Spectrophotometer

A spectrophotometer comprising:

• a specimen cell;
• a light source unit for emitting measurement light to said specimen cell;
• a photodetector for detecting the measurement light that has passed through said specimen cell;
• a light-shielding unit for preventing the incidence of the measurement light from said light source unit to the photodetector at predetermined periods;
• a reference cell;
• a switching unit for switchably guiding the measurement light to the reference cell instead of said specimen cell at predetermined periods;
• a memory unit for storing the output strength signals detected by said photodetector in association with a light-blocked duration when light is blocked by said light-shielding unit and switched duration when the optical path is switched by said switching unit;
• and
• a controller for calculating transmittance or absorbance based on output strength signal (S) of the specimen-side incidence duration, output strength signal (DS) of the specimen-side light-blocked duration, output strength signal (R) of the reference-side incidence duration and output strength signal (DR) of the reference-side light-blocked duration that are stored in said memory unit;
• wherein:
• the specimen-side incidence duration, specimen-side light-blocked duration, reference-side incidence duration and reference-side light-blocked duration occur in this sequence in a single period;
• and
• said controller calculates the true specimen-side output strength signal (sN) of the Nth period removed of the effects of output strength signal (RN−1, SN−1, RN−2, SN−2, . . . ) prior to the reference-side incidence duration of the (N−1)th period that are included in the output strength signal (SN) of the specimen-side incidence duration of the Nth period and calculates the true reference-side output strength signal (rN) of the Nth period removed of the effects of output strength signal (SN, RN−1, SN−1, RN−2, . . . ) prior to the specimen-side incidence duration of the Nth period that are included in the output strength signal (RN) of the reference-side incidence duration of the Nth period.