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!

Lee Introduces New Sodium Effect Filters

Oh holy cow – my man iSquint posted an article about Lee’s new range of Sodium Effect filters that were released recently.  This line of filters is excellent – if you ask “why on EARTH would I want a set of 3200 degree filters?” then you’ve never had to deal with daylight before.  Way to go, Lee, and iSquint, you’re awesome!

Lee has also developed a  new falight conversion gel that coverts daylight (5600k) to Tungsten (3200k) with a red bias.  The new gel colors is 604 Full CT Eight Five.


LIFI – Lumens per Watt of the Future

Have you heard of or seen the videos of the new “plasma” light sources that are out there?  People are talking about LIFI lamps and sources, and how many lumens per watt can be had with these new jewels.  Check out this video:

LIFI:  “Light Fidelity.”  LIFI lamps do not use a series of electrodes to attach to a filament to bring the lamp to a point where the source emits light.  Instead, a RF source is focused and pointed at a little column in the lamp, the plasma column, which then makes the lamp emit an intense, high output light.  We’re talking about having a little tiny source that can output 20K lumens.  The operating costs are significantly lower because of the technology that “plasmafies” the lamp itself, which requires a lot less electricity to operate the RF generator.

What about color rendering?  How white is the LIFI source?

LUXIM, the company who is pioneering this technology, has the following chart on their website:

Look at that first figure – 120 lumens per watt.  CRI of 80 (96 on the next model up), and 30,000 hours of operating life.  If you know about CRI and color rendering, you know that 100 is the best score, referring to a black-body radiator.  The STA-40-02 model renders at 96.

Luxim also makes LIFI lamps for entertainment purposes – one of their lamps runs at 7500°K, the other at 5600°K – both have a CRI of above 91, and both have 20,000 hours of life.  These lamps operate under 10A and run on around 28V – heat is lower, energy consumption is lower, which allows this source to be jammed into the back of a moving head without worry of overtemp.  According to Luxim, these lamps are also vibration resistant and rugged – all solid state components.

Do you have any experience with these sources?  Luxim people, do you read my blog?  Please post in the comments.