Lumen Method for Lighting Design

The Lumen Method is a widely-used approach in lighting design to determine the number of luminaires needed to achieve a specified level of illumination in a given space.

The Lumen Method is a technique used to calculate the number of luminaires required to achieve a desired level of illuminance in a space. It considers factors such as room dimensions, surface reflectance, and luminaire characteristics to determine the lighting layout.

How does the Lumen Method work?

The method begins by determining the desired illuminance level for the space, typically measured in lux or footcandles. This illuminance level is based on the type of activity being performed in the space and any relevant industry standards or guidelines.

Next, the total initial illuminance required for the space is calculated based on its size and surface reflectance values for walls, ceilings, and floors.

Using the total initial illuminance and the lumen output of the selected luminaires, the number of luminaires needed to achieve the desired illuminance level is determined.

The Lumen Method takes into account the inverse square law, which states that illuminance decreases as the distance from the light source increases. Therefore, the spacing and mounting height of the luminaires are important considerations in the calculation.

Lumen Method for Lighting Design

The Lumen method is the most frequently used technique to determine Lighting levels, and provides the average maintained level of illumination on a horizontal plane, usually designated as the work plane. This method is applicable to systems using any type of light source. The Lumen method is usually used for indoor lighting where uniformly distributed light is required over a large area.

In a lighting system, all lumens produced by the lamps do not reach the work plane due to losses in the luminaire and the absorption of light by the room surfaces. These losses are accounted for by the application of the coefficient of utilization for the luminaire being used.

The usual criteria is to design the lighting system to maintain a minimum level of illumination throughout its life. The light output of the lamps depreciates during service, so the initial light level of the system shall be increased to account for these losses. Causes of depreciation are:

a. Loss of lumens as a result of aging.
b. Decrease in lamp and luminaire output resulting from dust, dirt, insects and chemical changes in reflecting surface.
c. Increased absorption of the light by dust, dirt and changes in the rooms reflecting surfaces.
d. Differences between actual and design lamp voltages.

Lumen Method Formula:

The mathematical equation for the Lumen Method is as follows:

a. LUX = {LL x N x CU x LLD x LDD} divided by A

Where:
LL = Initial lamp lumens per luminaire
N = Number of luminaires (fixtures)
CU = Coefficient of utilization
LLD = Lamp lumen depreciation
LDD = Luminaire dirt depreciation
A = Area in square meters.

b. In most lighting design, the required maintained Lux intensity will be a known quantity. It will be either given in basic data or selected from standard tables. Therefore, by transposing the terms in equation above, the required number of luminaries can be computed.

Lumen Method Calculation

N = {LUX x A} divided by {LL x CU x LLD x LDD}

Table I can be used to determine the required lighting level. In some cases, the  recommended illumination levels are given for specific tasks, and apply only to the area where the task is to be accomplished. In these cases, the general area around the task should have one-third to one-fifth the illumination required for the task. This technique is known as task-ambient or nonuniform lighting.

Location of the fixtures shall then be determined.

a. Perfect uniformity of illumination is usually not feasible. Some deviation from the average is to be expected. Illumination nonuniformity shall be considered acceptable if the maximum and minimum values in the area (except for extremities, for example corners) do not exceed 10 percent above or below the calculated average.

b. To achieve acceptable uniformity, the luminaire spacing to mounting height ratio, from vendor’s data (catalogs), should not be exceeded. The commonly used practice of letting the distance from the luminaires to the walls equal one-half the distance between rows often results in inadequate illumination near the walls. In areas where desks or work benches are located along the walls, a distance of 760 mm from the wall to the center of the luminaire should be used to avoid excessive drop-off in illumination. This will locate the luminaires over the edge of the desks facing the walls or over the center of the desks that are perpendicular to the walls. In critical areas, for example drafting rooms, where tables are located against the walls, it is often desirable to use somewhat closer spacing between luminaires adjacent to the walls.

c. To prevent excessive reduction in illumination, the ends of fluorescent luminaires should not be located more than 610 mm from the wall. Often, increasing the number of lamps in the end units will satisfactorily prevent drop-off in intensity at the ends of a room.

d. Spacing closer than the maximum permissible is often highly desirable to reduce harsh shadows and ceiling reflections. This will also improve uniformity, particularly when using direct and indirect type of luminaires.

Advantages of the Lumen Method:

• The Lumen Method offers a simple and practical approach to lighting design, making it accessible to designers and engineers with varying levels of expertise.
• It provides a quick way to estimate lighting requirements without the need for complex calculations or detailed photometric data.
• The method is widely accepted and used in the lighting industry, with established guidelines and standards for its application.

Limitations of the Lumen Method:

• While the Lumen Method is suitable for many applications, it may not always provide precise results in complex or non-standard lighting scenarios.
• The method relies on certain assumptions and simplifications, such as uniform surface reflectance and fixed luminaire efficiency, which may not accurately represent real-world conditions.
• For more accurate results, the Lumen Method may need to be supplemented with other calculation methods or software tools that offer more detailed analysis.

Applications of the Lumen Method:

• The Lumen Method can be applied to a wide range of indoor and outdoor lighting projects, including commercial, residential, industrial, and outdoor lighting applications.
• It is commonly used in the design of office spaces, retail stores, educational facilities, healthcare facilities, and outdoor public spaces.

Overall, the Lumen Method serves as a valuable tool in lighting design, providing a practical and efficient approach to determining lighting requirements and achieving optimal lighting solutions for various environments.