The modern age of lighting has brought us many advances. Along with DMX and new light sources, there are new ways to measure and index the amount and quality of the light that those light sources produce. Measuring light output is pretty easy. Grab a light meter (or download one for your smart phone), point the light source at the eye of the meter (or camera of your smart phone) and look at the measurements. You will see a bunch of numbers come up — the mathematical computations of certain values that make up your light source.
Types of Light Meters
All light meters provide lux and/or foot candle measurements. Better meters also include Color Temperature, CIE chart X and Y values and CRI. Some may even include CQS.
The most common meters on the market, tristimulus meters, are great for full spectrum lamps, but not so good for measuring LEDs. Each of the three photodiodes in the meters are calibrated to read a specific wavelength of light and, by nature, LEDs are narrow band emitters. If those wavelengths are not present in the LEDs, the end result of your testing will be less than expected.
Another type, photospectrograph meters, are designed to evaluate the entire spectrum of available light. These are much better for LED fixtures because they do not discriminate against any wavelength. Photospectrograph meters are typically a bit more expensive, but they will give you much better results when testing LEDs.
To get a better understanding of all of these different measurements — what they mean, and how they impact how you might think about a light source — let’s look what each of these measurements mean.
Light Quantity
Foot candle and lux measurement tell us how much light is coming out of a light source and hitting an object.
A foot candle is a unit of light intensity — historically defined as the amount of light cast on a surface by a single candela source (like a candle) one foot away. Current usage is more specific — the amount of illumination that the inside surface of a one-foot-radius sphere would receive with a uniform point source of a single candela in the center of the sphere. A single foot candle is equal to one lumen per square foot. Foot candles are most often seen in architectural and commercial applications in the U.S. They are rarely used in entertainment lighting because they are not used internationally.
Lux is usually accepted as the metric equivalent of foot candles. A single foot candle is equal to 10.752 lux. It is also the international system of units standard of measurement of illuminance and luminous emittance. A single unit of lux is equal to one lumen per square meter. Therefore, lux is essentially the metric version of foot candles. For a chart showing how lux and foot candles are perceived in typical conditions found in nature, see Fig. 1.
When you are using a light meter, your light source is hitting the eye (or camera) of your light meter at any given distance. There are two ways to measure this information, illuminance and luminous emittance. Illuminance is the amount of light bouncing off of a surface like a wall or an actor, luminous emittance the amount of light that is hitting the wall or actor. Typically, I measure luminous emittance to find out how much light I am receiving at any given distance. Knowing how much actual luminous emittance a single lighting fixture will produce is really important to me when I am putting together a light rig. Without this information, I would be guessing on how many fixtures to use on lighting an area. When a lighting manufacturer writes a specification on any fixture, they should always provide you with the luminescence information for the fixture at full output. It is up to you as the user to determine how much of that light you will use either by changing the beam angle, adding color, or frost. Furthermore, one needs to understand that by adding color, gobos, prism or frost, the luminescence will be less than when the fixture was at full power with no color added or having the beam changed.
How White Is Your White?
Quality of light is as important as having good light levels. Having a lot of output is only good when you are getting the kind of light you are expecting to see. Being able to communicate these values is also important. Telling someone that they need their white warmer or cooler can be a little like asking for their water to be a little more or less wet. Having exact measurements of the value of your white light can be very important. Luckily, someone figured out how to give us a way to describe color temperature!
Color temperature measurement is defined by the electromagnetic radiation (color) emitted by an ideal black body radiator at a given temperature in the Kelvin scale. Basically, this means that if you had an imaginary blow torch that could heat up an imaginary item designed to absorb all wavelengths of electromagnetic radiation (black body) to 3200° Kelvin, the glow that was produced on the imaginary item from the heat of the imaginary blow torch would be a really pretty warm white.
Fig. 2, which pairs the Kelvin (K) temperature and a visual reference on the left with a light source example on the right, shows that there is a wide range of white light. The color temperature of you light source will be dictated typically by the type of show that you are working on. For example, most theatre is lit at 3,200K. This is because the dominant light source for theatre has historically been halogen, which produces a warmer, more orange/amber light. Rock ‘n’ roll shows have changed from 3,200K PAR lamps to being dominated by color temperatures of 6,500K and higher, some as much as 8,000K, because most of the light sources for intelligent lights have been short arc discharge lamps, which produce a cooler, bluish light. With the proliferation of color changing LED fixtures, we are no longer locked into having to choose a particular color temperature, so it is more important to be able to clearly communicate the design needs of a show. Using the Kelvin temperature scale makes that really easy.
Matching Colors
Matching color can be really difficult from fixture to fixture due do all the color variables that come into play, starting with (as noted above) the light source. If you were to pair up ellipsoidals, and one had halogen and the other an HID light source, and you were to gel them both with R26 Red, the surface you are lighting will appear to be two completely different shades of red. The starting color temperatures of the two light sources is different, and so is the end result.
Color correction gels can either boost the color temperature of the halogen source to match the HID or reduce the color temperature of the HID to match the halogen. With CMY (subtractive) and RGB (additive) color-changing fixtures, you might be able to use the fixtures’ color pickers and match by eye or, for more precision, use the X and Y values of a color meter to get an exact match.
A CIE (Commission International de L’eclairage) chart, with Y (vertical) and X (horizontal) values that represent all colors in the visible spectrum, can be used to match up any color in the visible spectrum. (The chart appears as an asymmetric triangle with Red, Blue and Green at the tips and a zone of white in the middle.) Using the X and Y measurements is really useful when trying to match dissimilar lighting fixtures. If I want to have a certain red, as long as I know the X and Y values of that color, I can spend some time finding out what combination of cyan, yellow, or magenta make up that color on any particular light. Same goes for red, green, and blue LED fixtures. It is just a matter of working it out from fixture type to fixture type.
CRI
CRI (Color Rendering Index) refers to the ability of any given light source to reproduce the colors of an object in comparison with a natural, or “perfect,” light source, like sunlight. Because sunlight possesses every wavelength of visible light, it has a CRI of 100. A lower CRI means that the light source has a lower number of wavelengths of visible light. Having a high CRI is important when trying to make scenery, costumes and actors look
natural. Think of it like this: If you buy a Hawaiian shirt in a shop that is lit with fluorescent shop lights, which might have a CRI as low as 60, the colors will not look vibrant at all. It is not until you get outside in sunlight that you realize how incredibly outrageously bright and full of color your new shirt really is.
CRI is measured by how well colors are rendered by any given source. Daylight, incandescent and halogen incandescent light sources tending to outperform even the best LED fixtures, fluorescents and sodium light sources on a traditional CRI basis.
CRI vs. CQS
Although CRI has been used since 1965, CQS (color quality scale) has been developed to address CRI’s shortcomings, particularly the visible differences when measuring the color quality of objects lit by LEDs.
The National Institute of Standards and Technology (NIST) launched CQS in mid-2010. Among other features, the new index is based on a broader palette of more saturated colors and takes into account such factors as chromatic discrimination, observer preferences and commercial availability of paints, pigments and fabrics (see Fig. 3).
Also, the way that CQS is calculated is different from CRI. Because CRI is averaged, a light source can have a high CRI because a few colors scored high, while others only did okay. CQS factors in the root-mean-square (RMS) of all color shifts rather than just averaging all of the samples. This means that there is no way that a light source can have a good number based on a few high scores. All of the scores have to be high in order to receive a high score. While CQS is still in testing, computer simulations combined with visual impression are confirming the theories behind this new scale. It’s expected that CQS will eventually replace CRI as a new industry standard.
The Human Element
While there are many light meters on the market, the best ones are still planted in your head. The human eye is capable of distinguishing about 10 million different colors. When you are looking a new technology, don’t just take the computer’s word for it, actually look at the light that your fixture is producing. Comparative analyses are still the best thing going. That’s right, no matter how good it looks on paper, nothing beats a good shootout
