Convex Mirror in Backyard to reflect sunlight onto the house

[jumpjack]

I live in a flat at ground floor, roof is 3 floors above...

 

[Nik_2213]

"... roof is 3 floors above..."
Fair enough.
FWIW, I'm adding a pair of 60 cm diameter mildly-convex (*) 'security' mirrors between cat-shelves on our garden wall to brighten this side of garden...

*) Reflected objects are closer than they appear, especially T-Rex...
;-)

 

[FOXGLOW]

How did your test go? It's about 2 years later? I am thinking about doing the same and in my situation, trial by error is the best way to go about. I am buying a couple 18" to 24" convex mirrors on Amazon and will mount them in my backyard on my tall fence posts which are always even in winter in the sun until 4:30 pm. A bit of a shade in summer is nice. So maybe I will have to cover the mirrors then. But in any case, the test will cost around $60 USD. And my wife might enjoy the few extra sun beams. Convex rather than flat is in my opinion and in my situation probably better because it will disperse the sun reflection rather than a flat mirror. Yesterday (and it is winter here right now - so a bit cold) I was walking to back of my house and caught a sun reflection right into my face which was actually very warm, almost summer warmth I would guess it was probably around 25 degrees heat in my face - the reflection came of a window from the building. That was enough to start my research. Please let me know. Thanks

 

[BlazyG]

Hello-

I am starting a project to get direct sunlight onto the house by placing convex mirrors on the outside stone fence of the backyard since it is the only place of my property that is not shaded by other houses in the neighborhood. For that, I need to calculate the size of the mirrors, given the distance from the stone fence on which they will be placed (on a swivel) to the house as well as the size of the glass doors that open up from the living room to the garden (I would rather that I cover the entire area of the doors with sunlight!). I tried to do some research on my own but I think I'm way over my head on this one. Is there some kind of formula I can use to calculate the area of the light that is reflected onto the house given the size of the mirror and the distance from the mirror to the house? Is there any other parameters that I have missed? Please feel free to ask me for more information if I have missed providing any!

Your project to use convex mirrors to direct sunlight into your house is quite innovative, but it does involve some complex optics. The behavior of light when it reflects off convex mirrors can be a bit tricky to calculate because convex mirrors diverge light rays, spreading them out over a larger area. This divergence means that the area of light that reflects onto your house from a convex mirror will be larger than the mirror itself, but also less intense.

To estimate the size of the mirrors needed, we need to understand a few key principles:

Mirror Equation: For convex mirrors, the mirror equation relates the object distance (
u
u), the image distance (
v
v), and the focal length (
f
f). The equation is given by
1
f
=
1
v
+
1
u
f
1

=
v
1

+
u
1

. However, for your purpose, this equation helps more with understanding the formation of images rather than the dispersion of light.
Field of View: The size of the area illuminated by the mirror will depend on its field of view, which is influenced by the mirror's curvature. Convex mirrors have a wider field of view than flat mirrors, allowing them to illuminate a larger area.
Geometry of Light Reflection: The geometry of how light reflects off the mirror and onto a target area (like your glass doors) is crucial. The angle of incidence equals the angle of reflection, but due to the convex nature of the mirror, rays diverge.
Distance and Size Relationship: The further the light has to travel from the mirror to the target, the more spread out (and thus, less intense) it will become. The size of the illuminated area on your house will depend on the mirror's curvature and the distance from the mirror to the house.
To estimate the size of the mirrors, consider these steps:

Mirror's Curvature (Focal Length): The curvature of the mirror determines its focal length, which affects how much it will spread out the light. A mirror with a shorter focal length will spread light more than one with a longer focal length.
Distance to the House: The further the mirror is from the house, the larger the mirror needs to be to illuminate the same area, due to the divergence of light rays.
Desired Illuminated Area: You want to cover the area of the glass doors. Knowing the dimensions of these doors helps in determining how wide the beam of light needs to be when it reaches the house.
A simple way to start is by using a practical approach, considering the angle at which sunlight hits the mirror and the angle needed to reflect it towards your doors. However, without specific measurements (size of the doors, distance from the mirror to the doors, and the amount of sunlight you wish to redirect), it's challenging to provide a precise formula.

As a practical experiment, you could use a small, movable mirror to test how different sizes and angles affect the sunlight's coverage on your doors. This hands-on method could give you a rough idea of the mirror size needed before committing to larger, more expensive mirrors.

 

[phinds]

How did your test go?

The OP has not been here for 2 years

 

[harborsparrow]

I hope you will post here about what kind of thing you end up trying, and how it worked out. I too have a couple rooms that need more natural light, but because I've got neighbors living upstairs, it's impossible to install any kind of sun tunnel.

 

[phinds]

I hope you will post here about what kind of thing you end up trying, and how it worked out. I too have a couple rooms that need more natural light, but because I've got neighbors living upstairs, it's impossible to install any kind of sun tunnel.

The OP has not been here for 2 years

 

[gary350]

Don't use convex mirror you will catch something on fire.

Summer sun is about 420 btu per sq ft. Winter sun is about 200 btu per sq ft. A mirror 2'x3' = 6x420=2500 btu of heat. If your aiming at a patio glass door 5'x6' buy a sheet of foam insulation that has aluminum foil on 1 side this makes a very low cost mirror. You don't get a good quality reflection but it works. 5'x6'=30 sq ft = 30 x 350= 10,000. btu of heat summer sun and 5000. btu winter sun. If you walk past the patio door inside the house you can feel a temperature increase of about 20°f. I turned our house furnace fan ON it circulated heat to the whole house, our house was 1500. ft then.

Build a turn table with the mirror on it. Put a solar cell in the bottom of a deep cardboard box when sun shines down inside the box solar cell has enough voltage to turn on a DC motor to rotate the turn table. When turn table mores our of the sun motor stops. The turn table will more forward little by little all day keeping the mirror reflection on the patio door. Auto return timer send mirror back to the start location at 11 pm.

As the sun elevation change the mirror angle needs to be changed. At my house Dec 21 our sun is at 32°. Then June 21 sun is higher, I can't remember without looking that up but I think our sun is 78°. Once a week manually re adjust the mirror. If you want to get fancy you can build another auto tracker to tilt the mirror from 32° to 78°.

I built this years ago. I had pics once but they are lost.













t

 

[Averagesupernova]

Don't use convex mirror you will catch something on fire.

You are thinking of concave.

 

[gary350]

You are thinking of concave.

YES concave. Time expired I can't correct it.

 

[sophiecentaur]

Don't use convex mirror

YES concave. Time expired I can't correct it.

That's a danger. The 'image' of the Sun that's required is actually a very fuzzy one, spread over the whole window area. What's needed is very much a DIY solution. A number of small plane mirrors, mounted where there's loads of sunlight can synthesise a de-focussing effect. This would be better than what a perfect concave mirror surface produces can be achieved . Choose a broad target area in the room and aim a reflection of the Sun from each mirror in turn at various locations in the target area, spreading the light safely. Each mirror can be adjusted separately by covering all the others.

A plane mirror array can cover a larger area than a single concave mirror, thus getting some sunlight over a longer time. Also, the mounting of small mirrors is much easier to achieve and they can be fitted in between plants and other features. Multiple reflectors are the solution for the very biggest telescopes so it's the way to go.

I know this is an ancient thread but our design brains have ben following it closely all the time. I don't really care that the OP may have moved to a different home by now.

 

[gary350]

That's a danger. The 'image' of the Sun that's required is actually a very fuzzy one, spread over the whole window area. What's needed is very much a DIY solution. A number of small plane mirrors, mounted where there's loads of sunlight can synthesise a de-focussing effect. This would be better than what a perfect concave mirror surface produces can be achieved . Choose a broad target area in the room and aim a reflection of the Sun from each mirror in turn at various locations in the target area, spreading the light safely. Each mirror can be adjusted separately by covering all the others.

A plane mirror array can cover a larger area than a single concave mirror, thus getting some sunlight over a longer time. Also, the mounting of small mirrors is much easier to achieve and they can be fitted in between plants and other features. Multiple reflectors are the solution for the very biggest telescopes so it's the way to go. Sun is at a higher angle June 21 that makes btu be about 47000/2=23000 btu in 90 deg weather.

I know this is an ancient thread but our design brains have ben following it closely all the time. I don't really care that the OP may have moved to a different home by now.

Foam insulation board in 4'x8' sheets is very easy to bend I tested it with, 1", 2", 3" bend it produces a hot spot too warm to put your hand if board is bent too much. 130°f is plenty warm no one will get hurt. I was aiming at a sliding glass patio door. Sun is in the south you can only do this with a reflector on the north side of the house. All you need to do is make the 4'x8' beam of sun light smaller about 4'x4' so none of the light missed the patio door. This cost much less than a 100 glass mirrors I checked prices and glass is too expensive and too much work to mount and aim 100 mirrors.

I painted the 2 garage doors on my work shop flat black 1 winter the 2 doors are 7'x8' each that = 112 sq ft x 200 btu in winter = 22400. btu of heat. WOW that heated up my work shop nice. Summer came it was to hot to go in the work shop sun is producing 47,000. btu in 90° weather I painted the 2 doors white again.

https://www.google.com/url?sa=i&url...ved=0CBMQjRxqFwoTCKDou8KRyYQDFQAAAAAdAAAAABAE

 

[sophiecentaur]

I would just not like the aesthetics of a shiny, white sheet. Mirrors are popular as trompes d’oeil ; white sheets are not. This is with good reason.
But you can do what you like, of course.

 

[jumpjack]

Found a very good, free and online tool to play with rays reflections:

https://phydemo.app/ray-optics/simulator/

I am currently using a couple of parabolic dishes for satellite TV, covered with small mirrors, to succesfully reflect sunlight to my north-facing window for 4-5 hours per day withount need for moving the mirrors; then I figured out that I could get exatly same result with a convex mirror: the only difference bewteen convex and concave is that one mirror turns the image upside doen (concave one), the other does no (jut try it with a spoon); so one could use, rather than a satellite dish manually covered by small mirrors, one of those convex mirrors used to check for incoming cars behind a turn: indeed, if you look at one of this mirror you can see the whole path that Sun will cover along a day!
So I think I will convert my system to convex-type, by moving the mirrors on the back side of the dishes: actually the parabolic mirror can only concentrate sun rays in its focus... but what does it happen if the mirror accidentally falls?!?

 

[sophiecentaur]

convex-type, by moving the mirrors on the back side of the dishes

Experimenting is good value. I would say that a convex mirror would probably be best because it will catch the Sun better throughout the day.
Enjoy yourself.