Colour temperature has to be one of the more counter intuitive aspects of understanding the nuances of lighting. Its purpose is to gives us a way to talk about the colour characteristics of light. The terminology used to describe colour temperature is what makes it confusing to the layperson. Colour temperature actually has nothing to do with the heat emitted from the lamp, but rather refers to the appearance of the light.
Each light source has its own unique colour, ranging from red (warm) to white (cool) and blue (daylight). Warm lights are those with low colour temperatures and kelvin numbers; these lights are more yellow or red in colour. Lights in the mid-range are referred to as cool and emit a whiter light. Colour temperature is typically measured in kelvin (K) or what’s known as absolute temperature. Light sources with the highest kelvin numbers are referred to as daylight or natural.
MEASURING COLOUR TEMPERATURE
The concept of measuring light colour comes from the idea that when heated to a certain temperature, an object will emit light and the colour of that light will change with varying degrees of heat. For example, when a black object (such as a lighting filament or radiator) is heated, it starts glowing. As it heats up, the color shifts, from deep reds (as in a fire) to oranges, yellows and white or blue light.
COLOUR TEMPERATURE AND LEDS
LEDs offer a number of benefits in terms of their colour temperature. In the early days, LEDs did not create a good Colour Rendering Index, but LEDs today have a much wider of light temperatures, from a warm white (2700k) to a white light (4100k).
COLOUR TEMPERATURE TERMINOLOGY
Kelvin (K) - Is the standard international measure of thermodynamic temperature (or absolute temperature). The Kelvin scale is named after a Scottish inventor and physicist, William Lord Kelvin who identified the need for an absolute thermometric scale in the 1800s.
The Colour Rendering Index (CRI) - Is basically a way to measure a light source’s ability to show objects naturally compared to an accepted reference. Light sources with a high CRI are typically used in photography or cinematography or other applications where light colour is critical. A light source with a CRI of 100 is well balanced in terms of its colour reproduction, shading and contrast.
Correlated Colour Temperature (CCT) – Is another term to describe colour temperature. It refers to the relative colour of light produced by a light source, using the kelvin measurement scale.
Lumen Depreciation - Light sources generally get dimmer over time, reducing their lumen output, a process referred to as lumen depreciation. While LED and fluorescent technology tend to last longer, they still lose brightness over time. In the lighting industry, L70 is the operating time (hours) until the LED Lumen output depreciates to 70% of its initial lumens. Lumen depreciation can be accelerated depending on lighting conditions such as heat, incorrect voltage or
WHAT COLOUR TEMPERATURE SHOULD YOU USE?
Choosing the right colour temperature depends on a number of variables such as individual taste, the desired mood or effect or a specific need, such as security or studying. Rooms designed for reading, such as a library will likely have lighting that is on the cool end of the kelvin scale.
When choosing lighting, it’s also important to consider the dominant colours in the space. If a space has a lot of green and blue colour, it would be better to use light sources with neutral to cool colour temperatures. If there is a mix of colours, natural white (4000k - 4500k) would be a good choice.
Residential Colour Temperature
Warm white lighting provides a calm, yellowish, relaxed light suitable for your home, a restaurant or hotel lobby or an application which requires a more intimate mood. Candlelight and wood flames range from 1500k to 2000k, while a sunrise or sunset is around 3200k, which explains why warm lights are popular in the home for the inviting and comforting light they provide.
Retail Colour Temperature
Generally retailers benefit from higher lighting temperatures (natural, cool or daylight) which highlight the appearance of products and display cases. Of course choosing the right light for a retail location depends on the product/service and the required mood.
Commercial Colour Temperature
In commercial lighting typically cool white or daylight light sources are used. Higher temperature colours produce a bright light with many applications, including display, security and garages. Studies show that certain light can be helpful for specific applications, for example in a classroom or office, reading improves when the lighting is near 5500k (daylight). For improved visibility, warehouses also might use bright, white lighting.
Until recently where LED has provided additional options, offices, warehouses, hospitals, big box retailers and other commercial spaces traditionally rely on fluorescent lighting which produces a cool white light, generally in the range of 4000 to 4500 kelvin. Outdoor lights might have different functions and colour temperatures but as an example, a standard metal halide streetlight is typically in the range of 4700 kelvin, while an HID light used for landscaping, might be 5000 kelvin.
Choosing the right colour temperature is both an art and a science and can't be done effectively without considering all the variables such as the application, purpose and desired effect of the lighting system.
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