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 (FEE) 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 12V 100W GY6.35 quartz tungsten halogen) output was assessed at 10 s intervals over a 4h period, and 10 mm intervals over approximately 3000 h. The environmental temperature of each component in a near infrared (NIR) spectroscopy system (lamp, FEE, spectrometer) was incrementally adjusted in 10 degrees C intervals between 10 degrees C and 60 degrees C. The lamp output was spectrally stable within the time of the first measurement (10s), although total illumination was not stable until approximately 40 min from start-up. Thus, the 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 the use of a single white reference resulted in a mean increase in root mean square error of prediction (RMSEP) as high as 0.22% TSS and individual increases as 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 10s), even during the last hours of lamp life. Spectral variation resulting from changes of NIR system components Ramp and FEE) also affected lamp output quantity rather than quality and thus did not affect the predictive performance owing to the second derivative absorbance pretreatment. Some lamps displayed start-up output characteristics on their first use that were not repeated in subsequent trials. This result indicates the need for a short Lamp "burn-in" period.