Common Lighting Terms used in Outdoor Lighting
Introduction: As we move forward with our advanced trainings, it's important that we use correct lighting terminology. Here are some of the common terms that we are likely to use. The definitions are adapted from the IES.
Adaptation: The process by which the eye accommodates to a change in light level. There is both 'Dark Adaptation' where the eye adjusts to a darker scene and 'Light Adaptation' where the eye adjusts to a brighter scene. For example, we try to fill in black holes in a lighting design so the eye is not subject to an adaptation period as it tries to adapt to changes in light level as it scans a scene.
Area Lighting: The practice of lighting a scene by lighting different areas of the scene, Technically, all landscape lighting is 'area lighting'.
Brightness: A subjective term referring to a 'perceived' luminance rather than the 'measured' luminance. For example, the light from a path light at dusk is perceived to have a low brightness, while the same path light at midnight is perceived to be very bright - the contrast between the illuminated area and the adjacent area is high so the perceptual sensation is one of increased brightness of the light source.
Diffused Light: Light that scatters in many directions to produce a beam with soft edges. Light can be diffused through refraction, such as with a frosted lens; or reflected, such as with our China Hats where light reflects off the slightly irregular underside of the hat to produce a diffuse beam.
Glare: The visual sensation produced when a light source is significantly brighter that its surroundings causing annoyance or discomfort.
- Direct Glare - refers to glare resulting from viewing a bare lamp
- Reflected Glare - refers to glare from a reflective surface
Illuminance: The density of luminance on a surface area - measured as footcandles (candelas/sq, ft.) or (candelas per meter).
Light Pollution: Excessive or obtrusive illumination that intrudes upon unintended or unwanted regions. This term includes Light Trespass, but is more usually used to describe light intrusion into the sky.
Light Trespass: Illumination that enters other properties or areas outside the intended region of illumination
Luminance: Light projected from a lamp source or reflected off a surface, measured at a point in space. Brightness is a term that includes luminance and the intensity of perceptual sensation; it is highly subjective and variable - luminance is a better term for lighting professionals because it can be measured and predicted - as candelas.
Reflectance: The ratio of luminance level that strikes a surface to the luminance level that is reflected off the surface. For example, a brick wall has a low reflectance value since much of the light is absorbed by the surface, while the reflectance of a white wall is very high since nearly all the light is reflected. Note: 'Reflectivity' is an incorrect term.
Reflectance Value: This is a value ranging from zero (for a flat black surface) to one (for a mirror).
Reflected Light: Light that reflects off a surface. For example, our path light project only reflected light. Light that bounces off walls, eaves, and plants is reflected light.
Refracted Light: Light that changes direction as it passes through a medium, such as glass. For example, the linear spread lens refracts light to produce an elongated beam.
Specularity vs. Diffusion: Specularity is a surface property that quantifies how perfectly light reflects off a surface. A mirror is perfectly specular. If the surface is rough or irregular than light bounces off the surface in many directions - this is referred to as diffusion. Glossy paint is highly specular while matte paint is diffuse. This is important when we talk about lighting surfaces. Rough building surfaces (and even matte paint) are diffuse to the extent that an uplight will reflect off the surface in every direction (including down into the garden bed).
Visual Angle: The angle, measured in degrees, of a light source from the observers point of view. For example, a tree light should be mounted at a minimum visual angle of 53º since lower angles present direct glare into the eye.
Visual Comfort: A subjective term that qualifies the presence or absence of visual discomfort caused by glare and highly contrasting and/or bright light levels. We can use this term in a general way, but keep in mind that the IESNA has developed an interior lighting standard for visual comfort (Visual Comfort Probability - VCP). This standard has not yet been defined for exterior lighting.
Visual Destination: The endpoint of a visual path.
Visual Path: The path that our vision takes as it glances from one place to another. Visual paths often start and end at focal points.
Created on 02/24/09