# Re: [APD] How much light actually gets down here?

```Hmm. Perfect parabolic puts light where we want it? WEll here's a jumping off point. Sometimes we want a very tightly focused "spot" of light (e.g, when working non small electronic circuits) but probably rarely and surely not often. Even over an 30 gallon aquarium a two or three inch wide swath of light running the length of the tank would be of rather limited utility -- well, you'd have some weird aquascapes, anyway. In fact a few of us have been known to flex an AHS reflector outward on some tanks to increase the spread of the light. I've done the narrow swath kind of things, sort of, using very narrowly focused LEDs but the effect is, well, disappointing is the best that can be said for it.

Image a car headlight that shown a nonspreading column of light only a few inches in diameter -- why, we'd kill even more folks on the road than we do now. Which, I guess, brings us around to the killin' subject again.

sh

----- Original Message ----
From: Vaughn Hopkins <hoppycalif at yahoo_com>
To: aquatic plants digest <aquatic-plants at actwin_com>
Sent: Thursday, May 3, 2007 1:46:56 PM
Subject: Re: [APD] How much light actually gets down here?

That a reflector catches light emitted in directions other than
towards where we want it to go is the reason for any reflector.  A
perfect parabolic reflector and point source does just that, but it
captures all of the light going in the wrong direction and perfectly
redirects it in the right direction.  A flat  reflector catches far
less of the light going the wrong way and directs far less of that
light in the right direction.  Our best reflectors are somewhere in
between.  It is the loss of light due to it being directed in the
wrong direction that is the reason light intensity drops off with the
square of the distance from the source.

One other problem with reflectors - they have to cover the entire
area between the source and the target to capture all of the light
going in the wrong direction.  This is a practical problem.  Try to
design a DIY reflector for your light, and you quickly see that you
can't capture anywhere near all of the misdirected light unless the
source of light is way, way back deep inside the reflector.  The
commercial light units that have the bulb visible below the outer
edge of the reflector are very inefficient.  At least AH Supply
reflectors extend much further in front of the plane of the bulb.

Vaughn

On May 3, 2007, at 10:33 AM, S. Hieber wrote:

> Agreed. With an ideally efficient reflector, i.e., purely reflected
> and didn't scatter or absorb, and was perfectly parabolic, etc,
> then all the reflected light from the reflector would be straight
> forward, plus an infinitesimally thin line of light directly form
> the bulb would be straight forward, ideally striking the same area
> at any depth.
>
> I thought we had reflectors because, all light bulbs (whether
> incasdescent, flo, led, etc.) emit light radially in all
> directions, generally at least half of it into a useless
> hemisphere. That benefit counts at any distance.
>
> sh
>
>
> ----- Original Message ----
> From: Vaughn Hopkins <hoppycalif at yahoo_com>
> To: aquatic plants digest <aquatic-plants at actwin_com>
> Sent: Thursday, May 3, 2007 11:54:27 AM
> Subject: Re: [APD] How much light actually gets down here?
>
>
> Ah, it is such a nice feeling to be able to argue about something
> other than how to kill a fish!
>
> If you put a point source of light in a parabolic reflector, right at
> the focal point, and the reflector is a perfect reflector, reflecting
> 100% of the incident light throughout the spectrum, there is no drop
> off in intensity with distance.  That light would shine as brightly
> on an object ten miles away as on your hand right in front of it.
> The problem is that there are no point sources of light, so it isn't
> possible to locate anything at the focal point.  Such reflectors are
> always less than perfect.  But, the light intensity most certainly
> does not drop off as the square of the distance, or even directly as
> the distance.  A line source, another practical impossibility, can be
> located in a linear parabolic shaped reflector, along the focal line,
> and be equally perfect.  No  light source we have is even close to
> being either a point or a line source, so our reflectors can't be
> perfect.  They can be very good, and that allows us to side step the
> loss of light with distance to some extent.  That is the only reason
> we even use reflectors.
>
> Vaughn
>
> On May 3, 2007, at 8:21 AM, S. Hieber wrote:
>
>> The loss of lightfal over a given size of area decreases as a
>> square of the distance from the point or origination --simply put,
>> if you draw lines fromthe pint source to the perimeter of the area,
>> then lower the area to twice the distance, the line will describe
>> an area that increases as a square of the original distance (for
>> example, iirc, an 8" diameter comprises about half the area of a
>> 10" diameter) . This rule of thumb is often cited and comes from
>>
>> The square of distance rule yields an easy calculation with a
>> single point source of light. However, with a line source, you have
>> an infinite number of point sources (all along the length and
>> around the tube), so the calculation becomes more complex. For any
>> one of these, the square of ditance rule holds, but as the area is
>> lowered, more point sources shed light on the area (an
>> ovesimplification but that's generally the way to picture it).
>> Taking all that into account, the lightfall roughly halves wtih
>> distance, ignoring side reflections, and a few other complications.
>>
>> With an MH, one could rely on the square of distance rule since the
>> light emits from a very small source, not quite a point, but close
>> enough for gardening purposes.
>>
>> The reflector won't make a terrific difference -- it does mean that
>> lgiht light come from more point sources (from a wider area) but
>> not tremendously so.
>>
>> sh
>>
>>
>> ----- Original Message ----
>> From: Vaughn Hopkins <hoppycalif at yahoo_com>
>> To: aquatic plants digest <aquatic-plants at actwin_com>
>> Sent: Thursday, May 3, 2007 11:06:26 AM
>> Subject: Re: [APD] How much light actually gets down here?
>>
>>
>> . . . the bulb sitting above the tank with no reflector loses light
>> proportional to the square of the distance the light travels . . ,
>> but
>> with a good reflector it loses less than that, and with a perfect
>> reflector it loses very little.
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>
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