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Colour temperature [K]

What is colour temperature?

Colour temperature indicates the appearance of the light supplied by a light bulb or an LED module. It is conventionally expressed in degrees of Kelvin (K) on a scale from 1000 to 10000. Kelvin temperatures for commercial and residential lighting applications generally fluctuate between 2700K and 4000K.

The reason why this colour measurement is actually using a temperature unit the Kelvin is because it is correlated by observing the colour of a piece of steel when heated to the prescribed temperature. The piece of steel will glow a different colour depending on the temperature it is heated to, varying from red hot through yellow to amber than white and finally a blueish white.

How to select a suitable colour temperature?

A good understanding of the Kelvin temperature (K) facilitates the selection of the most accommodating lighting solutions that ultimately provide you with the desired end effect and sense of well-being. So-called “warm white” light is located at the lower end of the scale (between 2700K and 3000K) and ranges from orange to yellow-white in appearance.

“Cool white” or “bright white” colour temperatures vary between 3100K and 4500K. Bulbs within this range will emit a more neutral white light and may even have a slightly blue tint. 4500K and above brings us into the “daylight” colour temperature. Light bulbs with colour temperatures of at least 4500K and above will give off a blue-white light that mimics daylight.

Bulb base

Light direction

IP Protection rating

Protection class

Class I incorporates appliances that have two levels of protection: the basic insulation and the earth connection. Such appliance consists of three wires (Live, Neutral, and Earth) connected to three different pins. Their usual colours are brown, blue, and green/yellow respectively.

Electricity travels from a power source to an appliance through a circuit. In case of an immaculate circuit, the power flows from the source to the appliance and returns to the source. The Live wire leads electric current to the appliance. The Neutral wire brings the current back to the power source. The Earth wire provides a way for the current to flow into the ground should there be shortcomings in a circuit.

The Live and Neutral wires are attached to the plastic connector which secures them to prevent any contact with the metal case. This is a so-called basic insulation. Should the Live or Neutral wire touch the metal case, there will be a consequential fault in the circuit.

Supposing that the basic insulation fails to perform, the earth connection will act as the next level of protection by using the Earth wire which is connected to the metal case. The Earth wire diverts the current into the ground and hereby prevents the current flowing through the end user’s body which can ultimately result in an electric shock. The fuse should then blow either in the plug or the fuse box, or there should be a power trip.

Class II appliances contain two layers of insulation. As in the case of Class I appliances, the plastic connector provides the basic insulation. The added layer of insulation is a plastic casing, which provides backup protection. The double insulation does not require an additional earth connection.

The only PAT test required is the insulation resistance test.

In some instances, the Class II classification is confused with the Class 2 designation; however, they are different. The Class 2 label is related to power supply, not safety.

Class III appliances use an isolating transformer that has two separate coil windings: the “Primary Winding” is connected to the power source, and the “Secondary Winding” is attached to the appliance. Each winding is wrapped around opposite sides of a common closed magnetic circuit called the “Core”. The windings have their own circuits, known as the Primary and Secondary circuits. The windings do not come in contact; hence, their isolation gives the transformer its name. Since the insulation is created by the isolated, non-touching windings, voltage needs to be passed through the windings via induction in order to carry a current.

Class III appliances do not require an earth connection. As a result, the current is cut off and cannot continue to flow in case of a circuit glitch. Therefore, the end user will not receive an electric shock.

Colour rendering index

The Colour Rendering Index (CRI) is the system utilized by the lighting industry to measure how the human eye perceives various colours. It represents the ability of an artificial light source to reveal colours of objects in comparison to a natural light source (the sun).

The colour rendering index (CRI) is measured as a number between 0 and 100. At zero (0), all colours look the same. A CRI of 100 shows the actual colours of an object. Incandescent and halogen light sources have a CRI of 100.

Typically, light sources with a CRI of 80 to 90 are regarded as adequate and those with a CRI of 90+ are excellent! The general rule is: The higher the CRI, the better the colour rendering capacity. A value below 60 represents substandard colour rendering.

CRI is entirely independent of colour temperature. They have absolutely nothing in common. For example, a 5000K (daylight colour temperature) fluorescent light source could have a CRI of 75, while its counterpart can have a CRI of 90.

This chart is a good depiction of differing CRIs, with each image having the same warm colour temperature (2700K):

How is CRI calculated?
The value of CRI for a light source is calculated by testing the colours. A scale of 8 CIE Standard Colour Samples was established by the “Commission Internationale de l’éclairage” (CIE) for the CRI method. There are 7 extra CIE Special Colours, adding up a total of 15 colour samples.

The general Colour Rendering Index examines the first 8 colour indices (Ri), with Ra being the average of the 8 Ri values and is stated as CRI with a maximum value of 100.

Special CRI refers to the remaining 7 colours (9 to 15) including the saturated colours and skin tones.

The first step of the test involves comparing either 8 or all 15 colour samples under the light source, which is followed by contrasting it with a reference light source, usually sunlight. The average differences are then subtracted by 100 to obtain the CRI value. Therefore, the light sources that display more “real” colours have higher CRI values, and the average differences between the light source and the reference sunlight are smaller.

At daylight, our lights are tested using all 15 colour samples, which include pastels, saturated colours, and skin tones for more accuracy.

Lumen output [lm]

Efficiency

A clear set of energy efficiency classes from A (most efficient) to G (the least efficient) meticulously determines the scope of energy efficiency for a particular appliance, i.e. the amount of energy a single light bulb uses. The lower the watts, the lower the electric bill.

The energy efficiency of any machine, device, or process accounts for performed activity volume for each measure of the energy. In lighting, the activity volume is measured in lumens. The electrical energy is expressed in watts. To determine the efficiency of a bulb, you need to acknowledge the amount of emitted light and the amount of electricity a bulb utilizes to produce that quantity of light.

How to Determine a Bulb’s Efficiency

The packaging should contain two crucial values that determine energy efficiency, i.e. the wattage and the lumens, or "initial lumens". Once you have found those two pieces of information, simply divide the number of lumens by the number of watts to obtain the standard measure of efficiency, which is lumens per watts.

It is best to use the actual wattage of the bulb, instead of the so-called "equivalency" value. The latter is a mere reference to the energy requirements of a standard incandescent light bulb that would produce the same amount of light.

The higher the number for lumens per watt, the more efficient an appliance: more light produced for less power. And vice versa, the lower the number for lumens per watt, the less efficient an appliance: more power is required for less light.

Here is an illustrative example.

A standard 40W incandescent light bulb uses 40 watts of electricity to produce 490 lumens. The efficiency of that light bulb is 12.75 lumens per watt.

An equivalent 40W spiral CFL light bulb utilizes 10 watts of electricity to put out 580 lumens, hence the energy efficiency is 58 lumens per watt. That is more than 4.5 times as efficient as the incandescent light bulb. Quite impressive.

A typical 40W-equivalent LED light bulb, the Cree Standard 40W Replacement LED, uses just 6 watts of electricity to produce 450 lumens. That's nearly 75 lumens per watt. That amounts to almost a 33 percent efficiency increase compared to the efficiency of the spiral CFL. In addition to that, it is nearly six times as efficient as the replaced incandescent light bulb. 

Why Lumens per Watt Matters

Higher efficiency of a light bulb leads to more energy saved, which helps reduce greenhouse emissions, decelerates the warming of our atmosphere, and most importantly, halves your monthly electricity bill. It may not make a significant reduction in cost, relative to your total utility bill, but it adds up over time.

You may install six 40 watt-equivalent LED light bulbs in place of each 40-watt incandescent bulb and get nearly six times as much light for the same price. Alternatively, you may replace the bulbs one-for-one and save at least 80 percent of the money originally intended to power your lights.

The accurate energy efficiency class shall be determined based on the lamp’s energy efficiency index (EEI) as set out in the Table below: 

Impact strength

IK ratings are defined as IKXX, where “XX” is a number from 00 to 10 indicating the degrees of protection provided by electrical enclosures (including luminaires) against external mechanical impacts. The IK rating scale identifies the ability of an enclosure to resist impact energy levels measured in joules (J). IEC 62262 specifies how the enclosure must be mounted for testing, the atmospheric conditions required, the quantity and distribution of the test impacts and the impact hammer to be used for each level of IK rating.

LED colour consistency

Figure: CIE Diagram with MacAdam ellipses (tenfold magnification of the ellipses provided a more precise view of the differences)

MacAdam ellipses are used to determine the consistency among colours from the human perception.

Specifically, one-step MacAdam ellipse is defined as the one in which interior most people are unable to recognize colour differences. If, on the contrary, we are within an area of colour variation twice as big, we will end up in a two-step MacAdam ellipse (also known as 2-SDCM), where we can start perceiving some variations between colours. Similarly, in a three-step MacAdam ellipse (3-SDCM) differences would be more noticeable.

Due to the variability of colours produced in the manufacturing of LED, the MacAdam ellipses prove to be an indispensable measuring tool to define colour dispersion between bins. Bins contained within the MacAdams ellipses inferior to the 3-steps (3 SDCM) offer exquisite results.

The method of light production changed with the introduction of the LED module. Millions of LED chips are produced daily from an assembly line during a manufacturing process, therefore colour inconsistencies between individual chips and their modules are unpreventable.

An ideal LED module assembly line will produce batches of modules that operate within a one-step MacAdam Ellipse. There will be no discernible difference between any of the module outputs. Such advanced LED modules are used in instances where colour performance and accuracy between fixtures are imperative. Typically, good LED modules are produced within a two to three MacAdam ellipse range, in which case you may spot a minor visual difference that is generally considered to be acceptable in commercial usage.

Inexpensive products will often use LED modules with MacAdam Ellipses ranging between four and all the way up to eight. Fixtures with such modules require careful handling. These budget LED modules may still be deemed acceptable and used in certain commercial or industrial areas, however they cannot completely eliminate colour sensitivity.

Light emission

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