Window Lighting Levels

window size

A recent question I received concerning my post on Window Size was “What size window will I need to receive reasonable light levels?”

Well this photo shows the window in one of our bedrooms.

This, South facing, window has an area of approximately 0.9mand provides adequate light to easily read a newspaper in the furthest corner of the 12m2 room on an overcast day.

Light Entering The Room

On a bright day, without direct sunlight, 10,000 – 25,000 lumens come through each m2 of Window.

Even on an overcast day you would get 1,000 lumens per m2 of window.

So on an overcast day the incoming light through this window will be around 900 lumens.

Room Lighting Level

With a room area of 12m the average overcast day lighting level will be 900/12 = 75 lumens/mfloor area

This level compares well with these suggested light levels for various tasks.

  • 10-20 lumens/sqm floor area – Conversation, Eating, Watching Television, General Circulation.
  • 20-50 lumens/sqm floor area – Cooking, Casual Reading, Bathrooms.
  • 50- 100 lumens/sqm floor area – Detailed Craft Work, Study.

Additional Considerations

The above calculations apply to my case where the window is plain glass and the walls and ceiling are white.

Using a tint or Reflective Film will reduce the lighting level by between 20 and 80% so this fairly small window may not be adequate.

With a less reflective decor the room lighting level will reduce significantly across the room.

In either of the above cases a 2mwindow (which is smaller than many windows I see in display homes) would provide adequate light in most conditions.