retrofit: How does the committee define “low vision”?
Woods: One of the ideas that came out of the workshop is that we needed to have a common lexicon. The lexicon that’s used clinically is somewhat different than what’s used in design, and we’re wrestling with that.
The committee has defined low vision as “chronic visual impairments that cause functional limitations or disability.” The committee further defines each of those terms. Chronic means low vision cannot be corrected with medical or surgical intervention or with refractive correction, so putting glasses on doesn’t correct it. Visual impairment means loss of visual acuity, loss of contrast sensitivity, loss of peripheral vision or occurrence of central blind spots. Functional limitations means increased difficulty in reading, mobility, visual motor activities and interpreting visual information. Disability means unable to perform usual or customary daily activities.
[Editor’s note: The committee also uses the term “normal vision” as a means of excluding the aging and low-vision populations.]
r: How is working with ophthalmologists and optometrists informing the guideline?
Woods: We’ve learned we don’t see with our eyes. We see with our brain. The eye is just the sensor. The signal the retina transmits based on light is converted and goes through several neurological processes before it gets to the visual cortex, which tells you about the color, depth, form and motion. A visually impaired person perceives the environment differently than a normal-vision person in the same room.
Furthermore, we found the term illuminance doesn’t mean anything, clinically. Illuminance is the amount of light that impinges on a surface, not on the eye. Luminance is the physiologic metric of the amount of light that enters the eye. It may come from a direct source, like a window or if you look into a lighting fixture or at a surface and get the reflected light off that surface. The direct
component and reflective component make up the total amount of luminance that goes into the eye.
Currently, lighting designers design based on illuminance. They only design based on luminance in special situations, like in a museum or a theater where everyone is looking at a screen. In addition, all the codes and standards are based on illuminance.
Meanwhile, contrast is a function of luminance—not illuminance—and glare is a function of contrast. We cannot design to suppress glare unless we understand luminance. Unfortunately, luminance is something the design community has not been taught to deal with.
r: Green and energy-efficiency goals are major drivers of lighting design right now. Are these goals helping or hurting building occupants with low vision?
Woods: Lighting power density [LPD], which is watts per square foot allowed for lighting, has been dropping in ASHRAE 90.1 since 1989. We’ve gone from incandescents and fluorescents to compact fluorescents and high-intensity lamps and now LEDs. We’ve been able to increase the amount of illumination for lower wattage levels. But there’s no direct linkage between LPDs and the quality and quantity of lighting in a space. The newer lamps and fixtures with lower LPDs have tended to reduce the quality of lighting by decreasing the color rendering index, shifting the correlated color temperature to higher temperatures—from “warm” to “cool” tones—and increasing glare because of the smaller sources of light. Because of these characteristics, some of the members of our committee have been working with ASHRAE to be more explicit about making an exception to the LPD requirements for populations where there’s low vision.