6 Quality and Quantity of Light

When choosing a light source for a particular installation, many factors must be taken into consideration, such as the purpose of the area you are illuminating, cost of installation and maintenance. Two of the most significant considerations in lighting installations are the Quantity and Quality of Light produced.

Some tasks or areas will require higher lighting levels than others, and so the Quantity of light produced may need to be increased, perhaps with higher wattage lamps. As a general rule, a single 150-watt incandescent lamp will produce more lumens than three 50 watt incandescent lamps. The amount of visible light produced by a lamp is measured in lumens or candela, which are discussed in a later chapter.

The Quality of the light produced is made of three general considerations:

• Glare: Reflective surfaces and unshielded lamps or filaments may be sources of glare, which is considered a level of brightness that interferes with vision and causes discomfort or eye fatigue. Glare may be reduced by shielding or repositioning the light source, or by decreasing the contrast between the source and it surroundings. Glare is more common at higher lighting levels than at lower ones. Frosted bulbs when compared to clear glass bulbs will have reduced glare effects.

• Diffusion: One of the ways to avoid annoying reflections of light and glare is to use diffusion to scatter light into many different directions. This is often achieved by using a variety of light sources, or indirect light sources, such as pointing a lamp at a white-painted ceiling to help scatter and reflect that light back into the room. This reduces the starkness of shadows and glare but does require higher lighting levels to achieve the same level of illumination.

• Colour: As discussed the chromaticity of a light source can have a psychological effect on people, with blues and greens said to be ‘cool’ (4000°K and above) while reds, oranges and yellow colours are said to be ‘warm’ (3000°K and below). If the task to be performed requires colour discrimination, then the CRI, or colour rendering index, should also be taken into consideration. For most tasks a CRI of 80 is sufficient.