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    Re: Nav light colors and ranges
    From: Trevor Kenchington
    Date: 2003 Oct 15, 23:52 -0300

    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{at}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

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