The Kruithof Curve – Color Temperature VS Illuminance


Have you ever heard of the Kruithof Curve?

Back in the early 1940’s when fluorescent sources were beginning to affect the way we thought about light and color rendering, a scientist that worked for Philips named Arie Andries Kruithof performed some informal tests on how the human eye relates the amount of light in a given time of day to the color temperature of the light source.  Typically, human beings like higher color temperature light sources during the daytime hours, and lower color temperature sources once the sun goes down.  People in warmer climates tend to favor cooler color temperature sources, and people in colder climates like warmer light.  It seems pretty intuitive, yes?

Is this an official guaranteed works-for-every-human-on-earth standard?  Of course not.  Everyone is different.  Eastern societies have different preferences than Western societies.  But – and this is a general but – there is a correlation between the amount of light from a light source (lux) and the color temperature of the light source (degrees Kelvin) that seems to be fairly common among us all in most situations.  This is the research that culminated in A. A. Kruithof’s color temperature VS illuminance curve, as seen above.  Kruithof was working on visually pleasing light sources, and was interested in how adjusting the amount of light altered the amount of illumination needed to maintain a pleasing sense to the human eye.

The rods and cones in the human eye work together, and once the amount of illumination reaches a certain low or high point, the rods (intensity sensors) lead the visual information to the brain.  At night, when dusk conditions occur, you might notice that most of the colors in your view tend to be monochromatic, usually blue – this has to do with the low level of illumination, and a phenomenon referred to as the Purkinje Effect.  The Purkinje Effect tries to explain why our brain switches to scotopic vision at dusk when illumination levels are very low, and color rendering is poor – as the brightness of the day decreases, the vibrancy of reds goes away a lot faster than the vibrancy of blues in our vision.

We might have some almost built-in tendencies towards color temperature and light levels – perhaps somehow tied to the cycles of the sun and our circadian cycles.  We might have a tendency to associate warm colors with fire light at night, and we might associate higher color temperatures with the mid-day illumination levels from the sun.  Who really knows.  Kruithof gave it a try, and the curve is what he determined.

The two sources in the graph are the color temperature of Western/Northern Europe at mid-day (D65), and a 2700 Kelvin MR-16 tungsten-halogen source, for reference.

Thanks, ArchLighting and SoLux!

Lighting 101: Luminance VS Illuminance

Have you signed up yet for the best f*cking lighting industries email newsletter that ever was?
It’s FREE!!!  Click here to sign up for THE INDUSTRY PULSE from!

Do you know the difference between “luminance” and “Illuminance?”

When talking with students, or people who just love light but don’t know much about it, there are a few terms that are used sometimes incorrectly – “That table has a high illuminance value,” or “you can measure the luminance coming from that light source.”  Not quite right.  Do you know how to use these integral lighting terms?

It’s easiest to remember if you think of it in terms of what is exactly happening when a beam of light strikes a surface.  Let’s look at this small image:

An incidence of light striking a surface.

Incident light.

In this example, we are looking at one beam of light hitting a surface.  The beam of light is red, and the surface, obviously is green.  The red arrow represents the incident light, or the beam(s) of light striking the surface.  Another time I’ll write about the Law of Reflection and what it means, but also look at this diagram of what happens when a beam of light is reflected:

Incident light and reflected light

Incident Ray vs. Reflected Ray

Figuring out the difference between luminance and IL-luminance is as simple as remembering what has happened to the beam of light; the incident of light hitting a surface – a beam of light that is travelling towards a surface – is what is measured when talking about illuminance.  Illuminance is measured as the amount of light striking a surface.  The incident light.  If we had a light meter and wanted to measure the amount of light that was striking the surface we were measuring, we would hold the little white bubble towards the device shooting the light.  These meters are also called “incident meters” for a reason.

On the other side of this, luminance, is what we measure off of the surface that has light hitting it.  Luminance is the measurement of the product of the incident light and the surface – anything that is reflected.  Luminance is also considered the human perception of brightness, or how bright we perceive the light that is reflected off of the surface.

Make sense?  Think of it like this – IL-Luminance, IL, I = Incident Light.  Illuminance is measuring the incident light.  Luminance is what’s leaving the surface – L = leaving.  Illuminance is measuring incident, luminance is measuring what’s leaving.

I hope that helps!