F
Peter Cole wrote:
> [email protected] wrote:
> >
> >
> > It's not obvious to me that LEDs would be mounted significantly more
> > accurately than a filament bulb.
>
> Incandescents have a fairly large filament, which makes them non-point
> sources, giving hot spots in the beam....
>
> LEDs are much closer to a point source. The actual radiating surface is
> tiny, usually a lens is built into the housing. White LEDs, like the
> high power ones in the better bike lights use phosphors (like
> fluorescents), so they have a somewhat larger radiating surface, but the
> radiation pattern is uniform and the surface is still small relative to
> incandescents.
>From what I've seen, LEDs are not, practically speaking, a point
source. Yes, deep inside the plastic housing, the emitting surface
itself is small, but ISTM the optics of the plastic are such that the
effective "source" is the plastic body. Certainly this is true with
the common white LEDs, where the white light output is dependent on
flourescing the compounds in the plastic. That plastic body is way
bigger than a point!
A typical halogen filament is a little wider (side to side) than a
typical LED, but is much smaller in the vertical direction. ISTM that
this would allow _more_ precise focusing, as long as the location of
the filament is adequately controlled.
Now I don't know if high output Luxeons are the same as other LEDs in
this regard. I don't own any, yet. But I suspect they are.
I notice that filament-based bike lights (and car headlights, etc.)
frequently have very sharp, well-defined beam shapes, characteristic of
very good optical control. Every LED-based light I've seen has lacked
the sharp cutoffs. To me, this is evidence that the LED isn't as
focusable.
> When riding with lights, you pick out much surface detail by the shadows
> cast. It isn't helpful when the light source casts its own shadows.
The question is, what level of nonuniformity is a problem? I know of
no evidence that any ordinary bike light has a level of nonuniformity
that rises beyond the "personal preference" level. And personal
preference is obviously a YMMV thing.
- Frank Krygowski
> [email protected] wrote:
> >
> >
> > It's not obvious to me that LEDs would be mounted significantly more
> > accurately than a filament bulb.
>
> Incandescents have a fairly large filament, which makes them non-point
> sources, giving hot spots in the beam....
>
> LEDs are much closer to a point source. The actual radiating surface is
> tiny, usually a lens is built into the housing. White LEDs, like the
> high power ones in the better bike lights use phosphors (like
> fluorescents), so they have a somewhat larger radiating surface, but the
> radiation pattern is uniform and the surface is still small relative to
> incandescents.
>From what I've seen, LEDs are not, practically speaking, a point
source. Yes, deep inside the plastic housing, the emitting surface
itself is small, but ISTM the optics of the plastic are such that the
effective "source" is the plastic body. Certainly this is true with
the common white LEDs, where the white light output is dependent on
flourescing the compounds in the plastic. That plastic body is way
bigger than a point!
A typical halogen filament is a little wider (side to side) than a
typical LED, but is much smaller in the vertical direction. ISTM that
this would allow _more_ precise focusing, as long as the location of
the filament is adequately controlled.
Now I don't know if high output Luxeons are the same as other LEDs in
this regard. I don't own any, yet. But I suspect they are.
I notice that filament-based bike lights (and car headlights, etc.)
frequently have very sharp, well-defined beam shapes, characteristic of
very good optical control. Every LED-based light I've seen has lacked
the sharp cutoffs. To me, this is evidence that the LED isn't as
focusable.
> When riding with lights, you pick out much surface detail by the shadows
> cast. It isn't helpful when the light source casts its own shadows.
The question is, what level of nonuniformity is a problem? I know of
no evidence that any ordinary bike light has a level of nonuniformity
that rises beyond the "personal preference" level. And personal
preference is obviously a YMMV thing.
- Frank Krygowski