Issues of Day Lighting and Combined Illumination

Light is the part of the electromagnetic radiation spectrum that can be seen by the human eye. The wavelength lies between 400 and 700 nm.

Light is divided into: •	Day light, artificial light and joint light •	Dazzles

Measurement and evaluation of light:

•           Luminous intensity I, unit W/sr L is the wavelength-weighted power emitted by a light source in a particular direction per solid angle. It is based on the average visual sensitivity of the human eye to light of different wavelengths. 1 Candela (cd), the SI unit for light strength, is defined as the intensity of monochromatic electromagnetic radiation in a given direction with the frequency of 5,40 × 1012 Hz and with the radiant intensity being 1/683 W per steradian.

Steradian (sr) is the SI unit of solid angle. It denotes the angle when the surface area equals to the square of the radius. Solid/spatial angle is a 2D angle in a 3D space, which is subtended by an object (subtended angle is an angle whose rays will pass through the end points of an object). It measure how large that object appears to an observer looking at it from a point. A small object nearby may subtend the same solid angle as a larger object farther away, that’s why the moon can cover the sun! •	Brightness B, unit cd/m2 B is the perception of the luminance of a visual target •	Luminous flux Φ, unit lm Φ is the quantity of the energy of the light emitted per second in all directions 1 Lumen (lm) is defined as the energy (W) of the uniform point light source that has an intensity of 1 cd and is contained in 1 unit of spatial angle (1 steradian)

•	Luminosity E, unit lux 1 lux (lx) is defined as the amount of illumination provided when one lumen is evenly distributed over an area of 1 m2.

Natural (day) lighting

Day lighting is essential for our every day light, since it provides us with sight which we are largely dependent to perform daily tasks. The fact that we are adapted to work during the day can be proved by the fact that light is a zeitgeber (German: time giver) for our circadian rhythm. Day light is not the same throughout the day; therefor the intensity is expressed as factor of day illumination. D = E internal/E external in % Day lighting of a room is mainly lateral in origin, even though humans are better adapted to light originating from above. Measurement of day lighting can be done at level of the whole room or at the point of an individual working area. At whole room level, the measurement is done at a comparable level in the whole room, e.g. 0.85-0.90 m above the floor. For individual working areas the measurement is done at that specific place.

The amount of daylight should ensure E to be sufficient for the supposed activity.

Visual stress (Meares-Irlen syndrome or Scotopic sensitivity syndrome)

Visual stress is the experience of unpleasant visual symptoms when reading, especially for prolonged periods. The reader may feel like the text changes in shape, color and clarity and even that the texts moves. This can cause sore eyes, headaches, and impaired comprehension. Visual stress can affect the child’s development of reading skills, because it decreases comprehension and makes reading an unpleasant and irritating activity that children will tend to avoid. Research has shown that 15 - 20% of people suffer visual stress to some extent, and they also tend to be hypersensitive to fluorescent lighting and flicker on computer monitors.

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