On 02/07/2018 14:30, GB wrote:
On 02/07/2018 14:16, Robin wrote:
On 02/07/2018 12:59, GB wrote:
On 02/07/2018 11:08, NY wrote:
"GB" wrote in message
news
I was just looking at this head torch, which claims to output 6000
lumens.
https://www.amazon.co.uk/gp/product/B078NCZ6KC?
That exceeds the output from a 5 foot fluorescent tube, which seems
unlikely. I know LEDs are more efficient, but not that much more,
and the tube is consuming 60w.
What's going on? Lies?
Maybe because it has a reflector so the light is all directed one
way, whereas a fluorescent tube is all round (apart from where the
fitting is).
Lumens are defined as the light per unit of solid angle. So, focusing
the same amount of light into a tight beam does indeed increase the
lumens. Still...
I may be mistaken but my memory is that lumens are a measure of the
total output (flux).Â* The measure of the "brightness" of a focused
beam is it's luminous intensity, for which the unit is candela (lumens
per steradian).Â* So the ad is unsurprisingly ********.
I thought much the same as you, but checked itÂ* before I wrote the above.
Lumen = the SI unit of luminous flux, equal to the amount of light
emitted per second in a unit solid angle of one steradian from a uniform
source of one candela.
Yes, that's why I said it's counter-intuitive. Let's try starting with
the definition you quote:
"Lumen = the amount of light emitted per second in a unit solid angle of
one steradian from a uniform source of one candela"
4 Pi lumens = the amount of light emitted per second from a uniform
source of one candela
8 Pi lumens = the amount of light emitted per second from a uniform
source of 2 candela
etc.
Now suppose all the light from the sources is focused by a mirror so it
is emitted over a hemisphere. Has the total amount of light emitted
increased because it's now all emitted into 2 Pi sr? Or is it that the
intensity has increased?
But it may be clearer at
https://en.wikipedia.org/wiki/Luminous_intensity
Of course, you are right in practice, because you very rarely need to
illuminate a tiny spot with intense light whilst leaving everything else
in darkness.
--
Robin
reply-to address is (intended to be) valid