Why is it marked 6500 K on fluorescent lamps (tube light)?

The value 6500 K marked on fluorescent lamps represents a parameter called correlated colour temperature. It means that the spectral (light colour) distribution from that lamp is similar to that of a black body at that temperature. Any black body when heated gives different copious at different temperatures—at 2000 K it emits red light, at 4000 K it is yellowish white. At 2700 K it gives warm light and at 6500 K it gives the impression of cool day light, according to illuminination engineers.

  Based on colour appearance fluorescent tubes come under three classifications, viz, daylight white (above 5000 K), neutral white (4000 K) and warm white (below 3300 k).

  The colour of light (and colour rendering index) depends on the fluorescent coating inside the tube. Three types of coatings are generally given – tri-phosphor, standard phosphor and multi-phosphor.

 Standard phosphor is used in ordinary tube lights. Tri-phosphor coated lamps, emit yellow light similar to sunlight. Standard tube lights render colour similar to daylight.