Transmittance Uncertainty Evaluation for a Broadband Spectrophotometer System

The spectrophotometer is one of the instruments designed and manufactured by the advanced opto-mechanical technologies. To maintain its accuracy and preciseness, the instruments are required to be calibrated frequently. The transmittance measurement uncertainty of a spectrophotometer with considerations of the normal incidence of light beam and operator manipulation factors is performed. The calibration data and regular measured data can be transferred to the servo data center via internet for further data analysis, instrument healthy examination and predicted maintenance period. A double beam spectrophotometer is utilized, taking advantage of the direct comparison of the luminous intensity to evaluate the uncertainty. To measure the transmittance of a sample with a few tenths percent in calibration, the sample is required to be smooth surface, uniform and homogeneous composition, and environmental stability. The measured transmitted beam should be focused inside and no light will be scattered outside of the collecting integrating sphere. A series of transparent thin films, including absorb Ni film, high index TiO2 film and low index SiO2 film alternatively deposited carefully on the fused quartz substrates, are the calibration samples. Equations are given to calculate the transmittance for beam being normally incident on the substrate. Factors such as material aging and non-uniform optical thickness can introduce systematic errors into the calibration, and the errors must be minimized by proper choice of materials and preparation techniques. The environmental range of the temperature and the relative humidity during the measuring process are controlled as 23 +/- 1.5 oC and 50%+/- 10%, respectively. The calibrated transmittance ranges are within 10% - 100%. The optimal expanded combination uncertainties of the system are found to be 0.23247 (T1-1: above T30 %), 0.13265 (T1-2: T30%similar to 8%), 0.04728 (T1-3: T8%similar to 0.92%) and 0.02910 (T1-4: T0.92%similar to 0.107%) in the wavelength ranges of 400 nm - 700 nm.