DX p7 reflectors

[danneR]

Hi there. I will start riding my mtb when it dark. So I have designed a Li-Ion pack, bought a DX p7 light, bought a buck driver... and all rocks!

Now my questions: I have a smaller DX light with 38mm diameter of the reflector. But there are 55mm reflectors as well. Are there any difference between the light with the same reflector diameter?

What DX light do you recommend for riding mountain bike in the dark?

I have this one http://www.dealextreme.com/details.dx/sku.22567, but I am not sure if this is the best alternative...

Anyone that can share their expericenses?

Cheers Daniel

 

[yellow]

usually the bigger the reflector, the more throw You get
which means the opposite to what one likes with bike lights

as to the light:
with the external pack and other driver, You get good service.
Of course the DX bike light pack (the Tesla clone) is a "complete" bike light setup
... but You have modded everything to be ideal, no real need to change, as long as You dont think You have to ...

 

[bstrickler]

usually the bigger the reflector, the more throw You get
which means the opposite to what one likes with bike lights

Actually, its more that the deeper the reflector, the more throw you get. The wider, or shallower it is (lower width-depth ratio), the more flood you get.

~Brian

 

[danneR]

Have you tested any DX lights?

Cheers DR

 

[a1penguin]

I have this same light on my handle bars. It's a lot brighter than a 7 year old 10W planet bike light it replaced. I also have a 15335 on my helmet, which has very good throw. All of the 900 lumen DX lights that take 1x18650 seem to be floody. The 230-250 lumen Q5 lights have more throw. I can see well enough with just the 15335, but the floodier P7 makes me more visible to cars. Two PBSF on the rear.

 

[jankj]

Actually, its more that the deeper the reflector, the more throw you get. The wider, or shallower it is (lower width-depth ratio), the more flood you get.

That is assuming that the reflector curve is optimized for throw. It could be optimized for a wider beam.... But that is hairsplitting, you are correct. But you still want large reflectors for good throw.


For maximum throw, the LED is placed in the focal point of the reflector. Ideally, you should have a point source here, but the LED is a bright area, not a point. You loose some throw due to the mismatch between the ideal point source and the real world light emitting surface area. The larger the reflector, the more the LED "looks like" a point source and the less of a mismatch. Also, a large reflector is less sensitive to small errors in mounting the LED in the focal point. So, all other things being equal, a large reflector will beat a small reflector for throw.

With the large multi-die (P7, MC-E) this effect is even more pronounced because the emitter is so much larger, plus you have the fact that the focal point typically will be in the center of the LED, where the P7 / MC-E has a dark spot (the cross separating the 4 individual dies). Thus, the part in the optimum throw position emits no light at all... Thus, (almost) all P7/MC-E lights designed for maximum throw has a dark spot in the middle of the beam. Typically, you reduce the throw a little to get rid of this spot.


Perhaps the characteristics of P7 can be used to create an asymmetric beam...? Place the focal point in the middle of one of the 4 dies, not in the dark cross...?