Re: What's the latest and greatest in LED headlamps?

Doug McLaren wrote:

Ryan Cousineau wrote:
|
|<http://www.cannondale.com/a_a/gear/08/FW/model-8lf01.html#gear_lights>
|
| It uses a 1W LED, presumably one of the new "super-LED" designs like the
| CREE XR-series.

Looks nice, but the math fails ...

3 AAA (1.5V-LR3) batteries included
1 Watt high power white LED
30 hours of burn time (steady mode)

The very best (and most expensive) AAA batteries I'm aware of have
about 1400 mAh of capacity.  Assuming that they stay at 1.5 volts
(which they don't, of course) and that we can extract all 1400 mAh of
that capacity 100% efficiently (which we can't!), that can only
provide 1 watt of power for 6.3 hours.  Take away my assumptions, and
the runtime is even smaller.

Obviously if the light is going to last 30 hours, the average power
would have to be around 0.2 watts.

So, either 1) the 30 hours figure comes from a greatly reduced power
setting or 2) you only get 1 watt for a short period of time (perhaps
two hours?) and then for the other 28 hours the power is greatly
reduced.

Most LED lights are designed to exploit the LED's increasing
efficiency as the voltage drops. Light output decreases
asymptotically over a very long period of time. While this means that
the wattage rating may not be accurate for most of the time the light
is operating, it also means that there is a very long window of
opportunity between noticeable dimming and dangerously reduced light
output, at least when compared to an incandescent light. I think it's
a fair tradeoff.

I also think it's fair for a light manufacturer to rate the light at
its initial power on fresh batteries, though without taking
measurements there is no good way to know whether the claim is based
in fact. Available LEDs vary wildly in lumens per watt, making
wattage relevant mostly as a gauge of runtime rather than
brightness.

It is almost always the case that claimed power of an LED light is a
reflection of the rating for the LED, not the power actually delivered
to it. Thus you can find "0.5 watt", "1 watt", and "3 watt" lights
aplenty, but no 2 watt lights-- because nobody rates their LED at 2
watts.

Chalo
.

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