Lighting Designs

3 (b). Lighting Designs

Visual field glare, colour - daylight calculations - daylight design of windows, measurement of day-light and use of models and artificial skies, principles of artificial lighting, supplementary artificial lighting.

VISUAL FIELD GLARE

Glare is difficulty of seeing in the presence of bright light such as sunlight or artificial light.

Glare is caused by a significant ratio of luminance between the task and the glare source. The angle between task and glare source and eye adaptation have significant impacts on glare.

Types of Glare and its remedies

The different types of Glare are

1. Direct Glare

2. Reflected Glare

3. Disability Glare

4. Discomfort Glare and

5. Distract Glare

1. Direct Glare

It is caused by bright areas, such as luminaires, ceilings and windows that are directly in the field of view.

Remedy: The solution for this type of glare is to avoid exposed to bright sources directly.

2. Reflected Glare

Glare from specular reflections of high brightness of polished or glossy surfaces and can result in completely blocked vision is called reflected glare.

Remedy: It can be reduced by using polarized lenses.

3. Disability Glare

It is the reduction in visibility caused by intense light sources in the field of view.

Remedy: The solution for this type of glare is to fix photochromic lenses.

4. Discomfort Glare

It occurs due to changing light conditions and can cause squinting, eye fatigue or may even make eyes water.

Remedy: Photochromic lenses can eliminate this type of glare.

5. Distract Glare

It is mainly an annoyance created by light reflecting from the surface of a lens, or internally reflecting within a spectacle lens.

The kind of glare may cause eye fatigue, annoyance and distraction. It is also experienced at night around headlights or street lights.

Remedy: An anti-reflective treatment is the solution for this problem.

Factors affecting visibility

1. Reduction of brightness of the scence by constriction of the pupils.

2. Reduction in contrast of the scence by scattering of the bright light within the eye.

3. Reduction in contrast by scattering light by particles in the air. (Fig. 3.6).

4. Bloom surrounding objects infront of glare.

5. Reduction in contrast by reflection of bright areas on the surface of a transparent medium (glass, plastic or water).

When the sky is reflected in a lake, so that the bottom below or objects in the water cannot be seen. This is known as veiling glare.

Methods to reduce glare

1. Sunglasses are often wore to reduce glare. The polarized sunglasses are designed to reduce glare caused by light.

2. An anti-reflective coating of an eyeglass reduces the oni glare at night, from inside lights and computer screens.

3. Some types of eyeglasses can reduce glare due to the imperfections on the surface of the eye.

Measurement of glare

Glare is typically measured with luminance meter or luminance camera. Both are able to determine the luminance of objects within small solid angles. The glare of a scene ie., visual field of view is then calculated from the luminance data of that scene.

Unified Glare Rating (UGR)

It is a measure of the glare in a given environment.

It is basically the logarithm of the glare of all visible lamps divided by the background illumination (L_b).

where log is the common logarithm (base 10).

L_b-background luminance (cd/m²)

L_n- luminance of each light source numbered (cd/m²)

ω_n - solid angle of the light source seen from the observer

(sr)

P_n - Guth position index, which depends on the distance from the line of sight of the viewer.

Daylight

Definition

Daylight is the combination of all direct and indirect sunlight during the day time. This includes direct sunlight, diffuse sky radiation.

Daytime is the period of time each day when daylight occurs.

Day lighting is lighting an indoor space with openings such as windows and skylights that allow daylight into the building. This type of lighting is chosen to save energy and also to avoid over-illumination by artificial light.

Table 3.3

Daylight intensity in different conditions