NavList:

   

Reply

With respect, Jared, your quote from the ColRegs did not address the
assumption and expectation noted in my posting. Nor did mine, or the one
to which it responded, refer to LEDs.

Not sure about your explanation of a steradian either. From unaided (and
so unreliable) memory, it is the 3-dimensional angle swept out by
rotating a 2-dimensional angle of one radian. A nice unit for a formal
definition of a candela but not, I would think, very relevant to the
actual measurement of the brightness of a light.

But thank you for referring us to Section 8 of Annex I. That provides
the formula defining the number of candelas corresponding to any range
of visibility specified elsewhere in the ColRegs but it does not say how
the brightness, in candelas, is to be measured. Nevertheless, I think it
  implicitly supports my expectation that the measurement is to be made
outside the light fitting, since the formula makes no allowance for
light absorption in a coloured glass, which it should do if the
measurement was to be made of the bulb or filament itself.


Trevor Kenchington


Jared Sherman wrote:

> Trevor-
>  No expectations or assumptions are needed, regulations exist to ensure that. the US COLREGS state:
>
> "The lights prescribed in these Rules shall have an intensity
> as specified in Section 8 of Annex I to these Regulations so as to be
> visible at the following minimum ranges:"
>
> The regulation calls for each light, each complete fixture or fitting or 
lantern, however it may be, to have a visibility in terms of range. Since 
measuring brightness of a small object many miles away is an exercise in 
expensive instruments, the rules also state that visibility can be determined 
by measuring the brightness of the lights in candela, rather than going off 
with a measuring chain to see if you really can see the light from xx miles 
away from the boat on a clear night.
>
> Which in turn is why they use candela rather than candlepower.
>
> http://physics.nist.gov/cuu/Units/candela.html:
>
> "Originally, each country had its own, and rather poorly reproducible, unit of luminous intensity;...
>
>        In 1979, because of the experimental difficulties in realizing a 
Planck radiator... adopted a new definition of the candela:
>
> The candela is the luminous intensity, in a given direction, of a source 
that emits monochromatic radiation of frequency 540 x 1012 hertz and that has 
a radiant intensity in that direction of 1/683 watt per steradian. "
>
> The steradian of course will be instantly recognized by all list members. It 
allows for a detection instrument which is placed xx meters away from the 
light source with an specific area of the light source being read, i.e. if a 
cone of light from that light source is allowed to form a circular area on a 
sphere xx meters away from that light source, the brightness of that circular 
area can be tightly defined and measured, requiring nothing more than placing 
a special light meter next to the lamp which is to be measured.
>
> The difference between "candle power" and candela all coming down to the 
tightly specified conditions under which the latter is measured, versus the 
total lack of standards for the first.
>
> Since an LED opeates in a tight directional cone, which is determined by the 
shape of the lens "encasing" the LED junction, the candela of the LED are 
easily specified by the supplier and no reflector or lens issues are normally 
relevant. With a raw bulb, the reflector and lens become important. A fresnel 
lens can do a great deal to confine and seemingly boost the power of a plain 
tungsten bulb. But for an LED? The use of a concentrating lens, like a 
fresnel, would be a waste of money when an LED with a tighter lens (the 
plastic housing itself) could have been specified.
>
>


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
                      http://home.istar.ca/~gadus

   

Reply