LED 101: Understanding the Fundamentals of LED Lighting

The CRI is a number quantification of the ability of the artificial source of reproducing colors, compared with reference standard illuminant modeled after daylight.

It was introduced by the International Commission on Illumination (CIE) in 1974, because of the wide variation in the ability to reproduce colors with the white light emitted by the many types of gas-lamps then on the market. Today, with over 40 years of use, the CRI index is firmly rooted in the lighting industry and among professionals.

From 2000s onward, LED technology has exposed the limits of the CRI index test method.

LED is the first lighting source that can be used for every application and have the full range of performance and quality level, including the ability to accurately reproduce colors. This comes from the fact that LEDs are built directly into fixtures, lamps and strips, as in the example image below:

Luminous flux is the measure of brightness of a light source in terms of the energy emitted in the form of visible light. Luminous flux, in SI units, is measured in the lumen (lm). 

Depending on the application, luminous flux can be measured per lamp, fixture, per linear meter or per square meter. For lamps the most known is the luminous flux of the light bulb, in all its variants (incandescent, fluorescent, LED).

The Color Rendering Index, commonly referred to as CRI, is a method by which we can measure how color looks to the human eye, depending on the light source compared to the sun. The CRI offers a scale of values up to 100, where 100 is the best color rendering light quality and a value below zero is a very poor color rendering. The higher the CRI value (also called CIE Ra), the more accurate the colors are.

If a luminaire has a CRI of 100, this means that there is no difference in colour rendering between the light and the reference light (the sun). Likewise, a CRI of 75 means that the light bulb reproduces a 75% replication of the visible colors that the sun shows, since both lights have the same color temperature. This means that if the reference light is the light of the sun during sunset, the light source to be measured must also have the same color temperature to allow the most accurate comparison with the CRI measurement.

To obtain white light from an artificial source, a combination of different emitted wavelengths is required, something that was discovered at the beginning of the 20th century.  From then on many "recipes" to produce white light and combine the  different wavelengths were invented and still are.

Product lifetime, Light Bulb vs LED

The typical product lifetime of an incandescent light bulb is 1000 hours. When the bulb reaches the end of its product lifetime, it cannot emit light anymore. Typically, just before, there is spark or pop, as the filament inside breaks down.

LEDs use a different meaning for product life. They are the only light sources that over time lose brightness, even up to 90% of initial flux. Eventually, LEDs will also fail completely. However, some emit visible light even after decades.

For example, a Nichia LED with 60,000 hours typical product lifetime will continue to light well beyond the 60,000 hours rated life. Under normal operating conditions, it will even after 200,000 hours.

LED lifetime is the time interval the product can still serve its intended purpose.  The time passed until a LED has 70% of the initial brightness is equal with the product life, L70 lifetime.

For all reputable lighting manufacturers and sellers, LED lifetime is equal with L70 lifetime. At this point, the LED is considered end-of-life and has to be replaced.

For LED technology, from the LED chip to related products such as LED lamps, modules and fixtures, high operating temperature can result in mechanical failure and significant drop of performance.

How a LED behaves when subjected to higher operating temperatures is directly related to its quality. High quality LEDs (such as Nichia or Cree) will function within parameters at high temperatures too, while low quality LEDs will break down, change their color, loose brightness or a combination of these. 

1. complete failure of the LED
2. light output is decreased permanently (Lumen Degradation) even if the issue with high temperature is solved
3. light output is decreased temporally while the LED functions at high temperature
4. the color temperature of the white LED changes

LED strips and modules used for lighting fixture use, in general, multiple LEDs.

As operating a single LED generates heat, more is generated when multiple LEDs are mounted on a PCB, due to the mutual effect. As such, in the case of LED strips or modules, the junction temperature (TJ) of each LED gets higher, compared to a single light source. This leads to the decrease in the LEDs lifetime and luminous flux.

For LED strips and modules, a better thermal management is required to minimize TJ and allow a longer lifetime of the installed products. For this purpose, the LED pitch, the PCB base material and the use of aluminum profile must be taken in consideration.

Linear LED light fixtures and LED tubes also feature a cover that protects the LEDs and diffuses the light. This cover is usually made of polycarbonate (a resin) and sometimes of glass. 

The cover has a certain light transmission rate that impacts the light's luminous flux and glare. If a cover has a high transmission rate, it will minimize the depreciation rate of the lamp’s luminous flux by reducing the light diffusion. However, the light of individual LED’s can be visible, increasing the glare effect.

Below, we present an evaluation of four covers to showcase light transmission and glare.