains real?

Colour reproduction by the eye is really false.  What we call "red", "green" and "blue" frequencies are detected to different degrees.  In addition the eye operates with a logarithmic response to intensity.  These facts are absolutely crucial in enabling the eye to see both light and dark objects of varying colours.  But in "reality" when you see a bright object next to a 'slightly' less bright object the number of photons actually being observed is not 'slightly' less... it is massively less!  If you saw things as they really are then bright things in your field of view would be dazzling and everything else would be in a dark gloom where all shadows look equally deep.  The eye is a phenomenal piece of kit, no camera system will ever come close to its capabilities.

[pauliem]

Both the interpretation of reality and the reality itself are real.

Just because your eyes and brain make an interpretation of what you are seeing, does not make those things any less real, or your perception of them.

A different set of eyes will see a different vision, furthermore another set of eyes from a different species will see something dramatically different.  But both are still valid.

One thing is for sure - my Golf GTI is not real yet, it's not a perception - it is just plain fact.  It does not exist.  Which is really annoying because if I could see that beautiful Oryx Paint with my eyes in the dazzling sunlight that is shining today, I would surely perceive it to be a beautiful thing.

[Bill_the_Bear]

One thing is for sure - my Golf GTI is not real yet

Heh heh