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How to Evaluate Light Bulbs for Quality

Choosing lighting for your workspace can be intimidating.  Most people now know that poor-quality bulbs affect your ability to distinguish colors correctly.  But when you're in a store, how do you know which ones are the best for your studio?

Everyone knows that watts correspond to light strength.  We now know, however, that different kinds of bulbs produce different light output with the same wattage. LED and compact fluorescent bulbs use fewer watts than incandescent and halogen for the same amount of output.  That light output is measured in lumens; a typical modern 60-watt incandescent bulb outputs 800 lumens.

Recently, consumer bulb packaging is beginning to list a specification called CRI, or Color Rendering Index.  This is a measurement of how colors from 8 standard pigment samples are rendered in that light.  A value of 100 means it is equivalent to sunlight or the classic incandescent bulbs everyone's trying to replace.  Bulbs with a CRI of less than 80 don't render well, and the California Energy Commission says that bulbs with a CRI of less than 90 aren't eligible for the planned rebates.

The problem with CRI is that it doesn't tell the whole story.  

The 8 pigments that are tested for CRI, named R1 through R8, are fairly pastel.  The other standard pigments, R9-R14 are are the saturated colors and the earth tones, useful for discerning between colors in artwork. R9, in particular, is a saturated red color that you really want to be rendered properly.

 

Standard pigments, with R9 highlighted

 

As a result, the R9 value (which you really want above 50) is starting to become important.  But it took years for CRI to be listed on consumber bulb packaging, and it will take years for R9 to be added.

Finally, there's color temperature. Over the course of the day, the sun's light shifts between a bluish-white at noon and the lovely amber color at surise and sunset, known to painters as the Golden Hour.  The noontime color quality is said to be "cool" because it's closer to blue, and the Golden Hour light is said to be warm.  We don't always perceive the shift in color temperature because our eyes compensate naturally.  But anyone who's taken a photograph on a sunny day with a camera whose white balance is set to "incandescent bulb" has seen the picture turn out bluish -- the camera was assuming the light would be warmer, and used that as "white," which gave it the bluish cast.

Color temperature is called this not because it's called "warm" or "cool", but because it's meant to correspond with the color of the light glow you'd get if you heated a piece of carbon to a certain Kelvin temperature. Confusingly, this means the higher tempeatures, more like the noontime sun, are referred to as "cooler" because of the shift toward blue!

Color temperature is not a measurement of quality. Color rendering can be excellent at many different color temperatures; it's just a matter of application. Most "daylight-adjusted" task lights have a very cool color temperature, about 6000-6500˚K. Incandescent bulbs and many bulbs used in galleries (like MR16 halogen bulbs) are in the 2700-3500 range.  In general you want to avoid mixing color temperatures; having a daylight-adjusted fluorescent task light and an incandescent lamp in the same workspacec an be unpleasant, and photographs taken under such conditions cannot be color-balanced correctly; there will always be a reddish or bluish cast in the shadows or highlights no matter what you do.  For this reason, if you are photographing with natural light, be sure any artificial light you add is of the same color temperature, or else block out the natural light entirely.

The final criteria is the spectral power distribution. This is a measure of the relative levels of the colors of the rainbow that make up the white-light output of a bulb.  gRaphed as a curve, the sun has a smooth slope, meaning that the colors are evenly represented across the spectrum. Old or cheap fluorescent bulbs have horrible-looking jagged curves with notches and spikes, which is why they made everyone appear unflattering and ill. CRI measurement is an attempt to turn that quality of light into a simple number, but with only 8 sample targets it could never be enough.  LED's and fluorescent lights have a notorious spike in the greens, which could make it difficult to correctly perceive greens and yellows.

Manufacturers have been dealing with this in a variety of ways; some use blue LEDs and coat the bulb with phosphors that get excited and put out the rest of the colors to make a nice white. Some mix LEDs with different color output characteristics ("spectral power distributions") to sum up to a nice smooth white curve.  Manufacturer Cree recently introduced bulbs with a neodymium coating, which effectively knocks back that particular hue to even it out.

You're never going to see spectral power distribution on standard consumer packaging.  If you care enough, you can usually download the spectral power distribution charts from manufacturers proud enough of the quality to make them available. 

So, when you're at the store, looking at bulbs, you're looking for a lot of specifications:

  • Wattage is the least useful, only matters for your electric bill and possibly heat considerations
  • Lumens of light output is better. 800 is about what you're used to from a bulb. The most lumens with the least watts is efficient but says nothing about the quality.
  • Color temperature is more often a question of application and preference, but most studio general lighting should be "cooler", closer to 6000˚K, as opposed to the 2700-3500 of incandescent and halogen lights. 5400˚K is a good balance.
  • CRI is where you really start talking about the quality. Never settle for less than 80, try to stay closer to 90.
  • R9 is not usually on the packaging, but is often in the specifications on the bulb manufacturer's web sites
  • Spectral power distribution is almost never on the packaging, but manufacturers making good bulbs make the charts available on the web.

One last consideration is price, of course.  It's easy to choke on the price of some of these High-CRI, High-R9 LED bulbs. (The Cree TrueWhiteLED bulbs are currently $109 for a 6-pack at Home Depot.)  But these things are built to last like ten years, making the savings on your electric bill and the savings on replacements (and time spent fussing with replacing them) a little more tolerable, especially in the long term.  Currently, it takes a little over a year for a bulb to pay for itself this way.  One other consideration is the rebates the California Energy Commission is proposing for bulbs that complies with California's stringent energy and quality (CRI and R9) standards.  If it meets California standards, it will not only be a very good bulb, it may be eligible for some kind of rebate.

 

 

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