D50 is the standard lightning condition of the printing industry. It has a degree of whiteness of 5,000 Kelvin, which is the color that glowing metal at 5,000 Kelvin would cast.
With the ISO 3664: 2009, the D50 standard lightning condition has undergone a change. It now contains a higher UV spectrum. Since this change had not been clearly communicated to all commercial printers and prepress businesses, even today there are many D50 lights with old neon pipes, causing confusion in matching color. In one printing studio, two D50 light units might be next to each other, that have different UV components and thus, for example, the optical brighteners in modern uncoated papers are visually completely different.