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.


Sharp Says “Here World, Have Nine Models of LED A-Lamps”

Electronics manufacturer Sharp has released a series of nine LED A-type lamps for the world to chew on – color temperatures from warm white to daylight (so that’s eight of the models) and a revolutionary color-changing remote controlled lamp that is controllable from warm white to daylight.

That’s pretty cool – my first question is obviously output, and Sharp has the daylight white model at around 560 lumens.  If you compare that to a 60w incandescent at 850 lumens it’s 35% less output, but it’s also got a lifetime of 40,000 hours (about 40 times the lifespan of an incandescent) and uses a minute fraction of the power.

Cost is obviously a factor, right?  The fixed color temperature sources are retailing around the $40 dollar mark ($42-$44) and the color changing model runs around $82 bucks.  Now theoretically if you were to use this six hours a day each day of the week, every week for a year, you’d use 2016 hours of the lamp’s life.  If you divide that into 40,000 hours of lamplife, this lamp will last you about 20 years.  If you compare power consumption costs with a 60w incandescent, there’s a clear winner, the LED source at 112 lumens per watt compared to the incandescent efficacy of around 14 lumens per watt.

Well, we will see, won’t we?  I do love these innovations!

About the adjustable color temperature and intensity source – from the press release from Sharp:

The model DL-L60AV LED Lamp features an Adjustable Color Function that enables users to change the color of the white light emitted from the lamp using an accessory remote control, an industry first for an LED lamp*1. Users can select from seven different shades of white ranging from a pleasing warm white to a cooler daylight white to match the weather, the season, time of day, purpose, or other preferences. This model also features a built-in Dimmer Function to adjust brightness. Together, these features allow users to select the illumination they like best to complement a diverse range of interior settings by using a single remote control to change the color and brightness of the light.

In addition, the model DL-L601N LED Lamp delivers a brightness of 560 lumens, among the highest in the industry for LED lamps*1 having nearly the same size and shape as ordinary incandescent lamps.

Check out some images:

sharp led

sharp led a lamp


Thanks Treehugger and DVICE!