CRI (color reproduction index) is the measurement of how accurately a light source can reproduce colors. My question is: what gives a light a good CRI? What do manufacturers do to accomplish such thing?
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Every element and molecule interacts with all the different wavelengths of energy and light differently but consistently. Manufactures can look at known data from previous experiments to see what elements and molecules have which interactive properties with that frequency of light.
The different colors of light or soft tones vs more natural tones are all produced by the light bulb and different filaments or gasses in the light will create certain frequencies of light and not all of them. Manufactures can put certain elements around the light being produced to make only certain wavelengths come out allowing them to further control the type of light emitting into the room. Being able to make different colored light from the bulb so you can mix that light with the already natural light in the room in a way that makes the space look “best” improves the reproduction score. You are trying to produce a light that mixes with the light already their to reproduce the index of lightwaves you need. Different types of light bulbs and bulb covers give them some choices to mix with the natural/yellow whatever light already present in the room.
A good light source has a smooth continuous spectrum similar to a material that is heated to a high temperature like the sun. The reflectivity of surfaces is also distributed along the spectrum of light, with sharper peaks corresponding to more saturated, festive colors.
Our attempts at creating efficient light emitters through excitiation of gases like mercury vapor or with light diodes produce radiation in a narrow band of frequencies. For example, sodium lights along highways have a sharp orange color, which is determined by the configuration of electrons in the chemical. If an object’s color falls in the gaps between these narrow emission bands, it will appear darker. Under a sodium light blue objects appear black. LEDs are slightly better than gases, giving off a broader spectrum.
To improve the light quality, parts of a light bulb are coated with materials that get excited when hit by incoming photons and re-emit light of a lower wavelength. Good color resolving ability is obtained by mixing multiple phosphor materials in precise proportions.
White light is a combination of all the different colors of light mixed together. But you can also get a “whitish” light by only mixing a few colors together.
Old incandescent light bulbs put out a full spectrum of light and so tend to have a pretty good CRI. But newer types of bulbs such as fluorescent or white LED bulbs generally put out only a few colors mixed together, creating a “whitish” light. This is because these bulbs create light by exciting fluorescent chemical compounds, each of which emits different frequencies. Depending on what mix of chemicals you use, you get different types of whitish light. By carefully mixing lots of different fluorescent chemicals, you can achieve something fairly close to a true white.
CRI is a measurement of how close to a true white light a bulb puts out. When a bulb has a good CRI, it is emitting a fairly well distributed amount of light across the visible spectrum. With a poor CRI, there are only a few colors mixed together. Because not all the colors in the spectrum are represented, shining the poor CRI light on objects may make their colors look weird.
CRI is important for things like photography and filming, where you use artificial light to make the scene brighter, but want natural colors so your subject doesn’t look weird in the photo or film. Flashlight nerds also obsess over CRI, but it isn’t really all that critical; wandering around at night doesn’t require you to know the exact shade of a green leaf or brown log your flashlight is shining on.
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