Let’s look at examples of blinds operations. The below image shows a space that has windows with daylight (upper) and view (lower) components. The daylight and view windows are separated into different window groups since the view window has an overhang above it, while the daylight window is exposed. This image shows, though, that the groups act independently, namely that the daylight window remains open while the view window is closed. As a result, some direct sunlight is cast back into the room, but not an excessive amount.
Let’s look at this in more detail, in accordance to measuring grids as prescribed in LM-83. Below is an Illuminance Grid of this same space with one point that is red, signifying that it is illuminated to more than 1,000 lux (ie it is receiving direct sunlight). This indicates at least one blind group is open. If all blind groups were closed, it would eliminate the red point. But since the sDA metric allows for up 2% of the grid to be hit by direct sun, this one point is permissible and the blinds can stay open
Now let’s see how the Illuminance Grid changes over the course of a day, with and without operable blinds. The image below shows how operable blinds affect illumination levels in a space on a simulation of December 21st. Row A shows a series of renderings of a building without operable blinds. Row B shows a plan view of the Illuminance Grid in a space without operable blinds. Row C shows a series of renderings of operable blinds opening and closing according the sDA metric. Row D shows the same plan view, but this time the space has operable blinds.
An animated simulation of operable blinds in the workspace can be seen below. As the area of direct light increases to over 2% of the total space, blinds close to decrease the area and create a more visually comfortable environment.
How is exposure to direct light affected by solar angle? The position of the sun in the sky will affect how much light gets into your space. For example, let’s say it’s June 21st and your building is on the equator. The room that you are measuring light in has one window on the southern exposure. The blinds will stay open all day even if it’s sunny, because the sun will pass directly over your building from east to west and never enter the space.
Now take that same building and move it to Boulder, CO (40 degrees north). In the same climate conditions (sunny all day) the blinds will close for a significant portion of the day, because the sun will be at a lower position in the sky, meaning its rays are more horizontal and will penetrate into the space.
Next up, we will discuss how to score the sDA metric.