Efficacy is also known as PPE, or Photosynthetic Photon Efficacy. Efficacy measures the amount of photons emitted by the grow light per input watt. You can calculate the efficacy (µmol/J) by taking the amount of PAR photon output (µmol/s) and dividing that by the input power (W). An higher efficacy means that the grow light is better at converting electrical energy (or watts) into usable photons, which are very important for photosynthesis.
Color Rendering Index
The Color Rendering Index, or CRI, indicates how the color of a grow light changes how realistic plants appear to the human eyes. A CRI value is given on a scale from 0 to 100 percent. The sun is rated at a CRI value of 100, the highest value a light source can get. Some grow lights, such as “blurple” LEDs (LEDs that only use red and blue light, making the light output look purple) have a low CRI value, because the plants look purple. However, Full Spectrum LEDs have a very high CRI, since they mimic sunlight and thus making your plants look very realistic. A high CRI is important, since it will make inspecting your plants and growing environment a lot easier. Imagine checking your plants for diseases in a purple, blue, or red environment versus checking them in broad daylight. CRI values above 90 are considered excellent and scores below 80 poor.
In the specifications of grow lights you may notice a value for Power Factor. The Power Factor indicates the ratio of Real Power (RP) to Apparent Power (AP). Real Power is the useful power needed for the light to work. Apparent Power is Real Power combined with wasted Reactive Power (power is sometimes wasted because of inefficiencies within the electronic circuit board for example). When dividing RP by AP you will get the Power Factor. In other words: Power Factor is the amount of power that is wasted by the bulb expressed in a 0-1 range. A Power Factor of 1 means no power is wasted. A lower Power Factor means there is more current needed.