HOW TO CHOOSE THE RIGHT LED LIGHTING PRODUCT COLOUR TEMPERATURE

LED lighting offers many benefits and features that were difficult, if not impossible to offer with other lighting technologies. Many of the obvious benefits, such as substantial operating energy and cost reductions, longer life, and lower overall heat generation are generally well known. Another feature that allows for dramatic appearance and productivity benefits now and in the future, involves the colour temperature of the light produced by LEDs.

LIGHTING COLOUR TEMPERATURE MEASUREMENT BEGINNINGS

"White light" is commonly described by its colour temperature. Measuring the hue of "white" light started in the late 1800s, when the British physicist William Kelvin heated a block of carbon. The block of carbon changed colour as it heated up, going from a dim red, through various shades of yellow, all the way up to a bright bluish white at its highest temperature. The measurement scale for colour temperatures, which was named after Kelvin as a result of his work, was based on Centigrade degrees. However, since the Kelvin scale starts at "absolute zero", which is -273°C, you can get the equivalent Centigrade temperature (compared to the visible colours of a heated black body) by subtracting 273 from the Kelvin colour temperature.

COLOUR TEMPERATURE SCALE APPLICATION

The term used in general illumination is correlated colour temperature (CCT). CCT relates to the colour of light produced by a light source, and uses the Kelvin temperature measurement scale (SI unit of absolute temperature). It describes the relative colour appearance of a white light source, indicating whether it appears more yellow/gold ("warm") or more blue ("cool"), in terms of the range of available shades of white.

Many people are now familiar with the idea of a "warm" white or a "cool" white being offered by fluorescent and other light bulbs. These bulbs have vastly different colour temperatures. The "warm" bulb often has a colour temperature of 3,000K and casts a more orange/red light on objects. Because you normally associate warmth with red or orange objects, this accounts for the "warm" descriptive name, even though it is a cooler (lower) temperature on the Kelvin scale. A "cool" white bulb commonly has a colour temperature of 4,100K and higher on the Kelvin scale. This is in the low range of blue colour, similar to ice, therefore earning the "cool" description.

COLOUR TEMPERATURE MEASUREMENT BASICS

Today, colour temperature can be measured simply using a handheld meter, such as a Sekonic C-500R meter (shown in figure) where a reading can be taken in seconds with the press of a button. When the button is pressed, readings are taken from a group of sensors (blue, green, and one of two red light sensors behind the shield) and processed through algorithms to produce a Kelvin temperature reading that can be used for photography, verification, or simple reference purposes.

It must be noted that when using a handheld meter such as a Sekonic C-500R, it must be held closely to the light source or in the most neutral (average colour, such as in the center of a room or space) in order to get an accurate reading of the colour temperature of the light source and not the surrounding environment. For instance, if the light is held close to a red wall, the red reflection from the wall will skew the reading to the red end of the scale. Likewise, if held next to a white object or wall, the reading will be skewed higher. These skewed readings are illustrated in the Skewed Colour Readings figure at right.

COLOUR TEMPERATURE SELECTION CONSIDERATIONS

So with choices now in lighting colour temperatures, the questions that often arise are: "How do I know what colour temperature I should choose? Should I have a certain colour temperature in my warehouse, a particular colour temperature in my offices, and then a specific colour temperature in my lobby?" In some cases the answer may be the same colour temperature for all of those applications, or it could indeed be different for each depending on the desired and/or required effect. Below are some suggestions on how to select the right colour for your application.

COLOUR TEMPERATURE AND PURPOSE

Colour temperature in a room or space can have a dramatic affect on the people that use that lighting to perform tasks. For example, rooms that are used for tasks such as reading can benefit by incorporating lighting on the cooler end of the scale. Many studies have been done on classroom and work settings where reading improves when done under lighting near 5500K (Day White - 5500-6000K). Effects on School Children with Lighting Colour Temperature Study" for additional information regarding the effect of colour temperature in a school classroom. Warehouses also typically benefit from brighter white lighting, helping to see objects better and easier.