A lighting solution is designed to achieve both quantitative and qualitative lighting requirements for the visual enjoyment of an indoor or outdoor environment (referred to as the application space).
The qualitative objective is achieved by ensuring the basic illumination requirements associated, for example, with changing light situations based on contextual needs, glare limitation, comfort and well-being of the operators, and energy efficiency.
The quantitative objective is realized by ensuring the right amount of luminous flux on the application space required by the specific visual task. The most common parameter is the illuminance (expressed in lux), the photometric quantity that represents the surface density of the luminous flux illuminating the visual task area.
The purpose of lighting calculation is:
- For design, as it allows defining the number and power of devices to be installed, according to current standards;
- For verification, as it ascertains the starting brightness level of an application space.
Lighting calculation meets the basic quantitative requirement to achieve a certain illuminance value. In this regard, national and international regulations provide recommendations on the required illuminance values, depending on the visual task area and the application space.
Lighting calculation is done by examining a series of parameters useful for classifying the application space.
The main parameters to consider are: the dimensions of the environment and its respective illuminance value, the type of device to install, the utilization and maintenance factor, the reflection coefficients of walls and ceilings, and the level of maintenance.
Another important parameter for defining the calculation is represented by the reference values from the technical sheets of the lighting devices to be installed.
Manufacturing companies provide detailed technical sheets of the lighting devices they design. They specifically indicate luminous flux, luminous efficiency, absorbed electric power (which helps measure the cost-effectiveness of the lighting body), emission power, and lifespan.
The lighting calculation methods to apply vary depending on the project’s lighting goal.
For the artificial light objective, the total luminous flux method and the point flux method are applied. For the natural light objective, the Daylight Factor (FLD) is considered.
The latter can be determined using numerical methods for evaluating natural light illuminance, which also allow obtaining photorealistic images of illuminated environments.
The “Utilization Factor” method allows for an initial sizing of the lighting system in an environment, determining the number of light sources needed to uniformly illuminate the application space.
The results obtained can later be subjected to a verification of the actual spatial distribution (point by point) of the achieved illuminance. This approach allows for a more precise sizing of the lighting system.
Lighting calculation is done using computer calculation tools that allow studying lighting conditions through the simulation of the lighting solution in a virtual environment. Once the model is created, changes, additions, and variations can be made, allowing for a tailored lighting solution.
The use of computer calculation tools for lighting calculation provides two types of results:
- qualitative because it allows for a realistic visualization of the object;
- quantitative or photometric because it allows determining photometric quantities (luminance and illuminance).
The calculation is done by applying a lighting calculation formula that, starting from the average illuminance values to be achieved, establishes the necessary luminous flux, that is, the number of devices to install.
The units of measurement required for lighting calculation are Lumen (lm), Lux (lx), and Nit (nt).
- Lumen (lm) calculates the luminous flux or the amount of light irradiated in all directions by a light source;
- Lux (lx) calculates illuminance, or the relationship between the luminous flux incident on the surface and the surface itself (lm/m2);
- Candela (cd) calculates luminance or the relationship between the light intensity emitted in that direction and the emitting surface (cd/m2).
For the total luminous flux method, lighting calculation is based on the following formula:
Ftot = A x Em / (u x ƞ x D)
In detail:
- Ftot corresponds to the flux emitted by all devices;
- A corresponds to the illuminated surface area;
- Em corresponds to the average illuminance on the surface;
- d corresponds to the depreciation coefficient;
- u corresponds to the utilization coefficient;
- ƞ corresponds to the efficiency of the device.
If we apply the total flux method to perform lighting calculation for an LED solution, the following formula would be used:
Ftot = A x Em / (u x MF)
- Ftot corresponds to the flux emitted by all devices;
- A corresponds to the illuminated surface area;
- Em corresponds to the average illuminance on the surface;
- u corresponds to the utilization coefficient;
- MF corresponds to the maintenance factor.
The factors that contribute to performing the lighting calculation are:
- The utilization factor, which depends on the characteristics of the light fixture, the geometry of the room in use, the reflective capacity of the walls, the type of chosen light source, and, in particular, the photometric curve, i.e., the angular spatial distribution of the luminous flux emitted by the source with its lighting fixture.
- The maintenance factor depends on both the degree and frequency of cleaning interventions in the environment to be illuminated and the aging process of lighting devices. The lifespan of lighting devices affects the maintenance factor. The maintenance interval closely depends on the environment in which the lighting system is located. For example, in extremely clean environments, like hospitals and offices, maintenance intervals are very long. In environments like restaurants and warehouses, the maintenance interval is more frequent. In industrial areas, interventions are more frequent.
- The reflection coefficient, crucial in determining the parameters defining the characteristics of the environment to be illuminated. It’s important to consider the contour surfaces that have the capacity to reflect the luminous flux. Such reflection coefficients depend on the nature of the wall materials, colors, and the surface treatment. Generally, reflection is indicated by a three-digit number: the first digit indicates the percentage reflection coefficient of the ceiling, the second that of the walls, and the third of the floor. The average reflection coefficients of the walls, ceiling, and floor become essential calculation factors.
The utilization and maintenance factors are defined as corrective factors as they allow adjusting the calculation of the number of lighting devices under normal operating conditions when the devices have undergone a decay or aging process.
