The ISO standard, as explained in this document produced by X-Rite, a company that produces hardware and software used for color calibration, is to view prints in light that is at D50 (full spectrum centered at 5,000K) in terms of color temperature. In terms of intensity, around 2,000 lux (roughly equivalent to an overcast day) should be used for color decisions and judging (2000lux +/-500lux is within the standard, but +/- 250 is preferred), but intensity levels as low as around 800 lux may need to be used to observe fine differences in tonal qualities. The ISO standard also recommends at least 500 lux in a display setting, such as an art gallery. The light should be even, with the darkest areas at least 60% of the nominal value. The print should be surrounded by matting that is neutral in color and between 10-60% in terms of luminous reflectivity (i.e. Munsell N8/gray). And the light source, image, and viewer should be arranged so that there is no glare reflected from the surface of the photo to the viewer.

Barring access to a photo viewing booth specifically designed to meet the standard, a location that uses diffused daylight of sufficient intensity to provide similar lighting conditions is recommended, and don't forget to view the print against a neutral background.