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!

The Bucha Effect

I posted an article about the Purkinje Effect a while ago – I wanted to share another light-related physiological effect, because they’re so fascinating!  Now obviously I have a high amount of fascination with the eye because, you know, it’s awesome – but it is such a complex component that is connected in so many ways to everything!

The Bucha Effect is a seizure-inducing phenomenon that was discovered in the 1950’s by a doctor (Dr. Bucha, of course) who was investigating a series of helicopter accidents.  The phenomenon is when someone gets dizzy or confused when exposed to flashing or strobing lights at between 1 and 20 Hz – in the case of the helicopter accidents, the rotor blades of the helicopter caused the sunlight to  strobe in the eyes of the pilots, causing them to lose control of the craft.  Dr. Bucha discovered that the strobing could cause flashes of light at the same frequency of brain waves, causing symptoms of epilepsy.

Not surprisingly, the Bucha Effect has also been refined and formulated into a crowd control device, and has been considered for non-lethal weaponizing.  You’re not surprised, right?

From a paper called The Bradford Non-Lethal Weapons Research Project (BNLWRP) – it’s a PDF link:

The U.S. military has funded development of various ‘dazzlers’ or ‘illuminators’ such as the Saber 20335, the Hinder Adversaries with Less-than-lethal Technology (HALT) system, and the Laser Dissuader all of which use red diode lasers36 to temporarily blind or obscure vision. The manufacturers of the Laser Dissuader also produce the LazerShield, which incorporates a red diode laser on a plastic shield and is designed for use in law enforcement for incapacitating prisoners.  Future plans for the HALT include the capability for dual red and blue wavelengths that flicker off and on to mitigate filtering by single-wavelength goggles.

The U.S. Government also funded a project to produce the Laser Dazzler38, which uses a random flashing green laser. There are concerns, however, over eye safety in relation to these devices.  A similar system under development by the U.S. Marine Corps Joint Non-Lethal Weapons Directorate (JNLWD)40 is the Veiling Glare Laser, which uses violet light to cause the human eye to fluoresce so that the subject can see only glare.  Some scientists are uncertain as to both the effectiveness and safety of using this technique.  So far it has only been tested on cadaver lenses and its potential for eye damage remains unclear.

Again, why can’t we cure cancer?

January’s Top Posts

Check out January 2009’s top posts – I love to see what the readers like!

1.  NYC’s Empire State Building Lit for the 2008 Presidential Election

2.’s Interview with Tony McGettigan from Luxim

3.  PRG’s Bad Boy

4.  The Purkinje Effect

5.  What Seems Like Ancient History:  The Justin Timberlake/Christina Aguillera Truss Collapse

The Purkinje Effect


When I teach beginning lighting design classes, I always have a week of learning about the eye – how your rods and cones balance each other, what their respective jobs are, how the images you see are translated to the brain, and how, as lighting designers, we can use the strengths and weaknesses of the eye to heighten the audience’s experience.  Physiology always comes into play when you’re designing a lighted environment, and students always seem to enjoy finding out what different colors of light do to the body – blues are soothing (even to both genders), reds increase blood pressure and respiration rate, yellows make your eyes tire faster, and black is a submissive color.  None of the really “alternative” students appreciate being told that their black fingernails, eyeliner, and dyed hair color are really signs of compliance.

Inevitably, a question that comes up is “why do we see blue and gray at dusk and when the sun is setting?”  It’s a great question, especially because it lends itself to explanation of the photopic (lots of light, optimal conditions) and scotopic (low light, monochromatic) vision systems, and how rods and cones operate and cooperate.

The Purkinje Effect, as it’s called, is when the sun goes down, and we see a whole lot of blued out and grayed out colors.  This happens primarily at dusk, and in very low light conditions, where the color receptors (cones) basically leave it to the light receptors (rods) for detail for the brain.  When the light goes down, the color information is lacking, so the rods have to compensate.  Our vision becomes monochromatic because our color receptors, which do not interpret intensity information and respond best to yellow-ish light pass the workload on to the light receptors, which respond best to green/blue light and allow us to see shape and contrast.  Just without color.

There’s the lecture for the day.