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CRI Explained

 

CRI is a very commonly confused topic in the LED lighting industry.  By definition, CRI stands for Color Rendering Index and is measure the ability of a light source to accurately render all frequencies of its color spectrum when compared to a perfect reference light (think sunlight). Lights with a higher CRI level more accurately represent true color of an object.

For example, a parking lot light (low pressure sodium with a CRI of less than 25) makes it impossible to distinguish most colors. If a policeman were to ask you if you saw a man in a blue jacket, you would not have been able to tell him, as all jackets would appear some shade of gray. 

CRI becomes important when color is important; map reading, charts & graphs, medical procedures, etc. CRIs above 80 are considered very good levels. Anything above 90 CRI is considered excellent and would only be necessary in very fine tasks requiring precise color discrimination.

CRI is often confused with kelvin color in the LED world.  Kelvin color is the apparent color temperature the source emits. This is often described in broad terms as warm white (2700 Kelvin approximate),  daylight or natural white (5000 Kelvin approximate), etc.

Kelvin (color temperature)

The term Kelvin to describe the color of white light has a very long and complicated history. The system was named after William Thomson, 1st Baron Kelvin an Irish physicist. The system has since been simplified and falls under the International System of Units (SI).  The system can be a bit confusing as redder light (warmer) is always a lesser number and bluer light (cooler) which is a higher number; and we usually think about warmer having a higher temperature. 

 

In practical terms Kelvin is a range from approximately 1800-10,000 that describes the color of white light. The following examples are self-explanatory:

1900 K: Candlelight

2800 K: Incandescent Light Bulb

3000 K: Halogen Light Bulb

4800 K: Direct Sunlight

6000 K: Cloudy Sky

10,000 K: Clear Blue Sky

Sometimes the color is chosen simply for aesthetic reasons, and some LED lighting systems are more efficient at a higher Kelvin. Most of our customers prefer 4000-5000K which mimics sunlight and is very neutral in color. This range of the spectrum typically has very high CRI as well. You are more likely to see lower Kelvin in residential applications.

Lumens & Lux

If you're looking to understand a light bulb's brightness, you may see two metrics that may confuse you - lux and lumens. Both are related to brightness, but measure slightly different things. 

  • Lux is a measure of illuminance, the total amount of light that falls on a surface.

  • Lumens is a measure of luminous flux, the total amount of light emitted in all directions.


The farther from a light source, the lower the lux reading. This is due to the dispersion of light as one moves away from the light source.

Therefore, when you look at a lux rating for a light source, you must always make sure there is a distance associated. For example, you may see "1000 lux at 5 feet" - if you only see a lux rating, you will not know what distance this is measured at, and you will not be able to make a valid comparison.

 

When to use lux vs when to use lumens


Lux is important for knowing how bright a particular surface appears. This is the crucial metric if you want to know how bright a surface will appear, such as a tabletop, reading material or photography subjects.

Without a sufficient lux level, it can be difficult to read or take good photographs.

 

Example lux levels are listed below (approximate):

Dark, cloudy day: 1,000 lux
Indirect daylight: 10,000 lux
Direct daylight: 100,000 lux

Lux is a measure of how much light falls on a particular surface, and can be the result of multiple light sources including daylight mixed in.

On the other hand, lumens is important for knowing how much light a single light source emits. This is useful for comparing the total amount of light source emits.

Measurement method differences between lux vs lumens


Since lux is a measure of how much light falls on a surface, handheld light meters can measure the amount of light that falls on a surface. 


Lumens, on the other hand measures the amount of light that is emitted by a single light source, and in all directions. Therefore, slightly more sophisticated instruments are required. Typically an integrating sphere is used. These devices capture the light emitted in all angles, and then sums the collective light emitted. Understand that light sources such as fluorescent tubes, emit lights in all directions around the tube. This is captured by the integrating sphere, but doesn't necessarily mean all the light will be useful as much of it may be lost with in the light fixture. LEDs, on the other hand, are very directional so nearly all of the emitted light is useful. Keep this in mind when comparing different sources with each other. Often lower lumen  LED lights will produce better useful light (lux) than higher lumen fluorescent or incandescent lights, as much of the light is lost and not useful.