PURPOSE: To enable the use both for measuring general samples and light source characteristic with a simple switch operation by retaining a light diffuser on a holder the same in the construction as a light source holder to form an exchange unit for measuring a spectroscopic radiation characteristic of any light source so as to be exchangeable for the light source holder unit.

CONSTITUTION: A light diffuser holder unit is mounted in place of a light holder unit in such a manner that the emission port Eo of an integrating sphere I is positioned as the luminous section of a tungsten lamp W when the light source holder unit is place there. Therefore, an emitted light emitted from the integrating sphere I enters a spectroscope when the mirror M is at the position as illustrated. A light source Q to be measured is set above the integrating sphere I and light of the light source to be measured is focused at the incident port Ei at the apex to enter the integrating sphere I. Under such a condition, as the mirror M is retreated in the direction of the arrow, light of a heavy hydrogen lamp D2 enters the spectroscope. Thus, switching is done over to the ultraviolet range measuring mode for the general sample whether or not the light diffuser holder unit is mounted.