Lighting for Devs

Setting your brain’s white-balance with full-spectrum lighting.

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What is the correct lighting environment for computer-based designers and developers? What does “full-spectrum” really mean? Why it important? This is not intended as a technical article, but a guide to practical and correct solutions.

Background

One sunny day I was driving along a country road, wearing blue-blocker sunglasses. I had to take them off for a second, and was amazed to see that the yellow stripes on the highway were bright orange! After a few moments, they returned to yellow. I realized that the brain has an “auto white-balance,” similar to modern automatic cameras. I thought, “Hmmm, am I designing web sites and adjusting photos in an atypical lighting environment? What are my clients seeing?”

For several years I’ve been developing web stuff using a CRT monitor and a 20-watt “cool white” compact fluorescent light (CFL), mounted in a reflector about three feet above the desk, pointed at the ceiling. This produced the low-light environment that I prefer, and to compensate, I ran the monitor at a correspondingly low level. Then I purchased a 19-inch LCD monitor, and that’s where this all started. The LCD panel will not render colors evenly nor accurately at low brightness levels. In fact, it must be set rather bright, as would be used in a bright office environment, to render white anywhere near accurately. So I decided to increase the light level of my office, and become accustomed to working in a more typical environment. That way, hopefully, when I choose web colors and adjust photographs they will appear more-or-less as intended, to the majority of end-users.

There are also health benefits to using proper lighting. To reduce eyestrain, the work area should be near the same perceived illuminance and color temperature as the computer screen. That is, reflective surfaces near and behind the monitor should be about 250 lux and 5000-6500°K.

I tried various fluorescent bulbs, and discovered that they all conflicted with the white-point of my monitor when set at 6500°K, the typical default setting known as the sRGB model. When I had to set my monitor’s RGB at 80, 70, 60 (%) to get “white” close to a white sheet of paper on the desk, I knew I needed better lighting!

So I did some research on the “proper” lighting for a web design environment. Without the proper ambient color temperature, the brain gradually shifts its white balance, and web designers may choose colors that are perceived differently by end-users in different lighting environments. The problem is compounded by the fact that many home users will be in an environment of 2700°K (incandescent), with the computer monitor set at 6500°K. Office workers typically work in 5000°K+, also with monitors at 6500°K. Of course, the environment shifts all over the place depending on the colors of reflective surfaces, drapes, windows, night, day, etc. So why bother?

The idea is to use the right illuminance (Lux), corrected color temperature (CCT) and color rendering index (CRI) in the work environment, then to set the computer monitor to render the same white as the ambient lighting. Without expensive measuring instruments, this may seem rather difficult, but let’s try anyway.

Several hours of research on the web yielded lots of colorimetric theory, a few recommendations for TV, photography, and artistic lighting, and nothing useful about the computer-based design environment (until now!).

Some Basic Theory

Lighting is usually measured in several axes:

  • Luminous Flux, in Lumens: total light power;
  • Illuminance, in lux: light emitted or reflected from a surface of a given area;
  • Corrected Color Temperature (CCT) (the white-point), in degrees Kelvin;
  • Color Rendering Index (CRI) in percent (a measure of spectral distribution).
“The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux.” Wikipedia, lux

Illuminance is the overall visible light in a given area, from both sources and reflections. Typical indoor illumination levels range from 80 lux (living room), to 500 lux (brightly-lit office). Samples:

Recommended illumination (nominal lux):
Minimal emergency light32
Living room80-120
Reading200
Workplace, minimum250
Computer workplace200-500 (ANSI/HFS Std. 10-1988)
Typical office400-500
Drafting750
Precise work1000
Surgery2500-5000

Corrected Color Temperature (CCT) is the white-point color of a “blackbody source” (e.g., a chunk of iron or carbon) heated to that temperature (in Kelvins). CCT provides an absolute measure to compare against the variability of human visual perception.

Color Rendering Index (CRI) is a measure of how closely a light source’s spectra matches a heated blackbody source. Incandescent lights have a CRI of 100%, with a smooth spectral distribution that rises toward the red end. Fluorescent lights have an uneven, “spiky” spectra, and typically have CRIs of 60-90%, meaning the spectral distribution is uneven. See the fine article, with spectral distribution graphics, at this Wikipedia article. For accurate color charts (on a sRGB/6500°K monitor) see this Techmind article.

Full-spectrum fluorescent tubes, running around 90-95% CRI, and costing around $12-$20 each, use proprietary and patented phosphor blends to even out the spectral distribution and raise the CRI to near that of the ideal incandescent. Note that this doesn’t mean the same perceived “white” color, but just the “smoothness” of the spectral distribution. This type of bulb is available in white-point colors from about 5200 to 6500° Kelvins.

Remember that the color temperature of a source refers to the peak of the spectra, and does not tell you anything about the distribution of the spectra, or CRI. Thus, when choosing good bulbs, look at both the CCT and the CRI. For example, a high-quality “full spectrum” fluorescent tube, as used in light panels for the treatment of seasonal affective disorder (s.a.d), will be 5700-6000°K with a CRI of 95% or higher. These should also be fine for the home office.

This is quite different from lighting for photography or television, where the lighting is designed to please the film or the TV imager, not necessarily the human eye. For example, TV cameras run “white” at 3400°K, while TVs run “white” at 6500°K.

Color Temperatures of various lights and environments
(these numbers are not absolute):
Candle1500°K
Incandescent bulb (new)2700-2900°K
Halogen bulb°K3000°K
TV studio lighting3200-3400°K
Indoor (tungsten) film3200°K
Outdoor (daylight) film5600°K
Cool-White fluorescent4200°K
Full Spectrum fluorescent5000-6500°K, with CRI > 90%
Typical office lighting5000°K, to 6500°K with lots
of unfiltered windows
Morning sun5000°K
Afternoon sun5500°K
Midday sun5700°K
Typical sRGB monitor6500°K
"Daylight" (D65)6500°K
Overcast sky6800°K
Blue sky11000°K
Warm white< 3300°K
Neutral white3300-5000°K
Daylight white> 5000°K

Remember that the Kelvin temperatures above mean that lower = more reddish, while higher = more bluish, as white-points go, referenced to blackbody sources heated to that temperature. The human eye/brain will perceive each as “white” under the existing environment.

What to do?

It seems that the ambient lighting should closely match the typical monitor (6500°K), but remember that most home users are probably working in an incandescent environment of well under 4000°K, shifting the brain’s white balance and making the 6500°K monitor look bluish.

I’m currently running a 27-watt, 6500°K CFL placed about 3.5 feet above the monitor and wrapped with a sheet of regular copy paper (on a wire hoop) as a shade. It’s working out very well, and the monitor’s 6500° white is very close to a white sheet of paper reflecting the overhead lighting. I am able to make better color corrections on photos, and also eyestrain is lower that it was with the old green/purple-spiked cool-white tube.

The G.E. “Sunshine” tubes, and their “Daylight 6500 K” CFLs are inexpensive and readily available. Better, and initially more expensive, are the BlueMax 30w CFL (5900°K, 94 CRI) CFLs, and LumiChrome 1XX (98CRI, 6500°K) tubes. Actually, they are not more expensive, because they will outlast 2 copies of the cheaper lights, and produce better light throughout their useful lives.

2011 Update: Now using an ALZO Digital brand 45 watt, 5500°K, >91 CRI compact fluorescent. It’s a photo/video light, and I have a piece of baker’s parchment around it as a diffuser. 2800 lumens of full-spectrum light! So much better!

Health and Mood

It is well known that the brain needs to perceive full-spectrum daylight for most of the day to produce a natural balance of neurotransmitters. Furthermore, the body cannot use calcium without calciferol (vitamin D), which is produced in the skin when exposed to direct sunlight.

The problem with natural light and computers is that (1) computers are frequently used at night, and (2) windows are usually in the wrong place, relative to the computer screen. Direct, natural light is also far too bright (35,000-100,000 lux) for viewing computer monitors.

The only practical solution is to provide broad-spectrum (high CRI) artificial light at the correct luminance, and place the source in the correct location to illuminate the work area but not the computer screen.

One final note: “Full-spectrum” has become a marketing buzzword, and many lights labeled as such do not actually meet the criteria. Always look for the actual CCT and CRI. Further, bulb lifetime claims are misleading—a fluorescent may produce light for 8,000 hours, but it’s poor quality light after perhaps half that much, and you’ll get tired of the dim, dingy light and probably replace it long before it quits. The premium-quality bulbs claim they produce better light for a higher percentage of their lifetimes, but that’s a difficult claim to test, therefore easy to make.

Reference Links

CIE Color Calculator

Named white points

Color-temp theory and graphics

Accurate color-temp graphics

Books and Papers on Daylight

College course on workplace lighting (PDF, lots of graphics)

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