Avoiding Light Flicker From Ceiling Fans

Flickering or strobing from ceiling fans is caused when a light source shines through the path of the moving blades of a ceiling fan.  It can be very annoying, but fortunately can usually easily be avoided by:

1. Raising your ceiling fans above the light dispersion zone
2. Separating the ceiling fan from the light source by moving them horizontally apart
3. Reducing the angle of dispersion of the light source

Estimating ceiling fan and light separation to avoid flickering is relatively straight forward.  The illustration below shows the three key variables:

1. Vertical separation between the lamp and the blade plane (V)
2. Horizontal distance between the lamp and the blade tip (H)
3. Angle of light dispersion (θ)

Note that if the lamp has a glass or shade it is prudent to measure from its edge rather than the globe because there may be considerable reflection and refraction of the light until it exits the glass or shade.

While avoiding light flicker from ceiling fans is an important factor in determining how to place your ceiling fans, for a more complete coverage on the subject we also recommend reading how high to hang a ceiling fan and ceiling fans for low ceilings.

Calculating the minimum horizontal distance (H)

The table below shows the minimum ratio of horizontal distance (H) to vertical height (V) including a 20% safety margin to ensure clearance.

Dispersion angle (degrees)	15	25	35	45	55	65
Ratio (1 unit V = X units H)	0.32	0.56	0.84	1.20	1.71	2.57

Example:

                Light height drop from ceiling = 100mm

                Fan blade plane drop from ceiling = 350mm

                Light angle of dispersion = 35 degrees  →  ratio = 0.84

 

                Vertical height from light to blade plane (V) = 350mm – 100mm = 250mm

                Minimum horizontal distance (H) = 250mm x 0.84 = 210mm

 

Calculating the maximum vertical height (V)

The table below shows the maximum ratio of vertical distance (V) to horizontal distance (H) including a 20% safety margin to ensure clearance.

Dispersion angle (degrees)	15	25	35	45	55	65
Ratio (1 unit V = X units H)	3.11	1.79	1.19	0.83	0.58	0.39

Example:

                Lamp drop from ceiling = 100mm

                Horizontal distance between blade tip and lamp (H) = 500m

                Light angle of dispersion = 35 degrees  →  ratio = 1.19

 

                Maximum vertical height from lamp (V) = 500mm x 1.19 = 595mm

                Maximum drop of fan blades from ceiling = 595mm + 100mm = 695mm

 

Skylights

Skylights can also cause flicker however this is typically less pronounced than the flicker caused by artificial lighting.  This is in part because during the day there is more ambient light reducing the contrast between light and dark, and in part because the light from a skylight is wider than the point source illumination typical of artificial lighting.  It is still recommended to avoid placing a ceiling fan beneath a skylight where possible.

 

Caveat

Please note that this is a guide only.  Every installation will have its own characteristics that should be discussed with your lighting designer or architect.