Fresnel lens for rear window - any good?
Bob Bashford
I'm newly arrved in the rv world ... starting wth a 1989
Telstar (27') ... and can't see a car immediately behind me through
the bathroom-size rear window ... is a fresnel lens of any help in
this situation? What size? Smallish? Or as large as possible?
Regards, Bob Bashford
Cactus Co
lens caused fires by magnifying the suns rays, maybe there made different now
but ask somebody with current experience.
Ed
wi...@epix.net
Bob, they work great. The longer your rig is, the bigger you should buy,
IMO.
Will KD3XR
sbo...@my-deja.com
olde...@pipeline.com wrote:
>
>
> I'm newly arrved in the rv world ... starting wth a 1989
> Telstar (27') ... and can't see a car immediately behind me through
> the bathroom-size rear window ... is a fresnel lens of any help in
> this situation? What size? Smallish? Or as large as possible?
>
>
In our 22' class-C I can see cars behind me with it that are invisible
without. Distance is impossible to judge, though (for maneuvering), so
leave plenty of plain window too.
Steve
Sent via Deja.com http://www.deja.com/
Share what you know. Learn what you don't.
bill horne
While I'm certain that I can roast an ant or start a fire with a
magnifying glass, I'm doubtful that I can do it with a Fresnel lens. Has
anyone ever tried it?
--
bill
Theory don't mean squat if it don't work.
Randy-V
I have heard of fires ruled to be caused by Fresnel Lenses. I went to
buy one for my 11.5' Coachman Camper years ago and there was a warning
label stuck to the smooth side of it that warned of the potential of
fire as well. This one was about 10" square and fit perfectly in the
window of the camper door (yes it did help rear vision too - but only to
see how close something was to you - it wasn't like you could read a
book with it). Further recall of the warning label said in order to
minimize the potential for overheating and/or fire, to cover the Fresnel
lens if the vehicle was to remain stationary for any length of time. I
remember thinking "how long is any length of time?". No matter we had
drapes that would just pull over the lens when we were parked.
There's my $.02 worth.
George Abbott
outside. IOW, the sun's image is SMALLER when viewed inside your rig. I
guess if the sun was coming in a window and going outward through the lens
and IF you were backed right up to something, it could concentrate the sun's
rays on that outside object {BIG GRIN}.
The lens starting fires is another urban legend.
To answer the original question, I love the rear window Fresnel lens. You
can see cars that are right behind you that you wouldn't see in your side
mirrors. A very good investment. Camping World and many RV dealers have
them.
George in MI
bill horne <who...@mail.dodge.public.lib.ga.us> wrote in message
news:37CB86F8...@mail.dodge.public.lib.ga.us...
Vince Wirth
>While I'm certain that I can roast an ant or start a fire with a
>magnifying glass, I'm doubtful that I can do it with a Fresnel lens. Has
>anyone ever tried it?
bill,
It can't be done. There is no One focal point that will concentrate
the Sun's energy. A Fresnel is really lots of magnifying lenses
merged together. Each ring is one and each has a different focal
point. The ant need not worry.
Vince Wirth
http://home.earthlink.net/~vincewirth
Jeffrey M Boyer
seconds!
Wouldn't trade the one from my van though, they REALLY help eliminate blind spots!
Jeff
bill horne wrote:
> Cactus Co wrote:
> >
> > Im not positive but I recall in my past life as a fireman that those little
> > lens caused fires by magnifying the suns rays, maybe there made different now
> > but ask somebody with current experience.
> > Ed
>
> While I'm certain that I can roast an ant or start a fire with a
> magnifying glass, I'm doubtful that I can do it with a Fresnel lens. Has
> anyone ever tried it?
>
Gerald Pearson & Sue Sarlette
http://www2.be.com/~dbg/antics/index.html
Among many other antics involving such things as carbon dioxide bombs
and a surgical tubing slingshot, it describes some antics performed with
the help of sunlight and a very large Fresnel lens. These include
setting blacktop on fire in seconds, and burning holes through the sides
of a can of spam nearly instantly.
Anyone who claims that a Fresnel lens cannot concentrate light enough to
burn things does not know jack shit about optics, and is merely trying
to inflict his ignorance on other people -- presumably so as to sound
knowledgable.
-- Gerry
Vince Wirth wrote:
>
> bill horne <who...@mail.dodge.public.lib.ga.us> wrote:
>
> >While I'm certain that I can roast an ant or start a fire with a
> >magnifying glass, I'm doubtful that I can do it with a Fresnel lens.
> >Has anyone ever tried it?
>
> bill,
> It can't be done.
<snip>
Dfrenchy
>burn things does not know jack shit about optics, and is merely trying
>to inflict his ignorance on other people -- presumably so as to sound
>knowledgable.
This guy sounds like a good candidate for the filter!
George Abbott
Why would you mount a Fresnel lens BACKWARDS on the outside of your RV
window? That is the only way it would stick to the glass for you to mount
it backwards. There is only one flat side that will stick to a window.
Only if used backwards would it concentrate, or focus, sunlight INSIDE the
coach.
Have you ever used one of these critters on an RV? I have and, unless you
mounted it backwards, it just can't cause a problem. If you don't believe
me, go into an RV store and look at one. Take it outside and hold the
SMOOTH side toward the sun and you will see that it disburses the sunlight,
NOT concentrating it. The packaging for these critters will let you do that
without removing it from the packaging.
George in MI
> Anyone who claims that a Fresnel lens cannot concentrate light enough to
> burn things does not know jack ****about optics, and is merely trying
> to inflict his ignorance on other people -- presumably so as to sound
> knowledgable.
>
> -- Gerry
Gerald Pearson & Sue Sarlette
>
> >Anyone who claims that a Fresnel lens cannot concentrate light enough to
> >to inflict his ignorance on other people -- presumably so as to sound
> >knowledgable.
>
If facts offend you, then by all means stop reading them whenever
possible.
-- Gerry
Nancy Freeman
of ants...burned a hole in his brothers shoe...the screen door button....
Cactus Co <cact...@aol.com> wrote in message
news:19990830195623...@ng-bj1.aol.com...
dana rasmussen
measured with a thermal couple. What I don't remember is which side went
up, it was a one way device.
--
Dana Rasmussen
drasm...@nospam.uswest.net
Seattle, Wa
Mike from San Jose
sbo...@my-deja.com wrote:
>
> In article <37cb0e31...@news.pipeline.com>,
> olde...@pipeline.com wrote:
> >
> >
> > I'm newly arrved in the rv world ... starting wth a 1989
> > Telstar (27') ... and can't see a car immediately behind me through
> > the bathroom-size rear window ... is a fresnel lens of any help in
> > this situation? What size? Smallish? Or as large as possible?
> >
> > Regards, Bob Bashford
> >
They're great, but distance is way off. Generally if you
see something, figure it's immediatly behind you. Leave
plenty of other window room so you don't lose the normal
view.
Also, if you're back window is so dirty you can't see
through it, with those lenses you can still see--don't know
why.
Mike (San Jose)
Jon Porter
>Im not positive but I recall in my past life as a fireman that those little
>lens caused fires by magnifying the suns rays, maybe there made different
now
>but ask somebody with current experience.
>Ed
I don't believe it! I used to work a tvarious theaters for several years
doing stage lighting. A lot of the instruments that we used then (and are
still used) are called
Fresnels because of the lens that they hav ein them.
Never cooked an actress yet. :-p
Vince Wirth
>Anyone who claims that a Fresnel lens cannot concentrate light enough to
>burn things does not know jack shit about optics, and is merely trying
>to inflict his ignorance on other people -- presumably so as to sound
>knowledgable.
Again I make the mistake of not going into deep detail. If You put the
source on the flat side (like a light house) you can concentrate the
energy. But that is not how they are used in RV's.
Thanks for the correction.
Vince Wirth
Linda & Jerry
I know this is probably a dumb questions but we have a new motorhome on
order and since we couldn't afford a backup camera they told us they
could put a magnifying lens on the back window. Is this the same as
the fresnel lens. How are you to know if they have it installed
correctly. From what I'm reading if its put on backwards it might start
a fire. We saw a magnifying lens on the back of a motorhome parked at a
service station and it had spots all over the lens. That's not the way
it's suppose to look, is it. What could have been the problem. Just
asking in case ours looks like this when we go to pick it up on Sept 8.
Thanks. Linda
sbo...@my-deja.com
The dealer is almost certainly throwing in a fresnel lens with your
purchase - they are cheap. The pattern is on one side, and they are
smooth on the other, so they cannot be installed backwards. They go on
the inside, and are held in place by static electricity. They can be
applied and removed in seconds.
The 'spots' you saw are probably air bubbles. A careful installation
(NOT in seconds) following the directions on the package can avoid most
or all of the bubbles. You may want to ask the dealer to just give you
the lens in its original packaging so you can apply it yourself.
The location of the lens is very non-critical for its function - it is
so wide angle that you'll see about the same view wherever it is on the
window - as long as you can see the lens itself. For that reason, try to
locate it so it obstructs as little of the straight-thru view as
possible. If you guess wrong, just go back and move it.
Steve
GBinNC
<mnov...@ix.netcom.com> wrote, regarding the wide-angle lenses
sometimes used on RV rear windows:
>Also, if you're back window is so dirty you can't see
>through it, with those lenses you can still see--don't know
>why.
Yeah, one would think they would disperse the dirt too!
GB in NC
[For e-mail reply, please remove the last two letters from "yahooey."]
GBinNC
>On Tue, 31 Aug 1999 23:28:22 -0700, Mike from San Jose
><mnov...@ix.netcom.com> wrote, regarding the wide-angle lenses
>sometimes used on RV rear windows:
>
>>Also, if you're back window is so dirty you can't see
>>through it, with those lenses you can still see--don't know
>>why.
And I responded, after reading it hurriedly:
>Yeah, one would think they would disperse the dirt too!
Turns out I misread the first post. I should have said, "Yeah, I guess
they disperse the dirt too!"
Haste makes waste. (Ben VI, was that one of "his" too?)
JOE NEAR
three years. No fires - wouldn't even heat up my coffee. Make sure it is on
the inside of the window. If installed correctly you should never see any
air bubbles between the window and the lens.
Joe
R Lindberg / E Winnie
Sorry, but you are both right.
A lens only has one focal point, if you put a source (light) at the
focal point you get a good spot-light. Be it a stage light or a
light-house (both use this type of lens).
But if you but the light source at an infinate distance, and put a
combustable object at the len's focal point... poof.
-----
Ralph Lindberg N7BSN <n7...@amsat.org> ICQ#5988954
RV and Camping FAQ <http://kendaco.telebyte.net/rlindber/rv/>
Remember, it's the same computer geeks that gave us the problem in the
first place that shortened "Year2000" into Y2K
Light, Ed
Roger Woods <dor...@your.com> wrote
>
> If the flat side is facing aft through the rear window, the sun is
> shining through it, and the focal point is inside the
> truck/van/whatever, would the owners be in deep doo doo?
>
> Anyone know what distance is the focal point of the lenses sold for
> RV's?
I have a couple. They are floppy flat plastic with a ring of ridges on one
side. I never thought they could concentrate light in a narrow beam, since
they show an image and at no point in the van does the image deteriorate
into a narrow beam of light. If it did, wouldn't you lose the image that's
on there like on a tv screen?
Well, tomorrow I'll take one out in the sun and see if I can get a spot out
of it.
They are very inexpensive if you feel like buying one and playing with it.
They come in different sizes.
Looking back out the van's back window, when the front edge of the hood of
a car dissappears it's about 3 feet from my bumper.
Roger Woods
<rlin...@kendaco.telebyte.com> wrote:
> Sorry, but you are both right.
>
> A lens only has one focal point, if you put a source (light) at the
>focal point you get a good spot-light. Be it a stage light or a
>light-house (both use this type of lens).
> But if you but the light source at an infinate distance, and put a
>combustable object at the len's focal point... poof.
Which brings up a question that might be of interest to only them
theoretical types but might have an application to RV's.
I remember reading years ago about the fresnel lenses used in
lighthouses. Over the years, delivering sailboats offshore, that
bright light cutting through mist when we were coming closer to shore,
preparatory to entering a port, sure made the crew feel good that we
were at least somewhere near where we wanted to go. Sometimes, we were
so tired from being beat to hell on the N. Atlantic, that we just
wished it was the correct lighthouse and had to come about, head
offshore, and sail some more miles south.
If I recall correctly, the flat side of the lens is on the side
nearest the light of the lighthouse. Yep, just checked at:
http://zuma.lib.utk.edu/lights/CA/bonita_fresnel.jpg and it is.
(Trivia Q: How do you pronounce Fresnel?)
If the flat side is facing aft through the rear window, the sun is
shining through it, and the focal point is inside the
truck/van/whatever, would the owners be in deep doo doo?
Anyone know what distance is the focal point of the lenses sold for
RV's?
Roger Woods, Norfolk, VA.
See my sailing stuff, fat stuff, or whatever tickles my fancy!
http://members.tripod.com/~PublisherRLW/
Email: rwo...@pinn.net
Ben Franklin VI
On Wed, 01 Sep 1999 15:02:08 GMT, GBi...@yahooey.com (GBinNC) wrote:
Turns out I misread the first post. I should have said, "Yeah, I guess
they disperse the dirt too!"
Haste makes waste. (Ben VI, was that one of "his" too?)
GB in NC
Ben 1 invented bifocals. Fresnel was only two years old when Ben 1 died. Much
of Ben's work and writing was done in France where Fresnel was born, Fresnel
studied his works and expanded on them.
Yes Ben 1 said Haste in every business brings failures. And he gave credit to
Herodotus circa 445 B.C. in 'Histories' then shortened it to, as you
remembered it "Haste makes waste."
So very applicable in the camping world where the extra time taken in the
preflight and recheck just after underway has prevented many a waste.
(who said we were OT, off topic) :)
Best regards,
,
,
.
.
"He that falls in love with himself will have no rivals."
(Franklin 1706-1790)
My way or the Highway! --- Ben ---
Price
(Trivia Q: How do you pronounce Fresnel?)
In the TV business, they're called Frezzies (Those are small, intense
lights mounted on the main camera bodies which can be focused using the
Fresnel type lens)... The lighthouse industry(?) probably has their own
designated nick...
--
Price Courtright
InterSat, LLC
SyndCast Media Group, Inc.
Base2Post, Inc.
Washington, DC
<!-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -->
Ol' Bab
1) Lenses (fresnel or not) come in 2 types
a) Positive focus -concentrates light, magnifies image through it, if
'strong' enough can concentrate sunlight to a point hot enough to start a
fire.
b) Negative focus -disperses light, reduces image through it, can never
cause a heating effect.
Lighthouses use positive focus. Window safety view-ports use negative.
No problem. Ol' Bab
Price wrote in message <37CE4909...@pressroom.com>...
Jeffrey M Boyer
>
>
> <snip>
> We saw a magnifying lens on the back of a motorhome parked at a
> service station and it had spots all over the lens. That's not the way
> it's suppose to look, is it. What could have been the problem.
When Installing mine it had instructions to:
Clean inside glass where lens was to be applied
mist the area with water
apply lens
wait a few days
After a few days the bubbles disappeared and it has been that way since
(3 years now). The fineness of the water mist helps I think and I used a
very fine mister for mine.
If you don't like how yours is after a week, think about re-applying it as I
have mentioned above.... or not..
Enjoy the new rig!
Jeff
GBinNC
>Q: How do you pronounce Fresnel?)
A. Fre-NELL.
(Forget the "s" -- this is, after all, French, wherein nothing is
pronounced as it appears! <g>)
Gerald Pearson & Sue Sarlette
>
> (Trivia Q: How do you pronounce Fresnel?)
freh-NELL ("freh" as in "fresh", but without the "sh" sound) Fresnel
was a French scientist who did a great deal of theoretical and practical
work in optics. French name -> French pronunciation.
> Anyone know what distance is the focal point of the lenses sold for
> RV's?
It's a negative number. In other words, it's a DIVERGING lens rather
than a converging lens -- roughly equivalent to a plano-conCAVE lens
instead of a plano-conVEX lens.
This is why the fresnel lenses sold for rear windows in RV's cannot
possibly focus the sun's rays to a hot spot -- they're diverging lenses
rather than converging lenses. This is also consistent with someone's
statement of fires caused by these lenses are an urban legend --
PROVIDED the stories are about DIverging fresnel lenses. CONverging
fresnel lenses whose focal lengths are sufficiently short for their
diameter (f/ratio < 4 is more than adequate) are quite capable of
focusing the sun's rays into hot spots -- and igniting asphalt in
seconds if the lens is large.
SOMEONE ELSE POSTING ON THIS NEWSGROUP CORRECTLY TOOK ME TO TASK FOR
OVERLOOKING THE FACT THAT FRESNEL LENSES CAN BE EITHER CONVERGING OR
DIVERGING, AND THE ONES USED IN RV'S ARE DIVERGING.
Incidently, it's NOT true that every zone of a fresnel lens has a
different focus. Each zone in fact has exactly the same focal length
(or as close to the same as the manufacturer can economically make them)
as all the others -- and this holds for both positive and negative focal
lengths.
It's also NOT true that a fresnel lens (or any other kind of simple
lens) can focus light in one direction and spread it out when you turn
it over and send the light through the other way. In fact, the
properties of a single-element lens are almost exactly the same in both
directions. If you mount a fresnel lens "backwards", its optical
properties will be exactly the same as if the lens were mounted
"correctly" -- the only difference is practical matters such as will it
stick to the window properly.
trivial quibble: A converging lens has TWO focal points -- one on each
side of the lens.
Trivia info: Diverging lenses are used in front of telescope eyepieces
to increase magnification [where they're called barlow lenses], behind
camera lenses to increase ["multiply"] the effective focal length of the
lens in front [where they're called telextenders], and as eyepieces in
"opera glasses" and in telescopes of very early (obsolete) design.
Galileo used a diverging eyepiece lens in the telescope he used to
discover the moons of Jupiter and thereby get himself in trouble with
the clergy.
-- Gerry
Linda & Jerry
that happens. Linda
Sylvan Butler
>I don't believe it! I used to work a tvarious theaters for several years
>doing stage lighting. A lot of the instruments that we used then (and are
>still used) are called
>Fresnels because of the lens that they hav ein them.
>Never cooked an actress yet. :-p
That's 'cause you were using them in reverse. I bet the lights got
really hot though! ;)
sdb
--
Do NOT send me unsolicited commercial e-mail (UCE)!
Watch out for munged e-mail address.
User should be sylvan and host is cyberhighway.net.
MSTRCLNR
>>doing stage lighting. A lot of the instruments that we used then (and are
>>still used) are called
>>Fresnels because of the lens that they have in them.
>>Never cooked an actress yet. :-p
I don't know - I've seen a few start to melt - under a Carbon Arc Super Trooper
<G>
TQ
kwrig...@gmail.com
> I'm newly arrved in the rv world ... starting wth a 1989
> Telstar (27') ... and can't see a car immediately behind me through
> the bathroom-size rear window ... is a fresnel lens of any help in
> this situation? What size? Smallish? Or as large as possible?
>
> Regards, Bob Bashford
Tom J
different vans, trucks and now the motorhome. I wouldn't be without one!!
1st I ever heard of one causing a fire?? Must have been an import
look-a-like or it was mounted on the outside of the glass instead of
inside???
Tom J
Janet Wilder
Also had one on the rear window of the Frieightliner RV hauler.
Great help for backing up to the hitch as well as when we were solo. In
all of our decades of RVing, never heard of one causing a fire.
Last and current motorhomes came with "back up" cameras.
--
Janet Wilder
Way-the-heck-south Texas
Spelling doesn't count. Cooking does.
Major Oz
Lone Haranguer
LZ
K Miller
bill horne
red shiftometer plugin.
--
bill
Theory don't mean squat if it don't work.
Bob Hatch
time the OP hit the enter key.
--
“If you're demanding a rodeo jester be thrown
in the dungeons for mocking the king, don't pretend you
support a free country”
Rep. Steve Stockman
http://www.bobhatch.com
http://www.tdsrvresort.com
K Miller
> On 11/1/2013 4:43 PM, K Miller wrote:
>> kwrig...@gmail.com wrote:
>>> On Monday, August 30, 1999 12:00:00 AM UTC-7, Bob Bashford wrote:
>>>> I'm newly arrved in the rv world ... starting wth a 1989
>>>> Telstar (27') ... and can't see a car immediately behind me through
>>>> the bathroom-size rear window ... is a fresnel lens of any help in
>>>> this situation? What size? Smallish? Or as large as possible?
>>>>
>>>> Regards, Bob Bashford
>>>
>>> They can and do cause fires. My Son-in-law put one on his camper
>>> and it did start a fire. They were lucky enought to see the smoke
>>> and put it out only after the floor, table and seat coushions were
>>> burned. They also have lots of smoke damage. You need to be at just
>>> the wrong angle for this to happen.
>>
>> Wow! Responding to a post from 1999. Way to dredge the past!
>>
>>
>
> When compared to the timeline of the Universe, he responded at the
> same time the OP hit the enter key.
K Miller
I kept expecting Canoli to show up, too.
bill horne
presence.
table...@gmail.com
> I'm newly arrved in the rv world ... starting wth a 1989
> Telstar (27') ... and can't see a car immediately behind me through
> the bathroom-size rear window ... is a fresnel lens of any help in
> this situation? What size? Smallish? Or as large as possible?
>
> Regards, Bob Bashford
bill horne
direction when the Sun is sinking below the horizon. And forget about
the melting plastic and burning shirt - those are insignificant
problems compared to what you're about encounter.
JerryOsage
goodness that the shirt was always parked in the shade.