Vocabulary of light
Read our vocabulary of light to understand what are the main units that compose it. She will have no more secrets for you at the end of this article. We will focus on the white light emitted by LED sources.
What is color temperature?
It is the unit of color expressed in Kelvin which ranges from warm white (yellow/orange tint) to cold white (bluish tint) via neutral white. There is no absolute truth but commonly warm white extends from 1800K up to around 3500K giving way to neutral white up to 5500K and beyond that corresponds to cold white.
Small counter-intuitive subtlety, the value corresponding to warm white is lower than that corresponding to cold white. In other words, in light, the hotter it is, the colder it is. This phenomenon is observed, for example, in nature with a flame. The higher its temperature, the more its tint will be bluish (therefore cold white).
Schematic representation of color temperature
What is the difference between lux and lumens?
Lux (lx) corresponds to the quantity of light received on a given surface, i.e. the illuminance. They vary according to the distance between the light source and the illuminated surface. The further away you go, the more the lux decreases but the more the illuminated surface increases.
Lumens (lm) correspond to the quantity of visible light emitted by a light source, i.e. the luminous flux.
The angle of the light source has a direct impact on the amount of lux. Indeed, two sources having the same power in lumens but a first with an angle of 180° and a second with an angle of 120° will not have the same illumination. Below is an explanatory diagram.
lux measured as a function of distance
What is CRI ?
The CRI (color rendering index) is a value that indicates the quality of the light, the fidelity of the color rendering of an artificial source compared to the sun. This value is scaled from 0 to 100 (the maximum being the same fidelity as that of the sun). Concretely, a CRI lower than 70 is synonymous of degraded colors.
Today, the vast majority of light sources on the market have a CRI of 80, which offers good visual comfort.
Why not use sources with an CRI of 100? First, the cost (the higher this value, the higher the price of the LED), then the efficiency (the latter decreases the higher the CRI) and finally the interest. We do not need to have a CRI higher than 80 in our daily life, a CRI 80 is more than enough and comfortable enough for everyday life. However, some professions require very high color fidelity, for example in the world of photography, cinema, art, fashion professions, shop windows, exhibitions, etc.
The TM-30, the evolution of IRC
To adapt to LED sources, a new measurement method has emerged, the TM-30. The CRI is measured on 16 points, the TM-30 on 100. This takes into account notions such as fidelity, distortion of the light spectrum, saturation and many more. To illustrate, find below extracts of photometric capture of our Sunlike LED ribbon, the best color rendering
Different aspects of the TM30 measured by our Spectrogoniometer
More accurate than CRI, TM30 result differs slightly
Light efficacy
The light output is expressed in lumen per watt (lm/W), i.e. the quantity of light emitted for a consumed quantity of energy. The latter is now a key element for obtaining the energy label and above all ensures the user of having an energy-efficient solution. A decent light output these days is 120 lm/W.
When you have to choose between two light sources, remember to compare the yields. However, the information is not always indicated. It is very easily calculated by dividing the luminous flux (lm) by the electrical consumption of the product (W).
The lifespan of an LED source
We often hear about lifespan when it comes to LED light sources, but what is it? Usually, this corresponds to a reduction in the luminous flux (lm) of 20% after a given period, for example after 50,000 hours (to be validated, however, with the supplier of the LED source in question).
In some cases, you can find the following notation L90B10 (or other values like L70B20, etc.) which is a calculation of the aging of the LED. This value indicates for L the quantity of lumen in % remaining after X hours, B corresponds to the percentage of LEDs having a flux lower than L. For example, for a value L80B10 at 50,000h, this means that after 50,000h of operation, 10% of the LEDs will have a luminous flux lower than 80% of the initial flux.
To optimize the lifetime of an LED, it is essential to dissipate the heat with an aluminum profile.
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