217c thought that was the temp you expose your room to get rid of spider mites. A few more definitions: The color rendering index (CRI), sometimes called color rendition index, is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as photography and cinematography, and even horticulture. It is defined by the International Commission on Illumination as follows: Color rendering: Effect of an illuminate on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminate Numerically, the highest possible CRI is 100, which is equivalent to natural sunlight. The higher the CRI, the better quality of light. The Kelvin Color Temperature Scale is a measurement of discriminating colors for the human eye. Color temperatures over 5,000K are called cool colors (blue/white), while lower color temperatures (2,700–3,000 K) are called warm colors (yellow/white through red). This is a measurement for the human eye, but serves as a reference for helping consumers know which lamp to buy for which stage of growth.