In my last post, I stumbled through an explanation of what Color Temperature is and why it doesn’t apply to fluorescent lighting. But, if Color Temperature doesn’t apply to fluorescents, why do they put a color temperature measurement on most florescent light packaging?
Well, they’re not deliberately trying to be deceitful, what they’re using is a Correlated Color Temperature or CCT.
Effectively, the rating listed on the packaging of a given bulb indicates the incandescent color temperature that the light will most closely mimic. Because our eyes are incredibly good at adapting to different qualities of illumination, this correlated color temperature measurement is essentially “close enough” for every day life. For example, “Warm White” bulbs are listed with a CCT between 2800 K to 3500 K, and for most purposes are similar in color tone to common tungsten-based interior lighting.
An important side note to consider is that the common tungsten color temperature of 3200 K is generally only found in photographic quality quartz tungsten lighting. The standard 60 watt light bulb used in many homes produces much warmer illumination in the range of 2800 K, which is why color film balanced for Tungsten can still appear quite warm when exposed under lower wattage household lights.
So while this CCT rating is acceptable for general lighting purposes, it does not convey the true color properties for photographic purposes. The illumination produced by a fluorescent light is a result of a gas discharge reaction producing ultraviolet radiation that causes phosphors deposited inside the lamp to fluoresce in the visible spectrum. While the light produced by these lamps within the visible spectrum and appears to our eyes to be white light, the actual spectrum is discontinuous in nature.
When viewed on a spectrograph, Tungsten and Daylight illumination both appear as a curve across the visible wavelengths – essentially, it is a rainbow that progresses smoothly through all the visible wavelengths. Conversely, fluorescents and other vapor-discharge lamps produce illumination that shows huge peaks and valleys across the visible spectrum. While our eyes are able “sum” these peaks and valleys into something we can perceive as white light, imaging chips and film will more accurately capture the higher intensity in narrow bands of the visible spectrum in reflected colors.
What does that mean in plain English? Essentially, the light produced by a fluorescent bulb can have intense slivers of certain colors along with incredibly dim expanses of other colors in-between. While the light may appear white, when it strikes a colored object, the color that is reflected from that object is tainted by these deficiencies in the spectrum. For example, certain Caucasian skin colors when photographed under a Cool White fluorescent source can often appear sickly and pasty. This is caused by the huge green spike inherent to the light source reflecting from the surface of the skin and negating a large portion of the magenta spectrum present in normal skin tones. In plain English, it makes people look more green then they really are and it’s the magenta in Caucasian skin that provides the “healthy glow”.
So from this, we can see that Fluorescent lighting provides a challenge when it comes to reproducing color, but where there is a challenge – there is often a half-assed solution already provided to the marketplace. Welcome the CRI number.
CRI stands for Color Rendering Index and it is a measurement used by lighting manufacturers in an attempt to describe how closely a fluorescent light can mimic daylight in reflected color reproduction. A CRI number of 100 (which is essentially impossible) would indicate that a discharge lamp perfectly equals daylight. Film generally requires a CRI of 95 or above to avoid any color casts, and any light below 90 will most likely exhibit distorted reflected colors when compared with daylight or tungsten.
It’s important to note that what we’re talking about is distorted colors, not distorted white points. While you can use software to correct distorted white point captured under fluorescent lighting, adjusting the white point does not balance the distorted colors within the scene. If an image exhibits a greenish color cast from a fluorescent lighting source, you can easily correct the overall white point, but what you can’t do as easily is correctly re-introduce the individual colors that were muted or enhanced under the discontinuous spectra of the fluorescent illumination.
For purposes of photography, it’s worth the effort to seek out the highest CRI fluorescent lamps to ensure that you’re starting out with the highest baseline color fidelity so that more of the actual color is recorded correctly in the initial capture.
So, now that we understand what a CCT measurement and a CRI number mean, we can drive out to Home Depot and see if we can build a suitable lighting solution from off-the-shelf parts.
Up next, adventures with CFLs.