CRI is a measurement of a light’s ability to reveal the actual color of objects as compared to an ideal light source (natural light). High CRI is generally a desirable characteristic (although of course, it depends on the required application). If your goal is to illuminate a scene such that the colors all reveal as they would naturally (typically an important requirement in the cinematography business for example) then you want a high CRI lightbulb. If, conversely, you’re lighting an underground tunnel and you don’t particularly care whether the shapes are black and white or in color, CRI might not be as important. We tend to think, however, that high quality color rendering is a benefit in almost every situation.
The value for CRI that is typically advertised on commercially available lighting products is known as the CIE Ra value (a standard set by the International Commission of Illumination based in Vienna, Austria). Another standard for determining the color rendering ability of a particular bulb is the Color Appearance Model (CAM). Various Color Appearance Models are preferred to CRI as a viable way to measure color (particularly for color temperatures below 5000K; a range that encompasses most light bulbs). One of the most popular models is the CIECAM02 (the Color Appearance Model that was published by the CIE in 2002). That said, CRI is still by far the most widely published index by which everyday consumers can make a judgment on the color rendering ability of a particular light.
Color Temperature is a method to describe the characteristics of visible light from different emitters. The color temperature scale works by comparing the visible light emissions from a given light bulb to those of a “blackbody” emitter (an object whose surface temperature is the same as its color temperature value).
Need an example? Incandescent bulbs provide a good approximation of blackbody emitters, so they are useful example to describe color temperature. As the filament of an incandescent bulb is heated it will eventually begin to glow. The glow, of course, has color characteristics. As the the surface temperature of the filament itself becomes hotter, its glow will change color. The glow will begin at relatively low temperatures (around 1500K) as a deep red color. As the filament’s surface temperature continues to rise the glow will transition from red to orange, orange to yellow, yellow to white, and if it were to get hot enough (typically not the case for incandescent bulbs), from white to a light blue.
Now for the confusing part: although the deep red glow occurs at a relatively lower surface temperature, it is typically referred to as a “warm” color temperature. That is because human beings normally associate the color red with fire and thus things that are hot (or “warm” in this case). Conversely, the white or light blue glow occurs at a relatively higher filament surface temperature, but it is typically referred to as a “cool” color temperature. That is because human beings normally associate the color blue with ice and thus things that are cold (or “cool” in this case).
The chart above shows the color temperature of different lights when they are operated at their typical operating power, but it’s important to realize that the actual color temperature will be a range depending on the amount of power that is applied to the light (more power will result in a hotter filament temperature and thus a “cooler” [higher K value] color temperature).
Correlated color temperature is a measurement used in lieu of color temperature for lights that do not approximate a black body radiator (that is, they emit light through processes other than thermal radiation). Both fluorescent lights and LED lights (light emitting diodes) fall under this category and thus are evaluated using CCT. Correlated Color Temperature is a specification used to describe the dominant color tone for non-blackbody light emitterssuch that they can be accurately compared and contrasted with those light emitters that do approximate blackbody radiation (like incandescent bulbs). Ratings at the lower end of the scale (~2000K) are generally referred to as “warm” (typically red and yellow colors) while those at the higher end of the scale (5000K+) are typically referred to as “cool” (typically white to light blue)...the same as for color temperature scales.
A foot-candle is a measure that describes the amount of light reaching a specified surface area as opposed to the total amount of light coming from a source (luminous flux). Foot-candles are measured in lumens per square foot as opposed to simply lumens (as in the case of luminous flux). Simply measuring lumens is deceiving because light that is illuminating an irrelevant area (such as the ceiling) is not productively used. In fact, it’s a waste. Further, luminous flux doesn’t indicate how focused the light beam is. Different optics and casings will focus the light more or less than others resulting in a more or less illuminated target area from the same type of light and the same amount of power. What you really care about is the amount of light actually illuminating the desired surface area. Therefore, from a user’s perspective, foot-candle is a much more important measurement than total luminous flux. In addition to the difference in units, some of the light coming from a source does not ever make it to the desired aim point (that is, lights aren’t 100% efficient). A certain amount of light is always lost to inefficiencies like light absorption, reflection, and/or dissipation. Foot-candle takes this into account while luminous flux does not. So forget luminous flux for now and focus on foot-candles as they are a much more relevant measure of lighting in the real world.
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