Building a Home Workshop, Part 6

Tag : Halides

On a flat surface where there are few reflections, illuminance canbe easily plotted using lines of equal illuminance. These lines arecalled isolux contours. Basically, it is a diagram of the“pool of light” found beneath outside street lights -the one so beloved of writers of detective fiction!
Such a diagram is useful when designing the lighting system of acar park, or in working out the spacing of lights to provide evenillumination in a large building.
Colour Temperature
An object at any temperature will emit radiation. At lowtemperatures the wavelengths of the radiation are mostly in theinfra-red region and so cannot be seen. However, if the temperatureis increased, the object (for example, a piece of steel) will startto glow. It is then emitting radiation that can be seen. Thetemperature of the object can be measured in Kelvin (K), which isits temperature in degrees Celsius plus 273.15.
The radiation properties of a hypothetical so-called black bodyradiator mean that at 1000K it will be red, at near 3000K it willbe yellow, at near 5000K white, blue-ish white near 10,000K andpale blue near 30,000K. This means that the colour of a lightsource can be specified in terms of its colour temperature - thecolour that a blackbody radiator would be if heated to thespecified temperature.
Electric lights have widely varying colour temperatures, butbecause your eyes are very tolerant of differing colourtemperatures, the perceived colour of different light sourcesvaries relatively little. Daylight has a colour temperature ofabout 5500K, while an incandescent light bulb is around 2800K.Fluorescent tubes are available with colour temperatures from2900-6500K.
Colour Rendering
Colour rendering refers to the appearance of an object when it isilluminated by the light source under consideration. Light sourcesof similar colour temperature can have completely differentwavelength compositions and so can provide great differences incolour rendering.
A low pressure sodium lamp produces light at just a singlewavelength and so the lamp reveals only that colour. Anincandescent lamp has an output that covers all wavelengths fairlyevenly - although there is an emphasis on red. A high pressuremercury vapour lamp has a mixture of some ‘lines’ (highoutputs at specific wavelengths) mixed with a continuous backgroundspectrum and a band of energy at the red end.
Of these light sources, the incandescent lamp gives the best colourrendering, followed by the high pressure mercury lamp and then thelow pressure sodium lamp.
Colour rendering is measured on a colour rendering index (expressedas Ra) scale of 1-100, where 100 provides the best colourrendering. The Ra scale for a lamp is based on the illuminatedappearance of fourteen different colour chips.
The colour rendering of incandescent lights is very good at 99Ra,while fluorescent lights vary from 85-90Ra.
When assessing how items will look under different lighting, colourrendering is much more important than colour temperature.