By Craig DiLouie, LC
In my last IES LightLogic post, I described the basics of the lighting design process that deals with calculating light levels and, subsequently, how many lights will be required to produce that light level. These are tools professional lighting designers use.
So here’s where we left off:
Application: Kitchen general lighting
Room dimensions: 10 ft. x 12 ft.
Light level target: 5 footcandles
Lumens required: 600, the result of (10 x 12) x 5
Luminaires required: 1 based on interest in using luminaires that produce 900 lumens each, the result of (5 x 120) / 900
Now we’re done, right?
Not quite. To do this right, we must account for two things:
- The size and dimensions of the room, as light’s intensity diminishes with distance, and walls and ceilings reflect light.
- The room finishes, as darker finishes will absorb more light than they reflect.
This means adjusting our calculation to account for a light loss factor with the rather technical name of coefficient of utilization. We’ll just call it CU.
Kitchen Lighting Calculations
To determine CU, first determine—here’s another handy lighting term—the room cavity ratio (RCR). Getting RCR involves more number crunching, but the calculations are simple, and it’s worth the extra effort to ensure we have enough light in the room.
If the room is a standard rectangular room, RCR is:
[5 x H x (L + W)] / (L x W)
H is the vertical distance between the task plane (an invisible horizontal line intersecting the space where the primary task activity occurs) and the center line of the luminaire. L and W are the room’s length and width.
Our kitchen is a typical rectangular room, so the above formula applies here. In this case, we’ll use the task plane at the floor, which we’re lighting to 5 footcandles. The ceiling height is 8 ft. We’d like to use recessed downlighting, and we’ll consider H to be the distance between the floor and the bottom of the luminaire. That’s [5 x 8 x (12 + 10)] / (12 x 10) = 7.3 RCR.
If the room is irregular-sized, RCR is done differently as shown below, with P being the room’s perimeter length and A being the room’s area.
(2.5 x H x P) / A
Now we can determine CU based on the RCR using the below table, which assumes a 20% floor reflectance (medium color such as light brown), 80% ceiling reflectance (white ceiling) and 50% wall reflectance (wall with pastel paint). Darker finishes would reduce the CU values. Lighter finishes would improve them.
|Room Cavity Ratio
|Coefficient of Utilization
Our RCR is 7.3, which is between 7 and 8. Rather than interpolate, we’ll round down to 7, which gives us a CU of 0.45. That’s our light loss factor. Instead of 600 lumens, we need 600 / 0.45 = 1,333 lumens. This figure should be combined with information on the luminaire that will be used to do the final calculation of the number of luminaires needed.
Previously, we did a calculation and determined we need one 900-lumen luminaire. But with CU in the mix, the calculation turns out a little differently:
Number of Luminaires = [Footcandle Target x Room Area] / Lumens per Luminaire x CU
If the target general light level in our 120-sq.ft. kitchen is 5 footcandles, and we assume 900 lumens/luminaire, then we would need (5 x 120) / (900 x 0.45) = 1.5 or 2 luminaires. These luminaires should be spaced appropriately according to manufacturer guidance.
This calculation was for general lighting, which is uniformly distributed throughout the room. Next would be task lighting, which if focused on specific tasks. Accent lighting could then be added, which focuses on the object or surface being accented.
What you need to know about how much kitchen lighting you need:
- Light level calculation must be adjusted based on coefficient of utilization
- CU takes into account room size and dimensions and luminaire
- Incorporating CU into light level calculation results in greater accuracy