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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.

   

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