BEFORE THE ARCHITECT – BACKGROUND – ARTICLES
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Avoiding Light Flutter: How Ceiling Fans and Ceiling Lights Can Get Along Together
Copyright 2007 Before The Architect
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Light flutter comes from ceiling fan blades interrupting a view of light rays – natural or mechanical source. This is about mechanical source interruption and its avoidance by lighting design.
Light flutter can be divided in two categories – primary and secondary.
 | Primary light flutter puts a fan blade directly across the beam at close range to the viewer – pulses light and shadow enough to hurt vision almost painfully. |
| Secondary light flutter puts a blade across more distant light rays to both blade and viewer, enough sometimes to be only barely noticeable for the viewing disruption, like death by duck nibble. |
Both flutters are inconvenient, signatures of poor lighting design in the author’s opinion. Both can be designed out reasonably.
Who cares? In our design experience – forces converge.
| Aging eyes - -if you don’t have them, you will – need more illuminance than generally designed. Light and shadow variance is no friend to ageing eyes. |
| Ceiling fans are a trademark of warmer climes – retirement climes. |
| Recessed lighting is broadly popular and applied. |
| Compact fluorescent lamps (CFLs) in downlighting are not only Green, they can produce ageing-extra illuminance that is tougher to match with incandescents. |
| Finally, by experience and the expressions of others, ceiling luminaires are commonly sited by tradesmen onsite, which, in the author’s opinion, is neither as it should be nor need be. Electrical installers are a segment of the building business, not the design business. |
The physics of it: a ceiling fan blade can intersect a ceiling-sourced light ray anywhere there’s a line of sight, not just right below or right by the spinning fan.
The physiology of it: height counts. The higher the base point for line of sight, the longer is the blade-intersected view, i.e., the less ceiling space on which to apply downlights. Our design uses a base point for line of sight 70.3” above floor level – upper end of U.S. male eye level (Architectural Design Standards, 10th Ed., Ramsey/Sleeper, p.2).
The geometry of it: apply room and fan dimensions to scale. Each space with a ceiling fan will return its own particular results for major and minor axes separately (unless the space is square). Let’s try on, as example, the 20’ major axis to a room with a 9’ ceiling, and a 44” diameter ceiling fan with a “standard” application, putting the blade plane 12” below ceiling level. The drawing to follow develops these metrics into prescripts of where and where not to install downlights at either end of the space.
Figure36: Light Flutter Avoidance, Section, Part 2B
Source: Home Design Standards-Home Building Standards 4Q08 desk copy draft, currently page 244 of 421 at 1/8/07
What’s learned? From 7’-4 5/16” outside the fan’s centerpoint and through 2’-7 11/16” to the major axis terminus at the space’s wall, recessed downlights appear to be applicable without primary or secondary flutter. Derivatively, with increasing length of downrod, ceiling-sourced opportunities for illuminance shrink, effectively expanding primary flutter ceiling surface area while reducing secondary flutter exposure.
This approach to avoiding light flutter has significant upsides –
| Calculations are not complex and can be executed quickly | ||||||||
| Methodology addresses a space’s major and minor axes separately and without an averaged generalization subject to useful space foregone | ||||||||
| Outcomes can be demonstrated and defined on the order of dead-reckoning | ||||||||
| Model relies on well-known values early-on in a house plan development – space dimensions and, by derivation, ceiling fans metrics | ||||||||
| Controllable variables abound, including height of base point, site of base point relative to the space, blade diameter (limitedly), downrod length (less limitedly) | ||||||||
| Practical refinements are possible, including adjusting eye level to owner’s physiometrics; adjusting the site of base points respectful of a given space’s realities, e.g., built-ins, steps up, wall-loaded furniture, etc.; rounding the corners of the rectilinear standoff space the model inevitably produces, giving up more useable space | ||||||||
| Physical dynamics are scuh that light flutter avoided will never be more closely defensed than at a space’s perimeters – the farther one travels into the space interior, the farther apart are lines of sight between blade diameter and light source | ||||||||
| None of the other our other
lighting options is diminished, including
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| Can apply to surface-mounted luminaires, too |
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