For the engineering types - Why are good lights large?

[me3@neuralfibre.com]

It's got me buggered.

I know it to the true. The bigger the reflector, the better they perform. Narva Big Bulls are huge - cheap and perform really well.
Small expensive lights often perform poorly.

Why is it so?

I've googled myself out, and I can't figure the diff between a small reflector and a large one.

Any ideas?

Paul

[PigDog]

larger reflectiv area reflects more light over a bigger area thats the way i c it

[Guy]

same reason a larger mirror on a telescope works better .. but opposite way around .. (try looking up stuff about newtonian telescopes and think in reverse)

[bogged]

I've googled myself out, and I can't figure the diff between a small reflector and a large one

I'll give it a go.

Well the diffrence is the small ones cant fit a large reflector in them..
HTH
YMMV.

[chimpboy]

I think it's because the light source is always only approximately at the focal point of the mirror, but it is less approximate (proportionally) with a large mirror than a small one.

(edit: there'd also be marginally more %age loss due to hardware that's in the way)

It's an interesting question, to me it seems different to just reversing the concept with a telescope because with a telescope, a larger reflector physically collects more light. With a spotlight the mirror is catching 100% of the light either way.

I would be interested in a serious answer if you find one. Have you visited the candlepower forum? Those guys are right into it.

[Struth]

A smaller reflector in a full semi circle has less reflective surface area to reflect/refract light with than a larger reflector in a semi circle does, coupled with this will be the amount of shaping/refaraction area built into the reflector or the lens, which again varies with the size of the light.


A small mirror will reflect it's own surface area of the available light source, a large mirror will do the same.

So to try and simplify, if you stick a 1" square mirror to the wall at home and aim a torch at it, the torch may well light up 2 sq Ft of the wall, but only 1" square of light will be reflected by the mirror.

Do the same with a 10" square mirror and 10" of the available light will be reflected.

Now apply that to the hemispherical light reflector, the bulb will radiate light almost 360 degrees and the smaller reflector may only reflect 100 degrees of that light forwards, whereby the larger reflector allows the light source to be deeper in the reflector and may allow 180 degrees or even 270 degrees of the available light to be utilised and reflected forward or outward in the case of a spread beam light.

No I am not a light engineer, simply think on these things

[macca81]

struth, if 2 reflectors, one 5" across and one 10" across, were both designed so that a given bulb has say 75% of its light output hit the reflector, would they both still give off an equally good light output? (assuming that lense is irrelevent)

[chimpboy]

I thought they were parabolic mirrors, not hemispherical. If they are hemispherical they are cheap junk.



Hence light from the focal point shines out (virtually) parallel. So in that sense the size should make no difference to how many degrees of light are collected and sent forward. I think this is what Paul is getting at - in pure physics theory a tiny mirror or a giant mirror should both throw exactly the same amount of light forward as long as they are the same shape and the light source is at the focal point.

Flat mirrors are a completely different kettle of fish.

It's an interesting question.

[Dirty]

I am betting that there is going to be minimal difference between the sizes and it more comes down the the quality of the reflector.

But I do know a light engineer (that actually also had some part in the lightforce lights) and I will ask him next time I see him.

- David.

[me3@neuralfibre.com]

I think it's because the light source is always only approximately at the focal point of the mirror, but it is less approximate (proportionally) with a large mirror than a small one.

I've been thinking similar Chimp.

Along with - if you read up on HID they talk about filament shape.
How do you project an even glow from an uneven light source. You need to "unfocus" it so you don't project the shape of the filament, but you need it focussed. Hmm - head hurts.
With a reflector and a fresnel - that might do it, but many lights are reflector only.

Paul

[rob.2571]

phone dr karl on jjj

[Struth]

I thought they were parabolic mirrors, not hemispherical. If they are hemispherical they are cheap junk.



Hence light from the focal point shines out (virtually) parallel. So in that sense the size should make no difference to how many degrees of light are collected and sent forward. I think this is what Paul is getting at - in pure physics theory a tiny mirror or a giant mirror should both throw exactly the same amount of light forward as long as they are the same shape and the light source is at the focal point.

Flat mirrors are a completely different kettle of fish.

It's an interesting question.

Parabolic is the correct term

But your diagram only shows the bulb emitting light in three points wheras it surely emits light in close to a 360 degree arc. So if the focal point is moved closer to point V, because the light is wider and there fore deeper, you will capture more of that 360 degrees of light emmision.

Also if you add light refraction surfaces to the parabolic surface and lets say they are of the same size in the small and large lights then the larger light will have more points of refraction than the smaller. Good lights have refraction in the reflector not the lens.

disclaimer: refraction may not in fact be the correct word

[chimpboy]

Parabolic is the correct term

But your diagram only shows the bulb emitting light in three points wheras it surely emits light in close to a 360 degree arc. So if the focal point is moved closer to point V, because the light is wider and there fore deeper, you will capture more of that 360 degrees of light emmision.

Also if you add light refraction surfaces to the parabolic surface and lets say they are of the same size in the small and large lights then the larger light will have more points of refraction than the smaller. Good lights have refraction in the reflector not the lens.

You can't move the focal point, it's the focal point of the parabola! If you moved it you would be out of focus and it wouldn't be a spotlight any more, it would be a floodlight or just a crap light.

So the scale of it doesn't change how many degrees of the light's arc are captured. If I put a 0-10cm scale on that drawing, or a 0-10km scale on it, the drawing would be exactly the same and every ray of light coming from the focal point would come out parallel like the three that are shown.

Mathematically there is nothing about the parabolic mirror that cares about scale.

So the question is just what is it that makes the real world set-up different from the theoretical one. And the only thing two things that I can think of that are really fudged in real life are (a) the fact that a lamp filament is not in fact a point of light and (b) the lamp itself and the wires to it have shadows that cut down on the light hitting the mirror.

I agree if you did something to the surface that could change things but I think we are talking about a normal curved mirror surface.

Not trying to be argumentative, I just think this is interesting!

[Struth]

^^^^^

OK I see what you are saying about the focal point being in the parabola, I was thinking it could be the globe position.

But must all parabolas be of identical shape in order to acheive parallel light spread, probably so.

Possibly the larger parabolas simply reflect the light across a wider area giving a greater spreads of useable light, maybe this can be related to a laser pen such as astronomers use. You can clearly point to a star in the sky and an observer can follow the laser light line directly to where you are pointing (these things are wicked by the way). But you don't get any real ilumination of anything other than what the extremely focused light beam hits if you for instance point it at a tree.

So if you widen the beam of light by widening the laser beam you can illuminate larger areas, it would stand to reason then that a larger parabola in a driving light would illuminate more of the landscape.

Probably the loss of focus/intensity would be near invisible to the human eye when put alongside a wider beam of light?

I will also say again that good lights have light refraction built into the reflector, they are not simply clean parabolas, and this IMO plays a large part in the difference between modern, good, smal and large lights.


Someone will come along at a point in this and prove that we are theorising from a base of pure ignorance, but it would be good to know the real answer.

Cheers

[chimpboy]

Someone will come along at a point in this and prove that we are theorising from a base of pure ignorance, but it would be good to know the real answer.

I agree, except that my ignorance is not very pure.

[RO8M]

I'm embarrassed to be admitting this (as an engineer) but I can't remember everything: specifically, can there be different parabolae? That is, I think but am not sure that you can have a steeper curve...

Search (Nah, just the wiki: http://en.wikipedia.org/wiki/Parabola)

Yeah, it seems you can have a steep or shallow parabola. So, would that have an impact on the "quality" of the light? A small light, with a Shallow parabola would not be as good as a larger light with a deeper parabola, so that that the filament on the larger parabola has a larger percentage of its emission reflected, and therefore going in the desired direction.

Like that pic you posted earlier, Jason. It would have a "Reflected angle" of say 220deg (everything below horizontal, and maybe 20 degrees above hor on each side). (talking 2D, but can be extrapolated to 3d.) If the focal point was further out, then even if the filament is at the focal point, if the reflector does not extend very far then your reflected (what you want) light percentage would decrease.

Am I making any sense?

[Struth]

I'm embarrassed to be admitting this (as an engineer) but I can't remember everything: specifically, can there be different parabolae? That is, I think but am not sure that you can have a steeper curve...

Search (Nah, just the wiki: http://en.wikipedia.org/wiki/Parabola)

Yeah, it seems you can have a steep or shallow parabola. So, would that have an impact on the "quality" of the light? A small light, with a Shallow parabola would not be as good as a larger light with a deeper parabola, so that that the filament on the larger parabola has a larger percentage of its emission reflected, and therefore going in the desired direction.

Like that pic you posted earlier, Jason. It would have a "Reflected angle" of say 220deg (everything below horizontal, and maybe 20 degrees above hor on each side). (talking 2D, but can be extrapolated to 3d.) If the focal point was further out, then even if the filament is at the focal point, if the reflector does not extend very far then your reflected (what you want) light percentage would decrease.

Am I making any sense?

I like that because it resembles some of my crude theories, as well as makes sense from a physics perspective.

cheers

[MightyMouse]

Just to throw a bit more "confusion" into the discussion many reflectors are now PES ( polly-elipsoid ). Parabolic was "easy" to design and manufacture tooling for in the days before significant computer power was available, but now its no bid deal to have the optimal reflector angle calculated and manufactured for lights of just about any general shape - look at the huge variations in car headlights.....

This isn't knocking the parabola.... just pointing out some more recent developments. BOSCH's lighing handbook has some interesting info.

Also discussions on reflectors need to consider the shape/optical emission profile of the light source - neither incandescent or HID are point sources and this has a large bearing on reflector design ( part of the arguement against HID conversion.... )

Keep up the very interesting discussion - we all "know" things but when it comes to the "why's" we often realize we aren;t quite as certain as we thought....

[hoser]

I know this thread is about reflectors but if you want small and bright, try these. Cheap, they are not though.

http://www.visionxusa.com/led/s1100/

Shown here for a perspective of their actual size.
http://www.rzrforums.net/lighting-stere ... -leds.html

[midi73]

I know this thread is about reflectors but if you want small and bright, try these. Cheap, they are not though.

http://www.visionxusa.com/led/s1100/

Shown here for a perspective of their actual size.
http://www.rzrforums.net/lighting-stere ... -leds.html

Not to bad. 4 of them are on par with a pair of hids from what I can gather. and as bright.

[me3@neuralfibre.com]

I know this thread is about reflectors but if you want small and bright, try these. Cheap, they are not though.

http://www.visionxusa.com/led/s1100/

Shown here for a perspective of their actual size.
http://www.rzrforums.net/lighting-stere ... -leds.html

Not to bad. 4 of them are on par with a pair of hids from what I can gather. and as bright.

Lumens 1/3 that of HID
Beam pattern unlikely to be much other than a spread. VERY complex to get a beam from an LED.

Paul

[chimpboy]

I'm embarrassed to be admitting this (as an engineer) but I can't remember everything: specifically, can there be different parabolae? That is, I think but am not sure that you can have a steeper curve...

Search (Nah, just the wiki: http://en.wikipedia.org/wiki/Parabola)

Yeah, it seems you can have a steep or shallow parabola. So, would that have an impact on the "quality" of the light? A small light, with a Shallow parabola would not be as good as a larger light with a deeper parabola, so that that the filament on the larger parabola has a larger percentage of its emission reflected, and therefore going in the desired direction.

Like that pic you posted earlier, Jason. It would have a "Reflected angle" of say 220deg (everything below horizontal, and maybe 20 degrees above hor on each side). (talking 2D, but can be extrapolated to 3d.) If the focal point was further out, then even if the filament is at the focal point, if the reflector does not extend very far then your reflected (what you want) light percentage would decrease.

Am I making any sense?

My answer is.... (just imho)... in this way you could catch more or less of the light from the lamp but it still doesn't explain why bigger lights are better - if the big light and the small light are the same shape and the smaller one is just scaled down from the big one. It only explains a difference between a deep-dished sort of lamp and a shallow one, again not size dependent just shape dependent.

Mulling it over more I'd almost now be willing to bet that the main factor is the fact that the light source is not a true point because of the size of the filament. My next guess would be that the "shadow" of the globe and wiring matters more in a smaller lamp.

[RO8M]

Yeah, I agree, Chimpy. I was really just wandering where my mind took me (something like Beatles lyrics?)

Many more complexity than i know about...

[Struth]

I don't think the wiring matters, it is generally outside of the parabola and has no effect on the light emitted, this is the way lights are designed.

Another thought is that the light does not lose luminesence or intensity over the small distance it travels to the parabola, so the larger the reflector, the more light is reflected in the desired direction, which is back to my larger beam of light without loss of visible intensity theory. I'm keen on that theory still.

We are all bombarded with lights matter all day every day in the form of radiation, but it is so small it passes through the gaps in our relatively large makeup with no harm, it's complex and beyond me but light does have matter or mass, but being made of such small particles it is difficult for any reflecting device to reflect all the light that is shone upon it, so light is lost/wasted, not redirected, call it what you will.

I also still maintain that if you shine a light on a small reflector only a small portion of the light will be reflected, use a larger reflector and a larger portion of the same light source will be reflected in the chosen direction. Because the reflected beam is wider. Now put that theory into a parabola that you can shape to reflect light in more than one direction and a larger parabola will reflect a larger beam of light.

My thoughts for the night.

Might ponder this again tomorrow night,

Might even read up a bit:D

One way to harness more light is to increase the reflector size,

[Bush65]

Something else to ponder on this subject is the system used in light houses.

They don't have particularly high power globes, but shine well past the horizon, and are made tall so their horizon is further away. If you approach a light house from seaward at night, you will notice the loom sweep overhead when you are outside the horizon, which is many miles from the light.

If the size of the reflector made no difference, why do they make them so large (surely more expensive than a small reflector).

[Struth]

This is difficult to follow but it may enlighten a bit,

http://www.physicsclassroom.com/class/refln/

Cheers

[me3@neuralfibre.com]

Something else to ponder on this subject is the system used in light houses.

They don't have particularly high power globes, but shine well past the horizon, and are made tall so their horizon is further away. If you approach a light house from seaward at night, you will notice the loom sweep overhead when you are outside the horizon, which is many miles from the light.

If the size of the reflector made no difference, why do they make them so large (surely more expensive than a small reflector).

Overhead sweep you see it dust / moisture.

Agree on the size bit.

Lighthouses are more complex. Generally they are tricky fresnel lenses wrapped around the lighht. This gives "pulses" of light to identify the lighthouse. At least - that's the old one's. Newer ones - who knows.

I asked on Candlepower forums - the only comments so are a the same as Chimpy's - due to lack of true point source, size allows the "error" to be less.
I'l keep digging.

Some guys there playing with mulit-million lumen lights - WWII searchlights. Impressive (and huge reflectors).

Paul

[chimpboy]

Some guys there playing with mulit-million lumen lights - WWII searchlights. Impressive (and huge reflectors).

At some point heat alone would make it necessary to go for bigger reflecters. But that is sort of an "outside factor" in relation to your question I think.

[me3@neuralfibre.com]

Some guys there playing with mulit-million lumen lights - WWII searchlights. Impressive (and huge reflectors).

At some point heat alone would make it necessary to go for bigger reflecters. But that is sort of an "outside factor" in relation to your question I think.

Assume same 55W H3 bulb.

In my experience performs better in larger light.

Ditto Lightforce - 3 lights, same globe. Bigger appears better.

Paul

[fester2au]

It's got me buggered.

I know it to the true. The bigger the reflector, the better they perform. Narva Big Bulls are huge - cheap and perform really well.
Small expensive lights often perform poorly.

Why is it so?

I've googled myself out, and I can't figure the diff between a small reflector and a large one.
Any ideas?

Paul

I reckon it's marketing. the general 4wding consumer will buy the bigger one cause it looks tuffer and corrects their phallic imbalance so this is where all the marketing and R&D goes. Size sells.