# Area lighting Design Calculations - Part One

Today, I will explain the Lighting design methods and steps for Outdoor Area as follows.

You can review the following previous article for more information and good following:

Methods for Design Outdoor Area Lighting (Floodlighting):

Area Lighting, an important aspect of outdoor lighting, is very effective for the following:
• Lighting wide open areas such as construction sites, railway marshalling yards, ship yards, docks, airport aprons, parking areas and filling stations.
• Security floodlighting to deter night-time pilfers in car parks, warehouses, industrial and commercial premises.
• Environmental lighting such as lighting of parks, gardens, sea shores, monuments and historical sites.
Area Lighting (Floodlighting) application is often considered the more complex and difficult of lighting techniques.

The most commonly used systems for floodlighting calculations are :

1. The point-by-point method,
2. The beam-lumen method,which include:
• IES method,
• CIE method.
1. Isolux Diagram Method,
2. Computer Aided Design.

IES: (Illuminating Engineering Society).

CIE: (International Commission on Illumination).

First: Point-By-Point Method

The point-by-point method permits the determination of footcandles at any point and orientation on a surface and the degree of lighting uniformity realized for any given set of conditions.

Based on the orientation of the surface, we have (5) cases in application of point to point method for lighting design as follows:

1. Illuminance directly below the luminaire on a horizontal surface,
2. Illuminance on horizontal surface but at angle to luminaire,
3. Illuminance on vertical surface at angle to luminaire,
4. Illuminanceon a tilted or sloped surface,
5. Illuminanceon for multiple point source calculations.

The (5) methods are summarized in the following image:

I explained this method before for indoor lighting calculations in the previous article "
Point by point method " , the same procedures for calculation will be followed in area lighting (floodlighting) design.

Second: The beam-lumen method

1- Beam Lumens (BL):
• The beam-lumen method is quite similar to the Lumen method (Zonal Cavity method ) for interior lighting except the following:
1. In Beam Lumens (BL) method, the utilization factors must take into consideration the fact that floodlights are not usually perpendicular to the surface and all of the useful light does not strike the task area.
2. Beam lumens are defined as the quantity of light that is contained within the beam limits described as “beam spread” Beam lumens equal the lamp lumens multiplied by the beam efficiency of the floodlight.

Note:
• It is recommended that sufficient point-by-point calculations be made for each job to check uniformity and coverage.

2- Light Loss Factor (LLF)
• The maintenance or light loss factor is an allowance for depreciation of lamp output with age and floodlight efficiency due to the collection of dirt on lamp, reflector, and cover glass.
• The total factor may vary from .65 to .85 depending on the type of lamp and luminaire used, and may include losses due to lamp orientation or “tilt.”

Part one: Design Procedure for The beam-lumen method as per IES method

Step- 1: Determine the dimensions of the area to be illuminated

Step- 2: Determine the type and location of floodlights

• The floodlights luminaires have the facility of symmetrical / asymmetrical and narrow / medium / wide beam light distribution and can be supplied either as integral or with separate weatherproof heavy duty type control gear box.
• The IES beam spreads indicate the angular ranges (horizontally and vertically) through which the intensity of the floodlight is greater than 10% of its maximum intensity. This angular range is referred to as the 'Field Angle'. See below image

For example:
Asymmetrical beam spread 138° (H) horizontal x 119° (V) vertical = IES 7 X 6

Note:
• The IES classification was formerly referred to as NEMA Type. I.e. IES 7 = NEMA type 7.

Recommended luminaires types as per IES
1. Luminaires located at or near the center of an area should be IES type 3, 4 or 5 depending on the coverage, intensity and uniformity required.
2. Luminaires located at or near the sides of an area should be IES type 2, 3 or 4 depending on the coverage, intensity and uniformity required.

The following general principles apply in the choice of beam spread:

• The greater the distance from the floodlight to the area to be lighted the narrower the beam spread desired.
• By definition, the “Field Angle” is used to determine the NEMA type. It is equal to the number of degrees between the 10% of maximum candlepower (near the center of the beam) locations. Since the 10% locations are generally near the edge of a floodlight’s beam, the illumination at the edge is 1/10 or less of that at the beam center.
• To obtain reasonable uniformity, the beams of individual floodlights must overlap each other as well as the edge of the surface to be lighted.
• The percentage of beam lumens falling outside the area to be lighted is usually lower with narrow-beam units than with wide-beam units. Thus narrow-beam floodlights are preferable where they will provide the necessary degree of uniformity of illumination and the proper footcandle level.

Step-3: Determine the Luminaire mounting height

Luminaire mounting height should be determined by source and wattage as in following image:

Note:
• shown values are the recommended minimum height (exceptions may be taken when diffuse lens or lamp is specified).

“Rule of thumb” of Mounting Height for pole-mounted floodlights:

The recommended mounting height is one half the distance across the area to be lighted.

For example:
If the area to be lighted is 40 feet across, the lowest recommended mounting height is 20 feet.
so, Mounting height = 1/2 distance to be lighted = 1/2 (40ft.) = 20 ft.

Step-4: Determine the coefficient of beam utilization

The factor, CBU, written as a decimal fraction, expressed in the following ratio:

CBU = Utilized Lumens / BL

The exact CBU can be determined graphically by projecting the outline of the area to be lighted upon the photometric data and totalling the utilized lumens. This procedure is detailed in the IESNA Handbook as shown in the following example:

Example:
By using the following CU chart in below, calculate the CU of floodlights luminaires needed for an area 200' x 75' wide given that the mounting height is 30 feet.

1. Ratio lateral distance mounting height = 75/30= 2.5(mounting heights)
2. Follow 2.5 ratio curve to street side (front) CU curve.
3. Trace intersection of CU ratio to left, determining 0.46 (percentage of bare lamp lumens on area) , then : CU = 0.46
See the below image for typical CU values for different outdoor areas:

• It should be noted that when luminaires are located along the sides of an area, the rear side (house side) CU is not contributing to the area.
• As an approximation, the average CBU of all the floodlights in an installation should fall within the range of 0.60 to 0.90.
• If less than 60% of the beam lumens are utilized, a more economical lighting plan should be possible by using different locations or narrower beam floodlights.
• If the CBU is over .90 it is probable that the beam spread selected is too narrow and the resultant illumination will be spotty.
• An estimated CBU can be determined by experience, or by making calculations for several potential aiming points and using the average figure thus obtained.

Step- 5: Determine the quantity of floodlights (N) or the level of illumination (fc)

Cas#1: Determine the quantity of floodlights (N) required if level of illumination (fc) is known:

When designing areas with cutoff or dropped lens type the quantity of floodlights (N)is determined by the following basic formula:

Where:

E = Average Maintained Illuminance (fc) of an area

N = Quantity of Luminaires

A = Area in Square Feet

BL = Beam Lumens

CBU = Coefficient of Beam Utilization

LLF = Light Loss Factor

Case#2: Determine the level of illumination (fc) if the number of floodlight luminaires is known:

Rearrange the basic formula given in step-5 above to get the level of illumination (fc) as follows:

Recommended Illumination Guideline as per IES:

Step-6: determine the suitable Luminaire spacing

• Luminaire spacing should always be determined in mounting heights. Typical distance from luminaire to the edge of an area should not exceed one half of the spacing between adjacent luminaires.see below image for recommended spacing guide:

“Rule of thumb” of Spacing for pole-mounted floodlights:

When more than one pole is added, pole placement is a concern. The “4 times“ rule of thumb for spacing indicates that a pole should be placed four times the mounting height from the adjacent poles.

For example: If a flood is mounted on a 20-foot pole, space the poles 80 feet apart.
Pole spacing = 4 x mounting height = 4 (20-ft. pole) = 80 feet between poles

Step-8: determine the suitable Aiming of Luminaires

See Step-2 in above for knowing the suitable field angle degree for selected luminaries.

“Rule of thumb” of Aiming for pole-mounted floodlights:

1- Vertical Aiming

A single floodlight uses the two-thirds rule of thumb for vertical aiming. The fixture is aimed two-thirds of the distance across the area to be lighted and at least 30° below horizontal.

For example:
If the area to be lighted is 40 feet across, the recommended aiming point is 27 feet.
Aiming point = 2/3 across distance to be lighted = 2/3 (40 ft.) = 27 ft. aiming point

• Additionally, to minimize glare, the recommended aiming point distance should never exceed twice the mounting height. If a pole is 20 feet high, the vertical aiming point should not exceed 40 feet out.

So, The vertical aiming point = 2 (20 ft. mtg. ht.) = 40 ft.

2- Horizontal Aiming
• When an additional floodlight is added to a single pole, horizontal aiming also must be considered.
• First, each floodlight should be vertically aimed according to the two-thirds rule. As long as the floodlight has a horizontal NEMA 6 or 7 beam spread, the floodlights can be aimed up to 90° apart.

In the next Article, I will explain Part Two: Design Procedure for The beam-lumen method as per CIE method. Please, keep following.