| Author | Message | ||
     Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 413 Registered: 9-2001 |
Christian - if the detector is linear and the various gain settings are well-enough controlled, then you could correct the detector readings computationally by dividing each reading by the gain setting used for it. Thus, if you increase the gain for a low-energy signal, and you can improve the S/N thereby, that improved S/N will be retained when you divide the signal by the gain to put all the data on a common basis. However, there are some other problems you are likely to run into, and will have to watch out for: 1) Drift in the readings and in the gain settings. 2) At high absorbances, stray light will generally become a limiting factor in how low a signal can be measured accurately. "Stray light" is somewhat of a catchall term here, and includes both physical stray light of several sorts, and electrical effects that show up as stray light. 3) If the S/N improves with gain, then the noise is not detector noise. If the limiting noise was detector noise, both it and the signal would be increased when the gain was changed and your S/N would be constant, therefore your instrument is not detector-noise limited, as is the usual expectation in mid-IR and near-IR work. The limiting noise source must be somewhere after the gain-control stage. 4) If the S/N doesn't improve in accordance with the sqrt(n) expectation, then your noise is not random noise. It may be that this is the same noise that is causeing the effects described in section 3, above. For both reasons, therefore, your efforts might be better spent in trying to locate and correct the noise source. This is not a task easily done or to be taken lightly, as it may well be a factor due to overall instrument design and architecture. \o/ /_\ | ||
| Christian Hansen (chha) Senior Member Username: chha Post Number: 26 Registered: 6-2010 |
Hi! To acquire a NIR transmission spectrum I would normally use air as reference. The instrument I use enables one to change the detector gain at different levels. For an air measurement I have to use the lowest gain setting or else the detector saturates. My sample is a wet ultrafiltration membrane, which absorbs heaviely e.g. 3.5 AU at 1940 nm. The SNR was also poor even with many scans. However if I increased the detector gain while measuring the membrane (not the air) the SNR greatly increased. I understand that then I could not use the absorbance scale as e.g. 2 AU = 1% transmitted light, but are there any other problems in using different gain settings for sample and reference? Best regards Christian |