Temporal and environmental sensitivity of a photodiode array spectrophometric system

The effect of the spectral variation in quartz tungsten lamp output with respect to elapsed time from power up and variation in environmental temperature, and the variation in readout in the front end electronics and spectrometer with temperature, on predictive model performance of total soluble solids (TSS) in intact fruit was assessed for a silicon photodiode spectrometer based system. Lamp (10 each of OSRAM HLX64623 and Sylvania 521995 12 V 100 W GY6.35 QTH) output was assessed at 10 s intervals over a 4 h period, and 10 min intervals over approximately 3,000 h. Environmental temperature of each component in an NIRS system (lamp, front end electronics, spectrometer) was incrementally adjusted in 10°C intervals between 10 and 60 °C. Lamp output was spectrally stable within the time of the first measurement (10 s), although total illumination was not stable until approximately 40 min from start-up. Thus performance of the predictive models based on second derivative of absorbance data was not significantly impacted by lamp warm-up time. Noise on measurement associated with use of a single white reference resulted in a mean increase in RMSEP as high as 0.22% TSS and individual increases high as 0.82. Averages of white reference measurements significantly improved performance. When predictive models were developed using second derivative absorbance data and averaged (10) white references, there was no statistically significant impact in RMSEPs on time of lamp warm up (after 10 s), even during the last hours of lamp life. Spectral variation resulting from changes of NIRS system components (lamp & FEE) also affected lamp output quantity rather than quality and thus did not affect the predictive performance due to the second derivative absorbance pre-treatment. Some lamps displayed start-up output characteristics on their first use, which were not repeated in subsequent trials. This result indicates the need for a short lamp ‘burn in’ period.