Daylight: Is it in the eye of the beholder?

The goal of this tutorial is to clarify issues that are relevant to a discussion about
daylight and the visual perception of it. The following topics will be discussed:

Daylight: What it is, and some surprising findings.
The Kruithof Curve
The Human Eye
Chromatic Adaptation

Daylight: What it is, and some surprising findings

The words synonymous with daylight are "natural" or "true light". Daylight is defined as the combination of sunlight and skylight The daylight condition most commonly associated with a clear blue cloudless day exists at about 6000K. Rarely, if ever, will you hear someone walk out into these conditions and say, "Today is too cool" or "too blue". However, if you take these same lighting conditions and view them in an indoor setting your perception will be that the same light you thought was "natural"outdoors now surprisingly appears bluish. In this paper we will explore the physiological and environmental reasons for this phenomenon and ways to overcome it.


Our first encounter with this phenomena occurred when SoLux was used to illuminate Vermeer paintings at the National Gallery of Art in Washington. An experiment was set up so the proper amount of footcandles (20-30) illuminated each painting. The observers were able to adjust the lighting to the most preferred color temperature using SoLux at 4700K and standard MR-16's at 3000K while maintaining a stable footcandle level. The color temperature that was most frequently chosen was 3500K. This was a little bit surprising because 3500 appears yellow-orange to the eye under normal viewing conditions, and the spectral power distribution supports this.

These findings could not blamed on untrained eyes because the people at the Vermeer test were professionals with many years experience in the lighting of art. (A curator from the National Gallery of Art and the NGA's chief lighting designer were amongst the group) However, it was noted that museum light levels are much lower than standard light indoor light levels. Does light levels at extreme
highs and lows and at points in between impact the eye's perception of color? Historical and empirical evidence suggests the answer to this question
is yes.

The Kruithof Curve

Early work that touched upon the relationship between light levels and color temperature was conducted by Kruithof . He developed a chart which defined a region of high and low levels of illumination for a range of color temperatures which were considered "pleasing" to a number of observers.

Kruithof provided the lighting designer with a breakthrough concept that has withstood the test of time because he used only the sun and incandescent sources for his study which yielded the purest possible spectral power distributions for his study. Based on the Kruithof Curve, SoLux low voltage daylight lamps should be used under the following illuminance guidelines:

Color Temperature Footcandles Lux
3500K 18-200 194-2,153
4100K 22-1500 240-16,147
4700K 27-5000+ 290-50,000+
5000K 40-5000+ 430-50,000+

Further refinements of the Kruithof Curve is currently being made by Weintraub et al. using SoLux and the lighting system used at the National Gallery of Art.

The Human Eye

So what causes this changing perception of color? Part of the answer lies with the level of lighting conditions outdoors, indoors and the way the human eye functions under these very different lighting conditions.

The number of footcandles measured during a typical cloudy day in Rochester, NY is about 3,200 footcandles with a color temperature of 6550K. A sunny day measures out at 13,600 footcandles and 5000K. The number of footcandles required for reading and displaying items in a retail store is between 75 and 150 footcandles and the number of footcandles required in a museum due to conservation issues is roughly 20 footcandles.

Lighting levels produced by outdoor, indoor, and museum lighting differ by a factor of about ten or more in each case. In order to respond to these changes the iris, the entrance into the human eye, is designed to contract and dilate rapidly. By expanding and contracting the iris controls the amount of light incident upon the retina. The retina contains the rods and cones responsible for vision. The light incident upon the retina is proportional the square of the pupil diameter. If the pupil doubles in size the amount of light entering the eye increases by a factor of four. The iris can expand to 8mm in dim light and contract to 2mm in bright light. This factor of four change in the diameter of the iris corresponds to a 16 times change of brightness on the retina, however the light level change from the museum to the sunny outdoors in Rochester is 680 times. The additional light level factor of 42.5 that the iris can't correct leads to a dynamic interplay between the two light receptors, rods and cones.

There are approximately six million cones and one hundred and nineteen million rods intermingled non-uniformly over the retina. The cones which are primarily located in the center of the retina in a region called the Fovea and have a responsivity as shown in figure 4 peak at 555nm (green region). Until recently, the cones have been primarily credited with color vision. The rods, whose responsivity peak at 508 nm (blue region), have traditionally been credited only with night vision.

In a 1996 paper entitled, "The Reengineering of Lighting Photometry," Dr. Sam Berman sets forth a new theory on the workings of the human eye where the function of the rods and cones are not mutually exclusive as previously believed.

To prove his theory that rod receptors were at work all the time, Dr. Berman measured pupil diameters which were exposed to light sources of equal total output but with different spectral power distributions. The sources which emitted more energy closer to 508nm and away from the peak cone sensitivity of 550nm resulted in smaller pupil sizes proving his theory that not only were rod receptors at work at all times, but also that the rods controlled pupil size and not the cones as previously thought.

What this all means to the observer looking at a painting outdoors, indoors or in a museum is this: Outdoors, the light is very bright causing the pupil to contract, however, the change in the pupil diameter is not great enough to offset the large increase in illumination. With the higher light levels the responsitivity of the cones is dominant yet the rods to a much smaller degree are still contributing to the overall response. With this large amount of light entering the eye, 6000K appears white.

Moving indoors, the pupil size grows allowing a larger proportion of light to pass. Once again, the iris is not capable of maintaining a constant level of illumination on the retina. Under these reduced lighting conditions, rods with blue sensitivity come more into play and hence the 6000K light that looked white outdoors now appears bluish and 4700K appears white. By traveling to the museum, we decrease our light level another factor of 10 times from about 200 to 20 footcandles. The rods are utilized even more and 4700K light which appeared white under normal indoor conditions now appears bluish or "cold" to the museum curator and 3500 appears white.

It is important to point out that as the iris contracts and less light is allowed to enter the eye, the size of the image on the retina does not change. Thus for a given field of view, the same amount of rods and cones are always exposed, it is the amount of light which triggers a larger visual influence of the cones for higher illumination and rods for lower illumination.

The interaction between the iris, the rods and the cones gives a plausible explanation to our observation that people will label direct sunlight at 6000K "white" yet say that 4700K at low light levels "looks a little blue".

Chromatic Adaptation

While the Kruithof curve characterizes a physiological condition that influences the perception of color, chromatic adaptation is a psychological condition that also plays an important role. Color adaptation is a rebalancing of the color response of the eye as the spectral composition of the scene changes. The brain is constantly working to process the information fed to it by our eyes. Sometimes the brain needs to "massage" the data; without this ability, most of the light sources we work and play under could quite possibly make most of us ill. Take for example fluorescent lights; to the eye, a room illuminated by cool white fluorescent lights appears white, however, a photograph taken in the same room reveals a green glow.

Combining the effects of chromatic adaptation with large illuminance fluctuations creates some interesting effects. For example, at night, the headlights on a car appear bright and white however, during the day, the same headlights appear dim and yellow. The inverse of this example is to introduce a small beam of daylight into a room illuminated by incandescent light. While the room appears white, the beam of daylight appears blue. Unless the same beam of daylight is viewed in an environment of daylight, it would be hard to convince the observer that incandescent light is yellow and daylight is "white" and not "blue"

Returning to the original question regarding daylight-is it "blue", or is it "white"? Obviously, there is not a single or simple answer. It depends on the level of intensity of the source and the surrounding environment. At high levels of intensity (i.e. out doors) daylight with color temperatures ranging from 4500-6000k all appear white to brain. At intermediate levels indoors, 6000k will tend to appear blue especially when directly compared to incandescent at 3000k and 4100K-4700K appears white. At lower intensities (i.e. museums) daylight anywhere in the 4500-6000k range will seem a little blue and 3500K-4100K will appear white.

For designers lighting an area or an object with daylight, they must take into consideration many aspects including- how brightly lit the room needs to be, and what adaptation have the viewer's eye's undergone and of course, what it is they are trying to illuminate. With the advent of the SoLux lamp at 4700K, 4100K and 3500K the lighting designer will have the ability to create the  perfect lighting condition for whatever the application.

Kevin P. McGuire Author/Inventor SoLux Technology


  • S.M Berman, "The Reengineering of Lighting Photometry," Publications of the Lighting Research Group, Lawrence Berkeley Laboratory, California, 1995
  • Roy S. Berns and Frank Grum, "Exhibiting Artwork: Consider the Illuminating Source, "Color Research and Application, vol. 12, no. 2, April 1987
  • Robert G. Davis and Dolores N. Ginther, "Correlated Color Temperature, Illuminance Level, and the Kruithof Curve," Journal of the Illuminating Engineering Society, Winter 1990.
  • IES Lighting Handbook, Lighting Handbook Eighth Edition, IESNA, New York, 1993
  • Optics, Hecht & Zajac, Addison-Wesley Publishing Company Inc., 1974

SoLux Technical Specifications

What our customers are saying...

"I used to push my clients towards GTI and Just Normlicht viewing booths but now only recommend SoLux. Even the best fluorescent bulbs aren't perfect for color critical work. SoLux bulbs are "

Scott Martin