Lux and Kelvin - An Overlooked Relationship Between Illuminance and Color Temperature

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When planning lighting for a space, many people take a disjointed, two-step approach in determining their lighting needs. The first step is typically about determining the quantity of light needed, and may involve questions like, "how many lumens do I need?" based on the activities being performed in the space, as well as personal preferences. Once the brightness needs are estimated, the second step, then, generally becomes about the quality of light: "which color temperature should I choose?" or "do I need a high CRI light bulb?"

While many people approach the questions of quantity and quality separately, research suggests that there is a very strong relationship between brightness and color temperature when it comes to lighting environments that we find pleasing or comfortable.

What exactly is the relationship, and how can you be sure that your lighting installation provides not only optimal levels of brightness, but the right levels of brightness given a particular color temperature? Read on to find out!

What is the difference between illuminance and color temperature?

Before we dive into the core question of the relationship between illuminance (lux) and color temperature (degrees Kelvin), we'll clarify exactly what we mean by these two terms as well as differences, since surprisingly, the two concepts are quite intertwined and can get a bit confusing.

Illuminance, measured in lux, tells us how much light falls onto a particular surface. When we refer to the term "brightness," the illuminance value is what we ultimately care about, because the amount of light reflecting off of objects is what determines whether or not the lighting levels are sufficient for activities such as reading, cooking, or artwork.

Keep in mind that illuminance is different from lumen output (e.g. 800 lumens) or incandescent wattage equivalent (e.g. 60 watt), which are commonly cited metrics that refer to a bulb's light output. Illuminance is measured at a particular point such as table top surface, and can be affected by things like the distance away from, as well as the orientation of the light bulb. Lumen output, on the other hand, is a measurement that concerns the light bulb itself. Just knowing the lumen output alone would not be enough to know if its brightness is sufficient; we also need to know more about the space such as the room's dimensions.

Color temperature, measured in degrees Kelvin (K), tells us about the apparent color of the light source. It is generally described as being "warmer" for values that are closer to 2700K that mimic the soft, warm glow of incandescent lighting, while being "cooler" for values higher than 4000K that mimic the crisper color tones of natural daylight.

The thing to keep in mind here, is that it can be tempting to describe higher color temperature light sources as "brighter" than warmer color temperature light sources, even if they have the same lumen output. This is completely understandable and even intuitive, given that incandescent bulbs exhibit a dimming curve that shows a clear, positive relationship between brightness and color temperature. As the bulb is dimmed, the color temperature gradually drops from 2700K, all the way down to 2000K and below.

From a technical lighting science perspective, brightness and color temperature are two distinct properties that separately describe quantity and quality, respectively. Unlike incandescent bulbs, for LED bulbs, brightness and color temperature are completely independent of each other in terms of their specifications. For example, we offer a line of A19 LED bulbs under our CENTRIC HOME™ line which feature 800 lumens at 2700K and 3000K, but also a very similar product under the CENTRIC DAYLIGHT™ line which have color temperatures of 4000K, 5000K and 6500K, also with the same 800 lumen output. In this example, both bulb families have the same brightness, but have different color temperature options, and the two specifications should be understood as two different concepts.

What is the relationship between illuminance and color temperature?

Theoretical and empirical research suggests that we generally find low brightness environments pleasing under low color temperatures, while high brightness environments tend to be more pleasing under higher color temperatures.

This relationship is named the Kruithof curve, named after the Dutch physicist Arie Anders Kruithof who studied this phenomenon.

A significant implication here is that when planning for lighting installations where a higher color temperature is desired, care must be taken to ensure that there is enough brightness across the space; otherwise, the lighting environment can be perceived as unnatural or displeasing.

For example, an illuminance level of 200 lux (20 footcandles) is good level of brightness for a kitchen or living room installation at 3000K. If, however, you were interested in a higher color temperature such as 5000K or higher, an illuminance level of 200 lux would appear unnaturally blue, dull and unpleasant.

On the other hand, illuminance levels that are too high for a given color temperature can lead to objects appearing unnaturally red.

What are the minimum illuminance levels recommend for each color temperature?

Based on some of the findings in the Kruithof studies, we have put together some rough guidelines below for common color temperature points and the recommended illuminance levels that correspond. Keep in mind that these are rough estimations only, and your personal preferences can also play a role.

  • 2400K: 40 - 80 lux

  • 2700K: 80 - 250 lux

  • 3000K: 100 - 400 lux

  • 4000K: 200 - 10,000 lux

  • 5000K: 300 lux or higher

  • 6500K: 450 lux or higher

Here is the same information, listed in footcandles:

  • 2400K: 4 - 8 footcandles

  • 2700K: 8 - 25 footcandles

  • 3000K: 10 - 40 footcandles

  • 4000K: 20 - 1,000 footcandles

  • 5000K: 30 footcandles or higher

  • 6500K: 45 footcandles or higher

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