What is full spectrum light? Full spectrum is not directly visible or observable!
Contributing to the confusion among consumers is the fact that the "fullness" of a light spectrum is not directly observable to the human eye. In other words, a non-full spectrum bulb and natural daylight could have the exact same emitted light color and appearance, despite having significantly different spectral properties. 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. In other words, as a consumer, you have no practical way to independently verify or confirm that a full spectrum bulb you purchased actually has a complete spectrum. 

How can that be?
Spectral speaking, there are many ways to create the same light color, and this also holds true for the color of natural daylight (commonly called daylight white). For starters, 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.

What is very important to remember is that both natural daylight and this fluorescent lamp have the same apparent light color - daylight white. In other words, despite a significant spectral difference, the light color emitted from the fluorescent lamp is indistinguishable from daylight, to our eyes.