There are no cut and dry definitions that draw the line between "full spectrum" and "not full spectrum." This makes evaluating and comparing full spectrum bulbs challenging for consumers, and has led to numerous manufacturers and brands making dubious marketing claims that their bulbs are "full spectrum" despite serious deficiencies in the spectral content of their lamps.
What is full spectrum light? Full spectrum is not directly visible or observable
Full spectrum typically refers to the completeness of a light source's spectral energy, particularly when compared to natural light sources such as natural daylight. The exact spectral composition of a light source can only be determined by specialized photometric equipment, such as a spectrometer.
Let's take a look at the light spectrum for natural daylight. You will notice that the light energy is distributed evenly across the entire visible spectrum, without any gaps, dips, or spikes.
Next, we take a look at the light spectrum for a daylight fluorescent lamp. Notice that despite its daylight color rating and emitted light color, the spectrum is very different from natural daylight. Specifically, there are numerous spikes and "valleys" as well as very little light energy emitted in the red wavelengths.
However, despite a significant spectral difference, the light color emitted from the fluorescent lamp is indistinguishable from daylight, to our eyes.
Why we need to go for full spectrum light?
1) Improved color rendition
Color rendition is concerned with the way objects' colors appear under the light source.
As such, full spectrum light sources are indispensable for applications requiring accurate or consistent color appearance. For example, workers in artwork, photography and graphic arts all require full spectrum light sources so that inaccuracies in color perceptions do not hinder their work.
2) Improved health or biological benefits
The health benefits of full spectrum lighting are not directly related to the way we see light or color. Instead, it relates to other biological processes, such as the way pigments and hormones such as melanopsin in the human body react to various wavelengths and intensities of light. These processes are not directly related to the vision system, but instead provide signals to our bodies to promote alertness, sleepiness and regulate our overall moods.
These processes are not limited to humans. Plants, who also rely on light energy, will also react to different light spectra differently.
How do we measure "fullness" of spectrum?
Color temperature and color rendering are two critical metrics that help determine a light source's similarity to natural daylight.
First, color temperature tells us about the emitted light color of a light source, and is represented by "temperature" value that describes the relative balance between yellow and blue. The higher the temperature, the bluer a light source is, and the lower the temperature, the yellower the light source.
Natural daylight, on the other hand, has a color temperature rating of 6500K. Therefore, in order to match the light color of natural daylight, a full spectrum light bulb should have a color temperature of 6500K as well.
Second, color rendering index (CRI) tells us about the way colors appear under a light source, and more generally, about the quality of its spectrum. CRI is measured as a score with 100 being the maximum possible score. Natural daylight itself has a CRI value of 100. A light source with a low CRI value will generally display (or "render") colors inaccurately, when compared to natural daylight.
Source: Waveform lighting