Throw and Aspherics?

[gcbryan]

I don't want to start a 50 page discussion of throw so I'd just like a few simple answers to simple questions.

These are practical questions related to practical projects. I've read most of the in-depth theoretical stuff.

Let's just deal with the XR-E R2 as the emitter so we aren't dealing with other emitter beam angles.

If you were building a dedicated thrower using a reflector you would drive the XR-E R2 as hard as you dared and therefore the emitter surface brightness is fixed. So the only thing left is a wider and deeper smooth reflector. Wider is the only thing that affects throw I think.

I understand why and just don't want to rehash that now. I also think that a deeper reflector while not affecting throw does make the center hotspot either bigger or overall brighter but not the focused center intensity.

Is this much right, yes, no, and why without a long background for your explanation please.

Now, with an aspheric the emitter facts don't change so for more throw you can increase...what?

The aspheric diameter just as with the reflector diameter or does it have more to do with shorter focal length to capture more of the light before it spreads or both?

Can someone briefly answer this?

Again, in the past the explanations has started at ground zero and I'm not looking for that here.

In a few sentences what are the variables that we have control of to increase throw with aspherics.

With reflectors it's just diameter and emitter brightness but I would like an accurate explanation of what exactly reflector depth is doing. I know that it is capturing more of the emitter beam but what is the exact effect of that?

Then with aspherics it's the same regarding the emitter part but the asperhic part is just diameter or focal length or what?

If you don't really know please don't jump in until after someone who really does know answers these simple questions then everyone else can take over the thread if you want to.

 

[paulr]

Deep reflector = less spill. Any reflector = light coming out of the emitter that doesn't actually hit the reflector, comes out as unfocused spill. Lenses are able to catch that light and focus it.

 

[Th232]

For an aspheric, longer focal length => tighter beam, better throw. The further away the aspheric is from the LED, the better the LED approximates a point source.

That said, as you've noted, for a given diameter aspheric the further the lens is from the LED, the less light it'll collect. Specifically, check the back focal length:diameter ratio (not the focal length!) to determine which aspheric will be better at gathering light. Other route is to get a secondary optic.

And (obviously), there's the issue of size. I highly doubt you'll want to use an aspheric lens that has a diameter or focal length of 200 mm in a "pocketable" light.

 

[gcbryan]

Deep reflector = less spill. Any reflector = light coming out of the emitter that doesn't actually hit the reflector, comes out as unfocused spill. Lenses are able to catch that light and focus it.

Thanks. I get that but less spill means more light in the hotspot and it means the hotspot is what...larger or more intense? I think hotspot size is just determined by the emitter size so it wouldn't be larger (I don't think) so it must be more intense. But if it were more intense you would think it would throw further. But I don't think a deeper reflector throws further.

So, if must not intensify the brightest most focused center of the hotspot but it must brighten the average intensity of the rest of the hotspot...is that how it works?

 

[gcbryan]

For an aspheric, longer focal length => tighter beam, better throw. The further away the aspheric is from the LED, the better the LED approximates a point source.

That said, as you've noted, for a given diameter aspheric the further the lens is from the LED, the less light it'll collect. Specifically, check the back focal length:diameter ratio (not the focal length!) to determine which aspheric will be better at gathering light. Other route is to get a secondary optic.

And (obviously), there's the issue of size. I highly doubt you'll want to use an aspheric lens that has a diameter or focal length of 200 mm in a "pocketable" light.

Is there some rule of thumb (is it linear?) that would give me some idea of how much the throw would increase with diameter increases from 30mm to 40mm to 50mm?

For the cheaper aspherics I'm using at the moment I don't really have specs for them other than diameter so it's just experiment to see what works.

With everything else being equal what would you expect the degree of magnitude to be between a flashlight using an aspheric in place of the front lens (assuming it focuses at that point) and varying the head size from 30mm to 40mm to 50mm ?

Just to put some numbers to this, using the 30 mm that I already have build it throws 200 feet effectively. It does more than that put to really use it effectively before the light starts to dim too much to be useful it's about 200 feet.

With that as a yardstick what would you expect from a 40mm and 50mm optic?

 

[Th232]

I can't say for sure, but I *think* that with an equal back focal length but an increased diameter, the increase would be more like that of a log curve, since the further off the axis you go, the lower the luminous intensity.

Of course, I could very well be wrong on that.

*Waits for an expert to correct me*

 

[Patriot]

Thanks. I get that but less spill means more light in the hotspot and it means the hotspot is what...larger or more intense? I think hotspot size is just determined by the emitter size so it wouldn't be larger (I don't think) so it must be more intense. But if it were more intense you would think it would throw further. But I don't think a deeper reflector throws further.

So, if must not intensify the brightest most focused center of the hotspot but it must brighten the average intensity of the rest of the hotspot...is that how it works?

We're using generic terms here so we can't say "deep reflector" without also specifying the type of reflector and it's width. For example, a 9" HID reflector is deeper than any LED reflector. So, we're speaking about ratio when it comes to the type of LED reflector subject that you're bringing up.

Less spill doesn't always equal more light in the hotspot but in practical terms it usually does. For example, if you had a reflector that was 4" deep and had an aperture of 1" much of the light would be absorbed or lost in the process of being redirected, therefore little or no increase with regards to the hotspot. If the reflector has been well matched to the source light and redirects in a parallel fashion, there will be an increase of throw with a deeper design. Someone with reflector design software could illustrate this but because there are an infinite amount of variables it's nearly impossible pin down in loose discussion.

 

[kengps]

I'm afraid you are searching for easy answers where there are none. Throw is a complicated compromise of many factors. Larger diameter reflectors don't neccessarily give more throw. A shorter focal length Aspheric doesn't neccessarily give more throw. Emitter transmission angle, light distribution within that angle, length from the emitter to the outer edge of the reflector (whether it be deep or wide). Curvature of the reflector, Texture, all make a difference. Compromises are made in many combinations to arrive at a conclusion. With an Aspheric...the focal length must match up to the various transmission factors of the emmitter. But given two aspheric lenses that collect the light in the same ratios, etc....the larger it is the better. The die image will be smaller and therefore more intense. Likewise two reflectors that have the same characteristics...the larger the better. Because the distance from the emitter to the outer edge of the reflector will produce a smaller image of the die and therefore be more focused. But again, there are many other factors that can change when you start increasing the size of the reflector/lens.

 

[kengps]

Now, with an aspheric the emitter facts don't change so for more throw you can increase...what?

The aspheric diameter just as with the reflector diameter or does it have more to do with shorter focal length to capture more of the light before it spreads or both?

Can someone briefly answer this?

PROVIDED the focal length matches the emitter parameters....the larger the diameter will give more throw. But consider that a shorter focal length could have the emmitter so close to the lens that only the very middle of the lens is being used. IT MUST MATCH...not too long, not too short. I know...I bought a big glass Aspheric lens, and it didn't work worth a crap. Lenses half its size out-throw it because the focal length was too short.