Matching north light with fluorescent bulbs

[Marvin Mattelson]

I am in the process of reevaluating my studio lights. I have been using 4 foot T-12 bulbs by Ott. The color temperature is 5000 degrees Kelvin and the CRI (color rendering index) is 91. The Color Rendering Index determines how accuratly color can be seen.

I feel the light is too warm and doesn

[Chris Saper]

Marvin, I unfortuntely do not know the specifcs with regard to CRI, although from what I have read is it extremely important.

There is enormous variability in the range of Kelvin temperatures, even controlling for the time of day. Temperatures are dramatically affected by atmospheric conditions, humidity, and altitude. Research I have done with respect to general Kelvin temperatures indicates generally that the light at noon is about 6500 degrees K, in sunlight.

North light conditions that would be as you are describing, would be light uninfluenced by direct sunlight. At noon the temperature could well exceed 15,000 degrees K, but that would be more characteristic of somewhere like Taos, with low humidity, high altitude and cloudless skies. In New York City, I would have to presume the temperature is much lower...salt air, lots of humidity, pollution...all things that influence the temperature of light.

If my memory serves me correctly, it is possible to measure Kelvin temperature for a specific geographic area (of course it would need to include assumtions about the atmospheric conditions), if you indeed want to pursue lighting that would mimic what you would normally have, given the location of your studio. This would be practical if you found yourself combining natural daylight and studio light. If I can recall how you might go about measuring specific Kelvin temperatures, I'll post it...it's been a while.

I am not aware of any manufactured bulbs that are rated at high K temperatures, but there certainly may be some. As I only paint in nautral daylight, I haven't had reason to look. Of the three bulbs you mentioned, I'd presume the higher rated temperature would be preferable to the 5000 K bulbs...it may not, however be enough a difference to appreciably impact choices you might make in your paintings.

[Michael Fournier]

Marvin I use GE Chroma 50 bulbs with a white frosted diffuser on the fluorescent fixture which are mounted directly over my window.

This window is not facing directly north it is kind of north east so in the early morning the light is slightly different but It is usually not a problem after 8 am.

I like the Chroma 50s they match the natural light color very well with the white diffuser on the fixture. (The diffuser may have some effect also.)

This is from GE's site about color temp.

Color Temperature

Originally, a term used to describe the "whiteness" of incandescent lamp light. Color temperature is directly related to the physical temperature of the filament in incandescent lamps so the Kelvin (absolute) temperature scale is used to describe color temperature. For discharge lamps where no hot filament is involved, the term "correlated color temperature" is used to indicate that the light appears "as if" the discharge is operating at a given color temperature. Chromaticity is expressed either in Kelvins (K) or as "x" and "y" coordinates on the CIE standard Chromaticity Diagram. Although it may not seem sensible, a higher temperature color (K) describes a visually cooler, bluer light source. Typical color temperatures are 2800K (incandescent), 3000K (halogen), 4100K (cool white or SP41 Fluorescent), and 5000K (daylight-simulating fluorescent colors such as Chroma 50 and SPX 50).

Now after reading this you would think that all 5000K bulbs would produce the same color light but they do not. I have tried many different 5000K and 5500 bulbs the Lite-A-Lux included.

In the GE line of so called daylight bulbs alone the Chroma 50 and SPX 50 which are both rated as 5000K bulbs you get two distinctly different colors if you hold a white card under each. The SPX which has a pinkish coating on the bulb produces a warmer light then does the Chroma 50 which has a grayish white color coating on the bulb.

So my advice is try a few and test them. (maybe you can find a lighting center that would let you return them if you did not like it but I would not count on it since these are usually special order at least for the lighting centers around here. Hope this helps.

P.S. This difference may be because florescent tubes have no filament so the color temperature has nothing to do with a actual filament temperature so the rating is some what arbitrary depending on how the manufacturer measures it. This is just a guess I do not know why one bulb has a different light vs. another of the supposedly same rating but they do.

[Michael Fournier]

Directly from GE's site:

There are two GE fluorescent lamp colors that are close approximations to outdoor daylight. One color is called Chroma 50 and Sunshine. It simulates the daylight/sunlight combination. The other is called Chroma 75 and it simulates north skylight. The lamps come in standard types - typically a 4-foot tube rated for 40 watts; so they fit into lighting fixtures that are easily found in hardware and home center stores. It is the phosphor or color of the tube that makes these lamps unique. And, yes, the light from these lamps mixes beautifully with outdoor daylight, so there should be no need to darken the room from natural light.

Note that natural light coming into a room changes constantly in color and brightness depending upon weather, season, cloud conditions and time-of-day whereas the light from the Chroma and Sunshine lamps is constant in color. So, sometimes you will see a difference between the light from the lamps and daylight; other times you will not. Note, too, that daylight is a powerful source. It produces abundant illumination, so use enough lamps to illuminate the work area properly.

To obtain the GE Chroma and Sunshine lamps and fixtures, try the large home center stores such as Home Depot. GE lamp distributors also carry them.

Well, since I have never noticed a big difference in light between the mix of daylight and my Chroma 50 bulbs and when I am painting after dark with the bulbs alone, it may be because I do not get direct skylight through my window - so I am matching a 5000K daylight and not 7500K sky light. Maybe with a higher window the Chroma 75's would work better for you, Marvin, if you have direct north sky light you are trying to match. Good luck.

[Michael Fournier]

I also found some more information on the GE site that explains the SPX 50's different color.

The SPX comes in two different colors: a red-enhanced SP30 or SPX30 for flowering and fruiting plants; or blue-enhanced SP50 or SPX50 for vegetative plants. The SP and SPX are plant grow-lights, so although they are rated as daylight bulbs, their color is not as important as the amount of energy for photosynthesis and other plant photoresponses they provide.

So I guess the GE Chroma 50 or Chroma 75s are the bulbs you want for studio lighting - not the SP or SPX bulbs. I hope this clears up the mistake I made in my first post.

Link to GE site http://www.gelighting.com/na/home/index.html

[Marvin Mattelson]

Thanks for the info Michael. I'll keep you posted on my search.

[Clive Fullagar]

Marvin,

I am interested in knowing whether you want to use these lights to paint under or to photograph your models, or both? As somebody who is trying to improve the quality of my source photographs, would you recommend using these lights to photograph and light subjects?

[Chris Saper]

Clive,

You might also check some of the threads on lighting the model here: http://forum.portraitartist.com/foru...?s=&forumid=72

[Marvin Mattelson]

Clive,

These lights are for the purpose of illuminating the area of my studio where I paint. My north light is obstructed by trees and sometimes I need more light than my windows provide.

When I take my reference photos I use strobe lights. The reason I use these lights is to compensate for the difference between the way film and the human eye perceive reality.

I have previously gone into some detail about this subject in other strings.

I wouldn't recommend using these lights to light the model unless you want a very diffused soft light.

[Michael Georges]

Marvin:

I have a mostly north facing window. While it is not nearly high enough, it serves and I love the natural light.

I have always wondered about temp and CRI and what the values were equivalent to. In other words, I have heard before that a 5000+K bulb better simulated natural sunlight - but sunlight has nothing to do with the light that you get from a north window and I wonder if it is too intense to simulate what you would get from a north window. Do you have any thoughts about this? What is the temp and CRI of the light coming into a typical north window? Unobstructed of course.

I have heard a lot of people talk about this and I wonder if they are getting the wrong info about temp because those companies seem to be selling bulbs that simulate sunlight, not the soft, cool reflective light of the sky that comes through a north window.