Light Levels, Lux and a Bright Sunny Window

For a change, the first part of today was clear, bright and sunny, and I was tinkering with a small chilli bonsai tree which currently lives in a little plastic Ikea mini greenhouse inside our patio doors.

It is doing ok, and this is by far the brightest place inside the house for a plant to spend the winter. But it’s situation led me to muse over how much light it was getting in it’s little greenhouse, in a window, in a house.

I wrote a paragraph or two in my Growing Chillies book on the subject of light levels and how brightness deteriorates very rapidly as you move away from a window, and how the human eye is so effective at compensating for this we don’t really know how cosiderable the change is. For a plant this can be the difference between life and death.

As an aside, a recent TV program on ‘The Body Clock’ had the presenter, Terry Wogan, playing around with a light meter to prove that to get a healthy dose of good quality light we need to be outside. A comfortable armchair by the window wasn’t enough to keep Terry’s body clock on the straight and narrow.

So today my inquisitive nature let me to reach for my light meter again. At this point I should say that my light meter is not as good as Terry Wogan’s. He had the bees knees I am sure, whereas I am stuck with my smartphone. But even though the Apps you can get to read light levels aren’t deemed accurate enough for real scientific study, they are still in the right ballpark, and enough for a little layman’s experiment.

I started by aligning my phone to the sun with the patio doors open, so there was nothing obscuring the view at all, the maxumum reading was about 80-85,000 lux. Lux is the unit of measurement for light. Great, that is pretty bright, a very sunny summer day might give you 120,000.

Then, without moving the phone, I closed the doors so that now the reading was taken from behind a double glazed window. The reading dropped to 38,000, less than half.

Then I moved to my little bonsai chilli plant, in it’s own little greenhouse, which added another thin layer of perspex to obscure the light. There the level dropped to 25,000 lux. Even though this little greenhouse sits not 8 inches from the outdoors, the light it receives is cut by more than 1/3rd. A quick trip out to my big single glazed glass greenhouse revealed a reading of 70,000.

How will this reduced light affect this plant and others that may be less lucky still? The first thing to remember is that this is very much a finger in the air experiment. I’m not going to win an honorary doctorate for my plant research here. Sunlight comprises of many of different types of light across the spectrum, infra red, through the visible colours to ultra violet and plants don’t need all of this, parts of spectral light are neessary and other parts aren’t. Sometimes glass is made to filter out some kinds of light but not others, and window glazing might do just that, so maybe it is filtering or reflecting some wavelengths of light but not others.

But what this experiment does illustrate is that the principle of light, regardless of it’s nature, diminishing rapidly with interference, is easy to prove. If you move further back into the room, away from the window and out of diret sunlight, the lux levels diminish into the hundreds very quickly, and a north facing window, even though it appeared bright, was less than 100 lux.

In summary my bonsai chilli is probably happy where it is, even though this spot isn’t perfect. At least it’s own personal greenhouse means tha cat can’t sit on it and on sunny days the light is adequate. But anywhere else in the house, and this goes for summer as well as winter, it probably isn’t going to do very well. It would reach for light and go straggly, the leaves will be pale green and it won’t get all the nourishment it needs from sunlight.

So the moral of this story is, and it particularly applies to chillies because they need high light levels, keep them right next to a south facing window, outdoors, or in a greenhouse if you can.

If you have a smartphone, try this experiment yourself. There are dedicated free light meter apps, or I have one called ‘GPS Status’ (Android) which does all sorts of gps stuff as well as light readings. The readings are exactly the same as ‘LuxMeter’, which implies they use the same internal module to take the readings.

Interesting, or not?

2 thoughts on “Light Levels, Lux and a Bright Sunny Window

  1. Pingback: Carolina Reaper – Growing the Hottest Chilli in the World | Chilli Books Blog

  2. Pingback: 8 tips on keeping your young chilli plants at the right temperature | Chilli Books Blog

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