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    Re: led lights
    From: George Huxtable
    Date: 2003 Nov 26, 14:41 +0000

    I welcome the contribution on LED lights from Ken James, who seems to know
    what he is talking about.
    
    I hope he won't mind if I cast a somewhat leary eye over his following
    assertion. Not that I disbelieve it, or think it to be impossible. Just
    that whenever anyone quotes "a well established fact", question-marks pop
    up in my sceptical brain. Have these observations been confirmed
    independently, and can Ken provide a reference? Were they made using the
    same type of LED emitter? Was the apparent brightness determined by
    human-eye assessment or by some instrument? Does the observation apply in
    circumstances (duty-cycle and period) in which the light APPEARS to the
    human eye to be steady and not pulsed (even though it IS pulsed) because
    otherwise the colreg requirements would not be met?
    
    Here's what he said-
    
    >There has been some question about intensity/brightness of pulsed LEDs. In
    >that regard, it is a well established fact that the phenomena known as
    >"flicker brightness enhancement" is real.
    >What it boils down to is that for the human eye, a periodic train of light
    >pulses of correct pulse duration and duty cycle (on-off %) can look
    >'brighter' than the same amount of light spread out uniformly over the same
    >time frame. The explanation seems to be that the eye does not have enough
    >time to tell the brain the light is off before it comes back on, and the
    >peak value gets taken for the average value, more or less.
    >
    >Now, does this mean that a LED can be made more 'intense' this way? NO!, in
    >most cases, the average "intensity" will be less than a continuously
    >operating led, so this may be where those folks who dis-regard the effect
    >may be coming from.
    >The advantage is that the pulsed LED will *LOOK* brighter "for the same
    >average power" as a continuously operating one, in other words, by pulsing
    >you can get a light that looks just as bright, or almost as bright, as a
    >continuously operating one that uses a lot more power (up to 80%
    >more...that's almost two months run time on one group 27 battery for an
    >anchor light that can be seen from at least two NM, instead of only one
    >month).
    >The catch is that it costs more money for this type of LED array 'driver',
    >and it may need to use more leds, due to needing to spread the thermal load
    >(at higher peak values) out over more leds, so it is a somewhat more
    >expensive design.
    
    =============
    
    If indeed the above assertion of 80% longer battery life is confirmed,
    which would imply a reduction of the average current drawn, for the same
    apparent brightness, to 55% of the current drawn in steady-state
    conditions, than such a pulsed design would be well worthwhile.
    
    Ken added-
    
    >As far as adequate intensity of LEDS for the application of boat navigation
    >lights, LED intensity has improved a lot recently. There are now five watt
    >LEDs (five watts for ONE LED...) that each one are as bright as a 60-80 watt
    >incandescent bulb...no joke!
    
    That's new, and interesting, information. Does it refer to mean power,
    rather than the peak power level achievable in a short pulse?
    
    
    >One thing in this regard NOT mentioned is color sector coverage and
    >cut-off...some LED lights are designed only for power boats, they do not
    >have enough vertical coverage, and there are some out there of questionable
    >color sector separation...it is not enough to use bare 'lamp' LEDs in a
    >fixture with a screen or filter, or even a vertical filter on the
    >array...neither scheme will  provide the required sector separation. You
    >must use some type of optics installed on the array itself (or a custom
    >designed LED, an animal which does not exist!).
    >For example, without proper separation, there will be a 'zone of confusion'
    >where you will look like a somewhat orangeish-white light for as much as
    >thirty degrees from dead ahead with leds in a bow combo light, way more than
    >the regulations permit.
    
    That question of sharpness of cutoff was touched on in the correspondence,
    and Ken is confirming here some of those suspicions that were expressed. Is
    Ken claiming that those problems, of sufficient light output at every
    azimuth and elevation within the defined sector (especially for sailing
    craft) and sufficiently sharp cutoff outside it, have now been resolved?
    Can he supply polar-diagram figures to convince us? Has any LED nav-light
    (bow or masthead) been given any sort of type-approval by any regulating
    authority? We small-boat sailors are living in hope, and as soon as the
    answer is "yes", we will be beating a path to the door of the supplier.
    
    Ken's mention of "a custom designed LED, an animal which does not exist" is
    intriguing. If a single LED is now capable of supplying enough light, there
    may be a big enough market to encapsulate it in an a special lens-moulding
    designed to meet the colregs requirement. And then that single light would
    be nearly a point-source, in which case only a small screen, in a
    reasonably-sized housing, would be sufficient to obstruct aany last vestige
    of crossover light across the bow.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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