FAQ

The purpose of UL Design Guideline 24480 is to help specifiers implement lighting schemes to promote circadian entrainment for day-active and night-inactive people occupying commercial, educational, and industrial spaces. The three goals of the document are to:

a. Define circadian-effective light.

b. Offer methods for measuring circadian-effective light.

c. Offer recommendations for the specification of circadian-effective lighting.

UL Design Guideline 24480 was developed using UL’s Collaborative Standards Development System (CSDS). CSDS is a consensus development framework that follows the essential standards development requirements defined by the American National Standards Institute (ANSI). This CSDS framework has been used by UL for the development of standards and guidelines in a wide range of areas for nearly 20 years. The development process for UL Design Guideline 24480 was led by a diverse task group of 14 expects that included researchers, scientists, designers, end users, and manufacturers.

Once the draft guideline document was developed it was sent out for two rounds of public comment between May and September 2019. All comments were addressed, and changes were incorporated into the guideline document as needed. The final document was published in December 2019.

UL Design Guideline 24480 is a living document subject to change based upon feedback on its ability to deliver daytime circadian-effective light to building occupants. Future versions of the document will continue to be produced following broad active participation by many diverse stakeholders.

Research indicates that light exposures at the eyes of occupants in many commercial and industrial buildings are too low during the day to support circadian entrainment. If, however, high levels of circadian-effective light are provided to the eyes of people in these spaces during the day, studies show that sleep quality and quantity increases at night, sleepiness during the day is reduced, and symptoms of depression are less prevalent.

According to the National Institutes of Health (NIH)[1], sleep is as impor­tant for good health as diet and exercise. Good sleep improves brain performance, mood, and health. Not getting enough quality sleep regularly raises the risk of many diseases and disorders. These range from heart disease and stroke to obesity and dementia. Circadian-effective lighting promotes good sleep, and therefore good health, for building occupants.

[1] National Institutes of Health. NIH News in Health: Good Sleep for Good Health. April 2021. ISSN 2375-6993. https://newsinhealth.nih.gov/

The UL Design Guideline 24480, Promoting Circadian Entrainment with Light for Day-Active People, provides a step-by-step process to help those who design or specify lighting in buildings to integrate circadian-effective lighting into their design and specification process. To specify circadian-effective light as part of a lighting design, two things are required. First, an accurate lighting simulation software package is needed to calculate the illuminance levels at locations where occupants are expected to work. Second, from those calculated illuminance levels and the spectral power distribution (SPD) of the light source, the online circadian stimulus (CS) calculator is needed to determine the circadian-effective light level at those locations.

Exposure is the combination of light level and duration. The UL Design Guideline 24480 uses a lighting metric called circadian stimulus (CS) to quantify circadian-effective light levels. The CS metric was derived from experiments using light-induced suppression of the nighttime hormone melatonin. CS values range from 0.0 to 0.7. A variety of laboratory and field research studies have shown that a CS = 0.3 is the minimum light level needed for circadian entrainment during the day if exposure durations persist for at least 2 hours. This design criterion of CS = 0.3 for two hours should be available to occupants at the same time every day. The regularity of circadian-effective light exposure is important to circadian entrainment.

Minimum amount of light: CS = 0.3

Minimum duration of light exposure: Two hours

Recommended timing of minimum light exposure: Morning hours; approximately the same time each day.

Circadian-effective lighting can be provided using the same daylighting techniques, electric light fixtures, and lighting controls as a specifier or designer would use to implement a lighting design to meet other modern visual and energy code criteria. No specialized lighting or control equipment is needed to provide circadian-effective lighting. Therefore, a lighting design meeting the criteria for circadian-effective lighting included in UL Design Guideline 24480 should not have a cost premium as compared to lighting designs that meet other applicable lighting guidelines, recommended practices, and building and energy code requirements.

Because circadian-effective lighting generally requires a lighting designer to increase light levels at the eye of building occupants, it does have the potential to use more lighting energy than lighting designed to meet recommendations for human vision alone. However, if a lighting design is done thoughtfully, this does not have to be the case. Recent studies have shown that lighting designs can provide circadian-effective lighting and stay within energy code lighting power density (LPD) limits.

A lighting specifier may need to try a number of different lighting configurations in order to meet circadian-effective lighting criteria while still meeting energy code requirements. This includes considerations to increase the availability of daylight for building occupants, including moving workers closer to windows or skylights where possible. It can include providing lighting from vertical surfaces within the field of view of building occupants. It might also include the provision of localized lighting at workstations where the light can more effectively reach the eye of the occupant. The spectral power distribution (SPD) of this localized lighting can also be selected to closely match the spectral sensitivity of the human circadian system, for example the use of blue light (between 440 and 480 nanometers), to provide greater circadian efficacy while using less energy.

If a lighting designer wishing to provide circadian-effective lighting is having difficulty staying below energy code LPD limits, an additional consideration could be to provide a lower CS level, for example a CS = 2.5, for a longer period of time over the day, for example for a period of 3 hours. In this case, lighting energy use may be similar to a design providing a CS = 3.0 for two hours, but the connected load of the lighting would likely be lower.

As with any lighting design, it is important to design circadian-effective lighting to minimize glare for building occupants. There are several strategies that a designer can use to help ensure that the lighting is visually comfortable. One strategy is to select lighting fixtures that provide good optical control of the light and limit direct view of the light emitting diode (LED) source within the fixture. The UL Design Guideline 24480 recommends that designers select a lighting fixture with a luminance below 8,500 candelas per square meter (c/m²) in order to limit the potential of the fixture causing glare for building occupants. A designer can also select light fixtures that distribute light indirectly onto building surfaces such as the ceiling and upper walls of the space. This will reduce the contrast between individual light fixtures and their surrounding surfaces, making them more comfortable to view.

Circadian-effective light must reach the eyes of building occupants. When calculating, specifying, or measuring light for circadian effectiveness, it is assumed that vertical illuminance at the expected location of the occupants’ eyes can be used to estimate circadian-light exposures. For example, if building occupants are expected to be seated at desks most of the day, a vertical illuminance in the direction of the expected view of the occupant, approximately 1 meter above the floor plane of the space, should be calculated or measured.

Specifiers may wish to use the International WELL Building Institute circadian lighting design recommendation or some other circadian lighting recommendation that requires a different lighting design criterion. The WELL recommendations use the metric equivalent melanopic lux, or EML, to characterize circadian-effective light. Functionally, the light levels recommendations provided by the WELL Building Institute are similar to, but not exactly like, those offered in UL Design Guideline 24480. The WELL recommendations, however, require a higher light level and a longer duration of exposure than the UL Design Guideline 24480. The CS calculator can be used to determine either CS or EML, allowing a designer to compare the two metrics.

In addition to its recommendation of CS = 0.3 as well as provisions to calculate EML, the UL Design Guideline 24480 offers circadian-effective light level recommendations in terms of vertical photopic illuminance (EV) at the eye. This simplification removes the need to use the CS calculator.

Without needing to determine the spectral power distribution (SPD) of the light source being used in a design, specifiers can provide 500 Iux of “white” light at the occupants' eyes. Typically, in an office where people are seated at desks, occupant eye level is approximately 1 meter above the floor plane. Therefore, using this simplified method, an EV of 500 lux should be provided at this height throughout the space continuously for at least 2 hours. An EV of 500 lux was selected based on an analysis of the SPDs of many available LED “white” light sources. This analysis has shown that this simplified EV recommendation of 500 Iux will provide a CS = 0.3 for about 90% of these light sources. Much lower values of EV can be used if the CS calculator is employed.

It is possible to determine the circadian-effectiveness of the lighting in an existing building and take steps to help ensure that the maximum number of building occupants have access to circadian-effective light. The first step in this process would be to measure the light at the eye level of the occupants throughout each space using a portable, hand-held device called a spectroradiometer to record vertical illuminance and the spectral power distribution (SPD) of the light at each measurement location. It is important that this device be properly calibrated and photometrically accurate. Once this lighting information is collected, the online CS calculator can be used to determine the CS at each measurement point, and a CS “map” can be created of each space. Alternatively, if a vertical illuminance of 500 lux is measured at eye-level locations within a space, it can be assumed that these locations provide circadian-effective lighting, using the simplified measurement method provided in UL Design Guideline 24480.

It is likely that some areas within spaces throughout a building will provide enough light to be considered circadian effective. In a well daylighted space, these areas will likely be near windows. Therefore, it may be possible to move workstations so that building occupants are closer to windows, and orient desks so that these windows fall within occupants’ typical daily fields of view. If this is not possible, a building manager could consider setting up a flexible work or meeting place in the area(s) with the highest circadian-effective light levels. Building occupants can then go to these areas for periods of time, for example for two hours each morning, to have access to this light. A workplace might also consider a program that would allow occupants to work for two-hour periods each morning outdoors when weather permits, or in an atrium or other well-daylighted area, if one exists.

The strategies to help ensure access to circadian-effective lighting while working from home are functionally the same as for any workplace. Moving workstations nearer to windows and orienting desks so that the worker faces the windowed area are good approaches. Of course, it is also important window blinds or shades are kept open as much as possible during daytime hours, especially in the morning, unless there is direct sunlight reaching the eyes. If possible, workers might also consider working outside for a couple hours every morning, weather permitting.

If daylight is not available in a home workspace, consider the use of task lighting, such as a table lamp or lamps with a translucent, light-colored shade, placed close to the work area (within arm’s length), for example on either side of a computer monitor, to help ensure that the light gets to the eyes of the worker. These table lamp(s) should provide a total of at least 3,000 lumens. If two table lamps are used, each should provide at least 1,500 lumens. The use of two lamps with lower lumen outputs would likely make them more visually comfortable for the worker.