Visual perception and its impact on emergency illumination design

Learning objectives

• Examine the basic mechanics of the human eye and how they relate to low-light vision.
• Review NFPA 101: Life Safety Code and International Building Code (IBC) requirements for emergency illumination.
• Examine common-sense emergency lighting design concepts that aren’t in lighting codes.
• Compare standards in wayfinding signage for various parts of the country.

The purpose of emergency lighting is to provide adequate illumination and wayfinding guides to allow occupants to quickly and safely evacuate a building. The speed with which a person can evacuate a building during a fire directly impacts that person’s risk of injury or death. This fact becomes even more pertinent in large buildings, like high-rises, where longer evacuation times with more convoluted egress routes out of the building are the norm.

Although the human eye has an incredible ability to see in a wide range of lighting conditions, a person’s ability to interpret what they see may be impaired due to any number of reasons: lack of familiarity with surroundings; fatigue; exposure to a high-stress situation, such as a fire or similar disaster; etc. With what’s at stake, emergency lighting designers must not only understand simple prescriptive code requirements, but they also must go beyond what is required and learn how to effectively leverage human-behavior concepts. Those concepts may seem to be common sense, but they are often ignored.

The human eye and low-light vision

It is true that the neurological responses associated with vision are extremely complicated and, even after years of research, still not fully understood. However, to design effective emergency lighting, it’s important to have a basic understanding of how the human eye responds to light.

There are two types of photoreceptors in the human eye: rods and cones. Through a combination of inputs from these two different types of photoreceptors, there are three primary modes of vision:

• Photopic vision: full-range color vision under normal illumination.
• Scotopic vision: monochromatic night vision.
• Mesopic vision: the midrange overlap of photopic and scotopic vision.

The level of luminance (amount of light emitted by a source) determines which of these vision modes is dominant.

Note that luminance is not the same as illuminance (amount of light falling on a given surface). While values of luminance (cd/m²) may appear to be used interchangeably here with illuminance (footcandles), the intent is to illustrate typical illuminance conditions at which any particular luminance is expected. This distinction between illuminance and luminance is extremely important and will be expanded on when discussing practical emergency lighting applications. Code requirements focus on illuminance levels, but the perception of brightness is dependent on the luminance of whatever you are looking at.  For example, both a black and a white surface can have the same illuminance (footcandles) under a given light source. However, one will be perceived as being brighter due to that fact that it reflects more of the light, therefore it has a higher luminance.

Photopic vision is attributed primarily to sensory input from cones when exposed to luminance above 3 to 10 (cd/m²) (illuminance ~0.3 fc). This lower threshold of luminance for photopic vision roughly corresponds to that from a bright full moon on a clear night. In this mode of vision, the eyes are sensitive to wavelengths of light from roughly 400 nm to 700 nm—this corresponds to a color-range spectrum from violet to red.

Scotopic vision is attributed primarily to sensory input from rods when exposed to a luminance of less than 0.001(cd/m²). This level of luminance roughly corresponds to the light present on a moonless night with only stars in the sky. In this mode of vision, the eyes are sensitive to wavelengths of light from roughly 380 to 650 nm (violet to orange). Rods are insensitive to red sources of light. Even with this spectral-range sensitivity from violet to orange, all sensory inputs are perceived as being monochrome. You are functionally color blind when this mode of vision is dominant. In addition, visual acuity becomes fuzzy, making it difficult to resolve fine details, such as text on signs. Scotopic vision primarily allows the identification of shapes and motion. While the temptation is to emphasize the primary characteristics of scotopic vision when considering how to illuminate an egress path, it should be noted that the eyes require an extended period of time to become fully dark-adapted (up to 30 minutes). This length of time is usually greater than the acceptable maximum amount of time required to evacuate a building.

Although the human eye can “see” luminance values spanning over 10 orders of magnitude, the vast majority of emergency lighting scenarios involve illumination levels under which mesopic vision is dominant. In mesopic vision, both rods and cones are active. The luminance range associated with mesopic vision spans roughly 3 orders of magnitude and overlaps the high end of scotopic vision and low end of photopic vision. This is a transitional state—the perception of motion, shapes, color, and field of vision will be unstable and unpredictable.

Codes and standards for emergency lighting 

The International Building Code’s (IBC-2018) guidance for general lighting is a brief, one-size-fits-all statement in Section 1204.3: “Artificial light shall be provided that is adequate to provide an average illumination of 10 fc over the area of the room at a height of 30 in. above the finished floor.” While the Illuminating Engineering Society Handbook may have expanded recommendations regarding the level of illumination that should be provided for a given task, ultimately, the Illuminating Engineering Society of North America’s (IESNA) illumination-level recommendations are not required by code. What is appropriate is a qualitative, and not quantitative, assessment.

However, emergency lighting is different because there are specific illumination levels dictated for the designated components of means of egress by both NFPA 101-2018: Life Safety Code and the IBC. The required illumination for emergency lighting is an order of magnitude less than what is required by IBC for general illumination. Table 1 summarizes the related minimum code requirements for illumination at the floor level when a building is occupied.

Figure 7: Edge glow type exit sign with chevron-type directional indicators in compliance with UL 924: Standard for Emergency Lighting and Power Equipment and NFPA 101: Life Safety Code.[/caption]

IBC, Section 1008, is slightly more specific than NFPA 101, Section 7.8, in designating emergency lighting for the additional following areas:

• Egress pathways in rooms and areas that require two or more means of egress.
• Vestibule and areas on the levels of discharge used for exit discharge.
• Exterior landings as required for exit doorways that lead directly to the exit discharge.
• Electrical equipment rooms.
• Fire command centers.
• Fire pump rooms.
• Generator rooms.
• Public restrooms larger than 300 sq ft.

The primary goal here is to illuminate the egress paths and provide wayfinding guides to allow occupants to quickly and safely evacuate a building. It is understood that under normal conditions, the general illumination in these areas will be dramatically greater than these minimum requirements. These minimum requirements only become the primary point of emphasis during an emergency condition when the primary source of power for the general lighting is lost and only emergency lighting is illuminated.

This is where the initial discussion of the three modes of vision becomes important. The code mandates an average illuminance level of 1 fc, a minimum of 0.1 fc, and a maximum-to-minimum ratio of 40:1. When operating under emergency power, emergency illumination levels are permitted to drop to 60% of those initial values after 90 minutes of operation. This is not an issue when operating from an auxiliary source with a stable voltage output, such as a diesel generator, but it is a concern for light fixtures with battery backup. Further complicating this is the fact that neither NFPA 101 or IBC requires that this drop -in illumination occur in a linear fashion. The initial illuminance level puts us at the low end of photopic vision, and after 90 minutes of emergency illumination, illuminance can decrease to a point that puts it firmly in the mesopic range. At that level, the range of luminance that can be perceived is significantly reduced as compared with normal photopic vision. This means that if unusually high levels of contrast (maximum-to-minimum ratio) are present in the path of egress, the ability for the eye to resolve any potential obstacles, identify signage, etc. becomes significantly impaired until scotopic vision takes over. Typically, in an emergency situation, the time required for this dark adaptation is unacceptably long.

Design considerations not in NFPA 101 or IBC

Simply specifying an illumination level for emergency lighting to meet the code requirement is not enough. While the code focuses on the level of illumination that needs to be provided on the floor, what good does that do if the occupants have trouble determining where the walls and doors are? This becomes a greater concern with highly directional sources of light that illuminate what they are pointed at and little else. Examples of such sources include sealed-beam halogen lamps associated with traditional wall-mounted unit battery lights. A guiding principle to remember is that wayfinding is generally accomplished by providing contrasting illumination on vertical surfaces to define the shape and volume of a space/path along with identifying any potential obstacles/hazards that may exist along that path. lThis will minimize a person’s uncertainty and increase the speed with which they can exit the building.

Shadows aren’t created only by stationary objects. When highly directional emergency light sources are used, careful consideration must be given to the quantity and location of lights in relationship to potential obstructions that may cast shadows and impact the level of illumination along the egress path. These obstructions may include items such as furniture and modular workstation panels. However, potential obstructions that are seldom considered are the occupants themselves—a person traveling along the path of egress will cast shadows as well. If there is only one source of light located behind a person, it is reasonable to expect that the person’s body will block that light along the direction of egress. If that person is in a stairwell trying to navigate down a flight of stairs, these types of considerations become even more important. The most appropriate design solution in most cases is to use more diffuse overhead light sources instead of highly directional wall-mounted unit battery lights. With the increased availability of UL 924: Standard for Emergency Lighting and Power Equipment -listed micro and mini lighting inverters and energy-efficient LED lighting, these types of solutions can be extremely cost-effective in comparison with traditional wall-mounted unit battery lights. Where the use of highly directional light sources cannot be avoided, resist the temptation to space them as far apart as possible. It is recommended that they be spaced more closely together and aimed so that the associated beams of light are slightly tangential to the path of travel. This will reduce the impact of shadows projected parallel to the path of egress and reduce the amount of glare from a light source pointed directly into a person’s eyes.

Good emergency lighting design should leverage that human tendency to be attracted to light. While there is a focus on exit sign placement to provide wayfinding along the path of egress, higher illumination levels for the emergency lighting should be used to help define the endpoints and changes in direction to the egress path. In combination with the code-required exit signage, this can dramatically simplify the decision-making process for someone attempting to escape a building. While photoluminescent tape on the floor can also be used to help define the egress path, note that the illumination provided by that tape is insufficient to meet the minimum code requirement and should only be considered as a supplemental wayfinding aid.

Exit signage visibility 

Exit signs provide easily identifiable directions to occupants on how to get out of a building. In many cases, without signage, it may not be clearly obvious to the occupants where the appropriate paths of egress are located. When appropriate signage is not present, most people will attempt to retrace their initial path. If that initial path leads back to an elevator instead of the nearest egress stairwell, how will the safety of the occupants be impacted? If there also happens to be a clear and immediate danger, such as a fire, limited visibility due to smoke, nonfunctional elevators, etc., any uncertainty or confusion in how to get out of a building may increase the potential for injury or death.

Chapter 7 of NFPA 101 provides requirements for the location of exit signs along the path of egress. The spacing between the signs, per 7.10.1.5.2, shall be “such that no point in an exit access corridor is in excess of the rated viewing distance or 100 ft, whichever is less, from the nearest sign.” Note that not all exit signs are rated for the industry standard viewing distance of 100 feet.  UL will also list exit signs for visibility from 50, 75 and 125 ft. For example, certain types of photoluminescent (glow-in-the dark) signs that require an external light source to recharge may only be rated for only a reduced 50-ft rated viewing distance and therefore require closer spacing between signs.

Many of the major requirements for locating exit signs are subjective, performance-based requirements and ultimately subject to interpretation by the AHJ:

• Exit doors, other than main exit doors that are obviously and clearly identifiable as exits, must be provided with an exit sign.
• Along horizontal egress paths where the continuation of the path is not obvious, such as an intersection of two corridors, a directional exit sign must be provided.
• Signs must be visible from the direction of travel.
• Where mounted above a door or other egress opening, the sign may not be mounted more than 6 ft 8 in. above the door. This is normally only an issue in an atrium or similar area with an unusually high ceiling.
• Where mounted to the side of a door or egress opening, the distance from the sign to the door/opening may not exceed the required width of the egress opening. For example, when mounted to the side of a 6-ft-wide double door, the sign may be offset no more than 6 ft.

One of the primary reasons why the visibility of an exit sign would be obstructed would be the presence of smoke. It is expected that as a smoke layer increases during a fire, the smoke will diffuse or absorb the light from a sign located at or near the ceiling. In some occupancy types (primarily assembly occupancies), floor-proximity exit signs are required to supplement those mounted at or near the ceiling. Where floor-proximity signs are required, the lower edge of the sign shall be mounted between 6 and 18 in. above the floor. The expectation is that if smoke is present, the floor signs will remain visible for a longer period of time and provide extra assistance for people trying to escape a smoke-filled environment, potentially while those people are crawling on their hands and knees to get below the smoke.

Exit signs must have enough visual contrast from the surrounding environment to allow quick identification. The three variables that directly impact this are the color of the letters in relation to their background, the size of the letters, and the luminance of the letters. These variables are governed by UL 924. The primary goal of the standard is to ensure that the sign is visible from at least 100 ft. While some of the tests associated with evaluating this visibility are subjective, the accepted industry standard for letters on internally illuminated signs is that they are either green or red on a contrasting background and are at least 6 in. high, 2 in. wide with a ¾-in. stroke and a spacing of 3/8-in. between letters. Where larger letters are used (i.e., 8-in. letters in New York City), sign elements shall be scaled proportionately to their height.

While the “EXIT” letters must be visible from 100 ft, when a directional indicator (chevron) is used, that indicator must be identifiable from only 40 ft. Some municipalities, such as Chicago, require indicators in excess of this standard. As opposed to a chevron adjacent to either side of the word EXIT, a full-length arrow under EXIT must extend from one side of the word to the other in Chicago.

Research by the National Institute of Standards and Technology and Lighting Research Center performed nearly 30 years ago had concluded that a minimum luminance level of 10 (cdm²) is required for reasonable exit sign visibility in clear and smoky conditions. Most modern LED exit signs are dramatically brighter than this. However, the UL 924 requirements for self-luminous signs (photoluminescent or tritium signs) have no requirements for letter color and dramatically reduced luminance values based on a subjective test (typically around 0.21 (cdm²)); they are only required to be visible from 50 ft. To put this in perspective, reference these luminance values to the three different modes of vision previously discussed.

There is no easy answer for which exit sign letter and light-source color is best. The part of the country in which you practice engineering usually determines the color of exit signs that you usually specify. While red letters and illumination are the most prevalent, advocates of green-illuminated exit signs point out that red is normally associated with “stop” or “danger” and green indicates safety. Some locations, like Chicago, don’t even allow green or red illumination. Instead, they require white LEDs with the belief that a white light source also has the benefit of providing additional ambient illumination. Ultimately, the acceptable color is at the discretion of the local AHJ.

Outside of the United States, having a sign that says EXIT has little meaning if the local population doesn’t speak English. In some parts of the world, it is entirely possible that multiple languages may be spoken by a building’s occupants, none of which may be English. While not acceptable for substitution in place of EXIT in most jurisdictions within the United States, the prevailing exit sign standard everywhere else in the world is affectionately called the “running man” (see Figure 2) and shows a green-colored stick running toward a door. The greatest benefit of this pictograph is that it is universally understood and not specific to a particular language. While an ISO standard, this pictographic is also referenced in NFPA 170-2018: Standard for Fire Safety and Emergency Symbols.

Case study: Why illuminating vertical surfaces is important

This article includes a discussion of emergency lighting strategies that are not dictated by code requirements. One of these strategies is to illuminate vertical surfaces to define the shape and volume of the egress path rather than focus strictly on illuminating the horizontal plane of the floor.

In the following example, there is a representative aisleway in a generic open-office environment. The general lighting consists of a mixture of center basket fluorescent troffers, fluorescent linear wallwashers, and triple tube compact fluorescent downlights. The downlights and wallwashers are generally clustered at the far end of the picture and the 2x4s are spaced roughly 8 ft on center toward the viewer. The downlights are highly directional, with most of the light distributed straight down, while the 2x4s are volumetric, giving a high level of vertical illumination.

The picture of the normal illumination within the space indicates that the downlights at the far end of the picture are more effective at illuminating the floor than the 2x4s. The measured illumination at floor level under the exit sign at the far end of the picture is roughly 8 fc. The measured illumination at roughly the midpoint between the viewer and the exit sign is only half of that, 4 fc—which is partially attributed to ceiling shades from the lateral files in the picture. These illumination levels may seem low, but remember that these measurements are taken at the floor per code requirements for emergency lighting, not at work-surface height, 30in. above the floor. Regardless, the illumination across the spaces looks reasonably uniform due to the high levels of light on the vertical surfaces.

The picture of the emergency illumination in the same space (Figure 3b) paints a much different story. The downlights that were intended for emergency lighting at the far end have burned out light bulbs and do not contribute any illumination. The failure of the emergency downlights was not intentional and as a result, the only sources of illumination in the picture are the 2×4 in the foreground, the exit signs, and some incidental borrowed light from a glass door leading to another corridor just around the corner at the far end of the picture. Simply comparing the visibility of the decorative pattern in the carpet between the two pictures makes it clear that the floor is very poorly illuminated at the far end of the room. The light meter barely registers 0.1 fc—the code minimum. Even near the 2×4 emergency light, illumination is only 0.7 fc at the floor. Based solely on this measured level of illumination on the floor, the emergency lighting is inadequate. However, despite the nonfunctional downlights and the associated reduction in illumination, the egress path is still surprisingly well-defined in the picture, and obstructions can be readily identified. This can be attributed mostly to the relatively high levels of contrasting illumination on vertical surfaces from the 2×4 light fixture. Even the far wall is reasonably visible due to the light from the 2×4 light fixture.  The failure of the downlights was not intentional, but it does help emphasize the point that focusing solely on the code-required illumination on the floor minimizes the role that illuminating vertical surfaces has in defining the egress path. The code doesn’t consider vertical illumination, but it is clear that light on vertical surfaces along the path of egress can significantly improve occupants’ wayfinding ability.

Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our WTWH Media editorial team and getting the recognition you and your company deserve. Click here to start this process.