Re: i got trapped in Wales because

On 2007-07-09, Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:
Whiskers <catwheezel@xxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:
Whiskers <catwheezel@xxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:
Whiskers <catwheezel@xxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:
Whiskers <catwheezel@xxxxxxxxxxxxx> wrote:

[...]

The 'white' LEDs currently in use that I've seen (and I own a couple of
torches that use them reasonably effectively) rely on the LED itsel
emitting at a blue/UV frequency (some visible blue) which is used to
excite a 'white' phosphor.

Righto. That's what I've gathered is the `usual up to date sort'.

They aren't as fuel-efficient as the simple
'one frequency' LEDs. The 'three colour matrix' idea is hampered by the
different amounts of light you get from LEDs tuned to different
frequencies, and the human eye's different sensitivities at different
frequencies, makes it difficult to get a real 'white'.

Umm. Ish, ish. A three colour matrix is how you get white on a monitor
screen - be it CRT, or LCD. It's tricky when you've got narrow band
emitters to do a good job with only three bands, mind - last time I
checked, the `high efficiency' light bulbs here had four or five bands
(look, it's daylight, I can't check easily now, but you can do it by
looking at the refraction pattern cast off a CD).

Yes. The 'high efficiency bulb' in my reading lamp has three distinct
colours: a strong red, a green, and a weak 'deep purple'. It gives a
'warm' tone generally, but is hopeless for any printed document whose inks
or dyes don't happen to match the frequencies from the lamp. The main
room lamp is a different make and has a green that is stronger than the
red, and a very weak (barely visible) 'violet' - even worse for seeing
coloured printing.

The LED lamp that runs off my laptop's USB power, has a continuous
spectrum as far as my eyes can tell in the grooves of a CD, but the blue
end of that spectrum is much stronger than either of the 'high efficiency
bulbs', or of a 'traditional' incancescent bulb or the overcast sky
outside my window. I find that the 'white' phosphor-type LED light is
almost as good as the old blue-coated 'daylight simulation' incandescent
bulbs for such activities as assessing colour photographs or other
colour-related tasks.

'Liquid crystal' computer and TV displays are usually back-lit by a
fluorescent lamp, which presumably has a 'continous spectrum' that is
filtered by the different coloured dyes in the individual 'dots'.

CRT dispays use phosphors tuned to three different 'colours' and a mask to
make sure that each colour of phosphor is activited only by the 'cathode
ray' related to that colour from the original signal.

Colour photography (chemical and 'digital') relies on the use of dyes in
precisely-defined colours known as 'red' 'green' and 'blue' ('additive'
colours, for 'slides' and displays such as TV or back-lit LCD) or
'magenta' 'yellow' and 'cyan' for printing etc ('subtractive') - usually
with the addition of 'black', in practice.

[...]

Now, when one does that, so I'm told, he is inclined to `blank' the
scene until his eyes slow down again. I have trained myself to not do
that (bike riding - I need to see!) - and I didn't do it that day.

Ah. Well most people probably don't even think of trying that let alone
actually train themselves to.

I don't think I really thought about doing it particularly - I just had
a need to see things, so I made sure I could. I'd expect that a lot of
people do the same thing - anyone who's got a need to `keep track of
things when everything's moving like billy-oh'. Sports, motor racing
perhaps, that sort of thing.

Hmmm. I'd say that selective attention based on experience, would work at
least as well. When experiments are done to discover 'where people look'
(using gadgets that superimpose the person's 'gaze' onto an image of the
scene in front of them), the 'selective attention' is clear to see;
whatever 'catches the eye' gets a lot more 'gaze' than the rest of
the scene, which gets cursory attention at most. So 'good' drivers can be
shown to direct their attention at potential hazards very quickly, whereas
'bad' drivers (who have a record of 'accidents') seem to let their
attention wander about all over the place indiscriminately or
inappropriately.

I do see the
'multiple narrow beam' effect from some vehicle rear lights, but not a
flicker. I doubt if the AC current driving those lamps is coming directly
from the 'alternator' driven by the engine; that would be far too slow and
erratic a frequency.

Certainly.

There must be an electronic circuit giving a
relatively smooth output frequency for the LEDs

Yeah, they'll have a fast chopper circuit somewhere, probably built into
the LED lighting unit itself. Such things are dead easy and dirt cheap.

- and I think it would
have to be a 'rectified square-wave DC' rather than AC, wouldn't it?

I'd expect so. Still, I wish I knew why they don't run 'em off a steady
current.

<http://www2.whidbey.net/opto/LEDFAQ/The%20LED%20FAQ%20Pages.html#Q21> -
the Agilent .pdf linked from there goes into the gory details (which I
don't pretend to understand beyond a very broad grasp). In summary, for
peak efficiency you should have a pulsed supply.

Erm?

`1) If you are trying to operate visible LEDs at maximum possible
brightness levels, you will always get better results using DC
operation. This is because luminous efficiency declines at high
currents.'

Pulsed DC; not a steady current. To let the diodes loose heat - and I
suspect also to let the holes and elctrons sort themselves out into the
'stasis' condition so that there are as many electrons jumping orbits as
possible for the next pulse - the jumping electrons being what give off
the light.

And other stuff. It's clearly complicated. I suspect it's got most to
do with:

`1) LEDs have more output at lower temperatures. Low duty factor pulsing
can, in some cases, lower the operating temperature of the LED.

2) The human visual system is non-linear. With the right choice of duty
factor and pulse rate, perception will correspond more to the peak
brightness than to the average brightness. This is especially true at
low pulse repetition rates. However, there are a couple of potential
problems with using low rep rates:'

I'm not sure I quite get it, either way. You've got to use high
repetition rates to get a decent light

Yes. Glowing wires are a lot easier to understand, aren't they? :))

and thus move your eye through what is effectively an array of
narrow beams rather than a solid beam such as you'd get from a
single incandescent bulb.

What it is is that you scan the beam across your retina and as you do
so, the signal flickers on and off and I *REALLY* notice the effect:
I see this alarming disorienting flicker.

Doesn't matter whether I'm on my bike or on foot on in a car: it's all
to do with eyeball movement.

This could be a physical difference between your eyes and mine.

I suspect that might be part of it - I've got pretty good resolving
power - but I suspect it's more down to me training myself to keep
looking when my eyes are moving around at high speed. I did that so I
could *see* properly when - I dunno, I really don't. Umm. I really
have no idea when I started doing it - not a clue. Must predate
motorcycling. I think it might even predate me finding out that this
`blanking' business is fairly normal. I just wanted to be able to see
properly, you know? So I made sure I could.

I don't think that's 'mormal' ;))

I think that both the blanking and what I do are normal - that is, while
`most folks' do the blanking, I reckon anyone who's got a need to
`unblank' (see below) can do so and usually does do so. You've just got
to pay attention.

It's highly likely that what I *THINK* I'm doing isn't what I'm reall
doing - if I were studied, it might well be found that my eyes do an
awful lot of very rapid stepping from one stationary position to
another. I don't think I could tell myself.

Perhaps you could get involved in a 'visual peception' experiment some
time, to find out.

I don't
know which is 'normal' - but I would expect the vehicle lighting
regulations to require that any lamp not meant to be 'flashing' will
appear 'steady' to at least the great majority of humans.

Unfortunately, they don't seem to have done the tests. And there are
plenty of people who think that high frequency flashing cannot be
detected by the human eye. Well, not if it's shining on a steady patch
of retina, it can't, but you try moving around...

They would
probably also take into account any well-known 'trigger' frequencies that
might set off a migraine or epileptic episode.

Oh yeah, but those frequencies are all pretty low, aren't they?

See the 'British Epilepsy Association Photosensitivity Info' link on the
whidbey.net URL above. "Most people with photosensitive epilepsy are
sensitive to flickering around 16-25Hz, although some people may be
sensitive to rates as low as 3Hz and as high as 60Hz."

Yeah, that's about what I thought.

"Bicycle lights
(red light emitting diodes) There have been cases where red flashing
lights (used on the rear of bicycles) have triggered seizures in people
with photosensitive epilepsy. This happened when the people were close to
the lights as they were setting them up."

Uhuh. The sight of white lines flashing past on the road at night can
do similar things.

Sunlight through tree branches is disturbing while travelling in a car.

[snip]

I wonder if your 'persistance of vision' is close to the 'short' end of
the human range?

I suspect it's got nothing to do with that - not exactly.

But I could be wrong - I nearly got killed one day because my brain
didn't pick up a car coming up behind me when I glanced in the mirror.
I was scared stiff and pulled over and did some trials: I didn't want to
go any futher until I had the problem well characterised, whatever the
hell it was.

It seems that the `length of time it took to resolve the image in my
brain after getting it to hit my eye' was much greater than normal -
certainly, I simply couldn't see anything at all until I'd looked in the
mirror for a lot longer than usual.

I had recently been prescribed beta-blockers for anxiety. The side
effect I met is not one I was warned about, and it's not listed in the
books either. But I stopped taking the drugs and my sight returned to
normal.

That sounds like brain rather than eye as such;

Is there a dividing line, really?

'Data parsing' rather than 'data gathering'.

the effect may be to give
you a 'shorter than normal persistence of vision' anyway.

I got the idea that something was running at slug speed.

This could be
an interesting area for a research physiologist to get into.

Yep.

[snip]

btw, the big DLP/DMMA projector rigs they use in cinemas for that sort
of thing have one mirror array for each of the three primary colours;
you can't get colour separation due to time effects, but you can get
mis-aligned optics causing steady colour fringing if it's not set up
right.

Digital projectors are still in their early development.

No they're not! They were quite highly developed back then in the early
1990s: 1993 was when I saw 'em (IIRC), and TI said they'd been working
on them for a decade by then.

The problem was getting the actual mirror array sorted out - sticky
mirrors (van der Waals forces, can you believe it?) were one problem. I
expect there were others - but at the time, they couldn't manufacture
the mirror arrays reliably enough to sell 'em.

That may be rather like saying that the propeller on a Hurricane is more
efficient than the propeller on a Bleriot - but propellers are not the way
to go ...

Erm, but they are the way to go - ever heard of turbofan and turboprop
engines? Fan, propellor, what's the difference? They're more efficient
than pure jet engines, if you're in a position to use the fan/prop
propulsion mode.

Anyway, thing about it, is that it's just engineering: the basic idea is
a bloody good one for all sorts of reasons, and they've just got to sort
out the details.

They got LCD monitors sorted out eventually - and LCD monitors are
*stupid*. <shrug> And they got the DLP/DMMA rig sorted eventually,
which is how come there are so many on sale.

How long before we are offered electrode implants to 'go to the next
level' in entertainment?

But the rest of it -
well, that's very simple optics. Logie Baird could have done that side
of the job perfectly well.

He didn't have LEDs, but his rotating wheel system was being developed to
produce colour images using mirrors, I think. Clever, and it works, but a
dead end of the sort that happens in the early stages of any technology
;))

Calling it a dead end is a bit much, I reckon. He had 600 line colour
telly back in the 1930s (or was it late 1920s? The answer is in my
filing cabinet, but I don't know where) - the chap from Wireless World
reckoned the colour quality matched that you could see in cinemas of the
time.

None of the purely electronic televisions of the time could get anywhere
near that - not the colour, not the resolution.

If he'd had someone working for him who understood electronics and (most
especially) wireless, I reckon he'd've got a lot further along. But he
didn't, and then all his gear got destroyed in a fire. A lot of his
methods were potty (which is why he needed people working with him), but
that's because he was a mechanical tinkerer who was bloody good at
optics and ideas but lousy at almost everything else.

Anyway, I saw the potential of mechanical colour telly as soon as I read
about it - it's much more sensible than CRTs or LCDs if you ask me, and
now that TI has come up with a sensible modern way of doing it, well,
I'm all in favour. Mind you, mechanical telly is always going to be a
projection technology, I reckon, so I don't see any obvious way to
replace a computer's monitor with any such arrangement. But I think I'd
like a flat panel CRT - you know that sort with `one triple gun per
pixel' they're developing? Damned if I know anything about the details,
mind.

That is a very promising line of development - which could do away with
many of the 'projector' installations where the imaging device isn't
required to be portable.

The 'colour' aspect has improved with 'white light' LEDs having
been invented and the tuning of output colours is getting pretty
good now. There are certainly LEDs being used in car etc rear lamps
and indicators now so presumably the 'flicker' has been accepted in
the current regulations

Yeah, but I'm not happy with the car gear. I've never noticed
`flicker' from bike lights set to `steady'.

The ones used on pedal-cycles (and in hand lamps nad torches) may operate
at a higher frequency of 'flicker' - just enough quicker for your short
persistence of vision to get the continupus effect?

I suspect it's more that I look at them closer


I meant `from further away'. Dur.

I'm glad you aren't tail-gating innocent cyclists :))

- when the light's
further away (think night-time motorway work), you can get a much faster
scan of the beam across your retina than you'll get with the bike light
50 yards ahead.

As my eyes and brain are not doing that, I'll have to take your word for
it.

Very strange to read about completely different sensory experiences from
identical input, isn't it?

A bit scary, actually.

[snip]

Oh, please stop it blinking! That's not at all helpful, it really isn't
- makes you harder to spot, I promise.

That too is not my experience; the blinking LEDs draw my eye towards the
lamp, making the cyclist a lot more 'visible' to me.

Yes, but you're not steaming up behind at 60mph, are you? The blinking
LEDs mean that I see unrelated blinks of light at unrelated places in
front of me, and cannot get any idea what the hell is going on - is that
one or multiple light sources, what are they, where are they, where are
they going? And very often, the light will be out when I glance in the
bike's direction so I won't see anything at all!

Please please please stop it blinking!

I do drive a car that can do the ton (just, and of course I never do,
officer)

It's perfectly legal - just not on the public highway on the mainland of
GB, as they say. Isle of Man? Autobahn? `Run wot you brung' track
day?

and I do sometimes encounter pedestrians and cyclists using
flashing LED lamps as an aid to their being seen - and it works as
described for me, whatever speed I'm doing. I don't seem to have any
difficulty recognising them for what they are.

I have trouble figuring out where they're going to be when I pass 'em.

Strange. I seldom have that difficulty, as long as I don't try to
double-guess what they might do next.

[...]

I find it almost impossible to work out where a flashing light is going
to go next - I have to stare until I can see the bike and rider.

That is very difficult for me to imagine; there seems to be work here for
psychologists and physiologists to argue about for a long time.

As you might have gathered, I *REALLY* hate bicycles with flashing
lights. They're trouble, they're bloody dangerous, and the riders need
educating.

I agree that they shouldn't rely solely on the flashing LED lamps.

I don't approve of cyclists relying solely on a flashing LED rear light
(or front light); there should be at least one 'steady' light to the rear
and another to the front. I suspect (hope) that anything less is still as
illegal as ever. But the flashing lights add information and draw
attention (which is exactly why they are used by emergency vehicles, of
course).

Flashing lights are a problem for me when used by emergency vehicles. I
find them disconcerting and they don't give me any idea where they're
coming from - they're nothing like as good at attracting attention as
the old fashioned moving lights they used to use.

I prefer them; the rotating or oscillating beams were very confusing,
especially when they started to bounce around inside my spectacle lenses.

I wonder if the current on/off flashing lights would be easier for you to
cope with if they emulated the gradual dimming and brightening that the
rotating reflectors produce?

The modern electronic
sirens have the same trouble compared to the old wide-band klaxons: you
just can't hear where they're coming from.

I think everyone has that problem. For a while the emergency vehicles in
London used sirens that included a lot of 'white noise' and 'chirruping' as
well as the 'siren' sounds; they were easy to locate in direction and
movement even if you couldn't see the flashing lights, but for some reason
they've stopped doing it.

I think they use 'em partly because they're flashier than the old kit,
you know? They're certainly less effective at their intended task, for
all that the modern kit is cheaper to buy and maintain.

Accountants rule the world these days.

(on the other hand, when it's heavy strobe lights and the music's
pounding and the smoke is thick, I just love it - when my whole field of
vision is in that state, I can put the pieces together and figure out
what's going on while all around me people are seriously disconcerted.
Unfortunately, what I like in that line isn't legal any more - they
daren't run strobes in nightclubs in the way they did back when I was
born, and haven't done since I became old enough to enter nightclubs
myself).

I'd get myself as far away from loud noises and bright lights as possible.
<shudder>

Moving lights, on the other hand - that's the badger! They use flashing
lights on emergency vehicles because flashing lights are `said to give
the same effect' as moving lights, but cheaper and more conveniently.
They don't.

Well, not quite, that's true.

Pedestrians move slowly enough to be even less confusing when
carrying flashing LEDs - to me.

btw, regarding the `flickering/steady' light lark - all fluorescent
ilghts flicker, and I've never heard of a gas discharge lamp being run
off anything but AC.

Battery-operated fluorescent tubes work very well - they're more efficient
than LEDs. I've never poked around inside mine (I have one fluorescent
hand-lamp dating from the '60s, running off two 6-volt lantern batteries;
you can get smaller ones now that run off torch batteries), but there must
be a sort of 'invertor' to get AC from the DC supply, if indeed AC is a
requirement - that isn't actually clear from this article:
<http://home.howstuffworks.com/fluorescent-lamp.htm> (they assume mains
AC; they say nothing about why a discharge lamp would /require/ AC).

So that means that many car headlights flicker -
but at frequencies that we can't see. I feel this has something to do
with the legality of bicycle lamps - I mean, what's good enough for cars
has to be good enough for bikes, right?

An incandescent bulb flickers if driven by AC; the main difference between
filament and discharge lamps is that the filament never 'goes out' between
flickers, whereas the discharge lamps (and LEDs, usually) do.

Most modern motor vehicles seem to have 'alternators' rather than
'dynamos' so there is certainly AC current available while the engine is
running, but the batteries need and produce only DC so I'd expect the
lighting circuits to be all DC all the time - but when the power is coming
mostly from the alternator it will have a 'pulse' depending in the engine
speed. Dynamos 'pulse' too, of course, but in a 'softer' wave-form I
think. I don't think any vehicle uses discharge lights for 'legal'
lighting purposes - too fragile, for a start.

My first moped had no battery, using a 'magneto' instead; that did have AC
lighting - which flickered noticeably at 'tickover'.

[snip]

We
seriously used plastic macs over old blazers or woolly jumpers,

And why not?

and
'school shirts' with collars - although we didn't often wear a tie while
hiking.

Shirts, okay; but ties? Come on, did anyone *ever* wear a tie out
hiking? You'll tell me now that they did, won't you?

Not seriously, but possibly for a joke.

<whew>

I suspect that Edwardian gents
with their own valets would have worn a tie or cravat even when 'walking'.

Hmm - a tie can be awkward if you're having to clamber around; you can
easily throttle yourself a bit. Cravat, maybe.

Poly-cotton trousers were generally available, although some
chaps still stuck with cotton or 'flannel' or 'cavalry twill' - hence their
preference for shorts, I suspect.

Woollen trousers are best. I had a pair of `old man's trousers' that my
mother bought from the charity shop and cut down into plus fours for me.
I do recall the looks I attracted from my peers when I turned up in that
kind of gear - you could see them starting to frame a piss-take, and
then you'd see them think `Ah, erm, yes, he's going to be a lot better
off wearing those, erm, yes, *woollen* trousers that stop just below the
knee and well above soggy undergrowth height and okay, he'll just be
smug, won't he?'

Unfortunately, while I'm not a lot taller now than I was when they were
converted for me, I am quite a bit thicker around the middle.

In my experience, British army poly-cotton works best; it dries quickly.

Horrible stuff as far as I'm concerned - wet polycotton against the
skin? Blech - and it tugs and pulls and provides bugger all protection
generally. Wool might not dry out that quickly, but it doesn't have to:
keeps you warm when wet. And it's great protection against all that
greenery that stabs you or tries to flay the flesh from your legs when
wearing lesser clothing. *AND* it's a bit more use when things are on
fire. I like woollen clothing from the practical point of view.

Oh yeah, once you've got plus fours tied neatly just below the knee,
you've got quite amazing freedom of movement - no tugging at the knee,
no matter what.

[snip]

.... and a bit less draughty than a kilt; but I'd expect less chafing from a
kilt.

--
-- ^^^^^^^^^^
-- Whiskers
-- ~~~~~~~~~~
.

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