sunlight, at the equator at noon is usually said to be about 6,700k. however the kelvin temp can range drastically depending on the time of day and where you are, i've seen an article that said it ranged between 5,000k to ,iirc, 32,000k. if i were looking for a daylight tube i'd choose something between 67k and 12k, probably in the little bit higher range because i like a crisp white light with a slightly blueish hue.