The quality of a visual environment considers a wide range of variables including luminance balance, color appearance, visibility of multiple visual tasks (often accomplished by a layered lighting system), visual comfort, daylight and views, control and finally, user acceptance.
Luminance balance refers to the brightness of surfaces (vertical and horizontal) within the view of an occupant. When excessive brightness and darkness are combined in the same view, the contrast leads to visual discomfort and even headaches as the eyes continually adjust between the extremes. Balancing these luminance values means maintaining good uniformity between all the surfaces that make up a field of view. Lighting the vertical wall surfaces is one of the best ways to improve luminance balance. Since luminance, especially for interior areas, is difficult to calculate, the IES 10th Edition Handbook recommends illuminance and uniformity values for vertical surfaces such as walls. If surfaces have very low reflectance values such as dark wooden walls, these illuminance values may need to be increased.
Color appearance is made up of the correlated color temperature (CCT - the color of the light source) the color rendering index (CRI - how well the light portrays the color of objects) as well as the individual wavelengths that make up the light (the Spectral Power Distribution [SPD]).The R9 value refers to a specific rating of the red component of the light color. Office and industrial building occupants generally prefer warm to neutral color temperatures in the 3000-4100K range. Cooler colors than this can produce slightly higher visual acuity in some applications but may also create a sense of starkness and institutionalism.
Layering the lighting system with ambient, task, and accent light creates variety in spaces and makes it easier to maintain the luminance balance. An ambient layer provides a low level of diffuse lighting that can help illuminate objects and people's faces, which is helpful for general wayfinding and for face-to-face communication, a sense of brightness and illuminates the architectural surfaces, such as the walls and ceiling. Task lighting provides the actual quantity of light necessary to perform a job (such as reading paperwork or filing) and draws the eye to the work surface. Accent lighting adds emphasis and visual variety in a space. Luminance balance means keeping enough brightness variety in the space to make it interesting and to highlight work areas, without making the lighting appear spotty or full of shadows.
Visual comfort relates mostly to glare from any light source — a luminaire, window, skylight, etc. Unfortunately, glare tolerance is often subjective and varies between individuals. Glare is controlled by selecting luminaires that direct their light toward the ceiling, walls, or work areas, but not into the employee's eyes. Louvers, shields, fins, overhangs, and blinds all reduce glare from windows and skylights; louvers, lenses, baffles, and the optics of the luminaire control glare from electric sources.
Closely related to visual comfort, views to the outdoors are an essential part of visual comfort — providing a link to activity, changes in time and weather, more organic forms, and other variety for the eyes. However, care must be taken to eliminate unwanted glare and excessive brightness that can occur at some times of day at some times during the year. Glare from windows must be minimized and balanced with the other views of the space.
Lighting control is essential for the users. View-preserving blinds or shades minimize direct sun and glare from windows or skylights. Personal controls give the individual user the ability to adjust task lighting and balance brightness in their own workstation. Automatic controls for daylight dimming and occupancy save energy.
While most of these attributes also apply to exterior lighting, exterior lighting equipment must also address backlight (light trespass) uplight (light pollution) and glare. Tables 6-1 through 6-3 outline the allowable ratings based on lighting zone.
The true test of a high quality lighting system is customer feedback. Post occupancy evaluation of the daylighting, views, electric lighting, and controls will be best evaluator of the lighting quality success.
All of these components result in some level of user acceptance of the lighting system.
- Baseline: Does not have any requirements for vertical surface illuminance or color appearance, allows for CCT up to 4100K and minimum CRI of 70 with R9>0. The baseline also has no minimal requirements for task lighting, glare control, and access to views.
- ★: This level requires maximum illuminance uniformity ratios of 40:1, CCT of no more than 4100K, and CRI of at least 80. It also requires wall lighting, task lighting, luminous intensity limigs, view-preserving shades, and views for at least 50% of regularly occupied areas. Exterior luminaires also have baclight, uplight, and glare limitations.
- ★★: Raises uniformity ratios to 20:1 and CRI to 80 and R9 of at least 50. For outdoor lighting, that could have a negative effect on flora and fauna, CCT cannot exceed 4100K. Occupants should have personal control over their entire work area or task lighting and access to views is increased to 80% of regularly occupied area.
- ★★★:This performance level continues ★★ but allows for tunable CCT.Additionally, at this level, color rendering may be measured with a new color quality scale. Also at this level, views are provided for 90% of regularly occupied spaces.
Table 6-1 Backlight Ratings for Exterior Luminaires
Allowed Backlight Rating* | Lighting Zone 0 | Lighting Zone 1 | Lighting Zone 2 | Lighting Zone 3 | Lighting Zone 4 |
---|---|---|---|---|---|
Greater than 2 mounting heights from property line | B4 | B4 | B4 | B5 | B5 |
1 to less than 2 mounting heights from the property line and properly oriented** | B1 | B2 | B3 | B4 | B4 |
0.5 to less than 1 mounting height from the property line and properly oriented** | B0 | B1 | B2 | B3 | B3 |
Less than 0.5 mounting height from the property line and properly oriented** | B0 | B0 | B0 | B1 | B2 |
*For property lines that abut public walkways, bikeways, plazas, and parking lots, the property line may be considered to be 5 feet beyond the actual property line for purpose of determining compliance with this section. For property lines that abut public roadways and public transit corridors, the property line may be considered to be the centerline of the public roadway or public transit corridor for the purpose of determining compliance with this section. NOTE: This adjustment is relative to Table 6-1 and 6-3 only and shall not be used to increase the lighting area of the site.
** To be considered 'ideally oriented', the luminaire must be mounted with the backlight portion of the light output oriented perpendicular and towards the property line of concern.
Table 6-2 Uplight Ratings for Exterior Luminaires
Lighting Zone 0 | Lighting Zone 1 | Lighting Zone 2 | Lighting Zone 3 | Lighting Zone 4 | |
---|---|---|---|---|---|
Allowed Uplight Rating for Street or Area Lighting | U0 | U0 | U0 | U0 | U0 |
Allowed Uplight Rating for ornamental parking lighting and luminaires not used for street or area lighting | U0 | U1 | U2 | U3 | U4 |
Table 6-3 Glare Ratings for Exterior Luminaires
Lighting Zone 0 | Lighting Zone 1 | Lighting Zone 2 | Lighting Zone 3 | Lighting Zone 4 | |
---|---|---|---|---|---|
Allowed Glare Rating | G0 | G1 | G2 | G3 | G4 |
Any luminaire not ideally oriented* with 1 to less than 2 mounting heights to any property line of concern | G0 | G0 | G1 | G1 | G2 |
Any luminaire not ideally oriented* with 0.5 to 1 mounting height to any property line of concern | G0 | G0 | G0 | G1 | G1 |
Any luminaire not ideally oriented* with less than 0.5 mounting height to any property line of concern | G0 | G0 | G0 | G0 | G1 |
* To be considered 'ideally oriented', the luminaire must be mounted with the forward portion of the light output oriented perpendicular and away from the property line of concern.