Daylight Factor

Last updated: May 5, 2019

What Does Daylight Factor Mean?

Daylight factor is an architectural term that describes the amount of natural light within a room or structure. It is given as a ratio of the light level inside the structure to the light level outside the structure (indoor light/outside light).

Due to the negative effects of long-term exposure to certain types of artificial lighting—including those used in many office buildings—a space’s daylight factor is relevant to the occupational health of those working within it.

Safeopedia Explains Daylight Factor

Daylight factor is one of several factors that must be considered by employers to ensure that workers have access to enough light to do their job safely and without the negative health effects that come from working in poor lighting for extended periods. These considerations position daylight factor within the realm of occupational ergonomics, a field that is devoted to ensuring that workers can perform their responsibilities in an efficient manner, without negative health consequences or increased risk of injury.

The daylight factor of a given structure is relevant to two factors that are tied to occupational health and safety. First, it is used as part of an employer’s obligation to ensure that workers have enough light to operate safely (i.e., to be able to see everything in their environment) and without stress due to eye strain. Second, it is used to ensure that workers have access to sufficient amounts of natural light. This latter usage reflects research that demonstrates the importance of natural light to worker health and productivity.

Workplaces with low daylight factors and high levels of artificial light have been demonstrated to have lower productivity and reduced employee vitality, and thus they result in poorer sleep quality among employees. These effects are related to the role of natural light in regulating human circadian rhythms, as well as to the disruptive effects of artificial light on those same rhythms. The resulting loss in employee performance includes reduced safety performance, and it has been linked to serious accidents.

In some jurisdictions, many new buildings are legally required to provide inhabitants with access to a certain amount of natural light (i.e., a minimum daylight factor). Jurisdictions with this legislation include Portugal, Germany, and Singapore, but not the United States, the UK, or Ireland. These requirements are chiefly provided in terms of daylight factor but may also be provided in terms of “lux” (the amount of light available to be received by a human eye within a given space).

The importance of daylight factor as an element of workplace lighting is enshrined as part of the EU Workplace (Health, Safety, and Welfare) Regulations (1992), which require that “every workplace shall have suitable and sufficient lighting” and that this lighting “shall, as far as is reasonably practicable, be by natural light.” The amount of light necessary to be considered sufficient is not defined by this regulation.


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