Light, Reflection and Redirection

All the shade

I have a few issues in the garden.  HA that’s an understatement.   As a grower whose enthusiasm for planting, weeding, watering and maintenance wanes by the 3rd week in June, you can imagine all the problems my garden faces!  But mainly I have issues with light. In a backyard whose privacy I love for the numerous trees that edge my yard, I also have deep shade in a good part of my yard and the rooms facing into the backyard get no direct sun leaving the kitchen a dark space.  While my cave dwelling husband loves this, I thirst for light.  So this has taken me on a search for how to use reflective materials in my garden to direct more light to my rooms and deep shade.


One idea has been a reflector called a heliostat.  The advantage of a heliostat is that its not stationary like a mirror, but moves with the sun to consistently direct sun to the spot you would like.  Downsides?  It looks rather utilitarian and bulky.  I have contacted the company to get more information on its dimensions.  But the greatest worry I have is, will we be blinded by the light?   The area with the greatest amount of sun is at the farthest north-western point of the yard and using that area to direct light back to the E/SE. would direct light all the way across the yard to get light into the back windows.  I can’t imagine it would even work with the kitchen on the first floor without sending a beam directly into the windows at a nearly horizontal angle.  The glare would be terrible!  If moved closer to the house, the deck may block it at times.  Another consideration is how well an electronic piece of equipment will stand up to the heat of summer and snow of winter.


So I’m imagining a series of mirrors that will direct light first to a spot in the shady part of the yard and then on to another mirror lower to the ground close to the sliding glass door that would then direct light upward to the ceiling in the kitchen.  The first mirror may remain low to the ground and reflect up to a mirror that secured to a tree limb that would reflect it back down to another low mirror and up to the ceiling.  The problem being, how long will that one mirror reflect the light to the second mirror and to the third?  If it only lasts a 1/2 hour whats the point right?


The amount of light loss from multiple reflections of mirrors can only be measured if you know the index for loss of each mirror you use.  For high quality, expensive optical mirrors, loss can be as much as 10%.  So if you start with the assumption that your standard bathroom mirror would lose at least 20% of light through refraction etc, and air is the medium through which the light is travelling to reach each mirror, and loss =1-(1-l1)*(1-l2)   where l1 = air loss (1%) and l2 = mirror loss (20%) then after 3 reflections you may end up with only 40% of the original light from the sun.

For a more exacting measure you can follow Fresnel’s equations.

But for my purpose, I just wanted to know, if I were to setup a complex set of mirror reflections how could I redirect the most light?

I would probably need to start with a concave mirror that concentrates light from a larger area into a single area and at the end use a material that diffuses the light to spreads it out from a single source, for example a piece of white marble.  Trying to use cheaper materials like mylar will not hold up to weather and UV degradation.


Another option is the use of solar tubes on the ceiling.  The expense would be more, but the light more reliable!  Solatube sells 14″ tubes that cover about 300 sq ft.

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