| Author | Message | ||
     Stephen Robert Holland (Steve) |
"We have a processed based AOTF NIR instrument with a fiber optic reflectance probe which requires external performance verification with certified spectralon photometric standards (to verify absorbance accuracy and repeatability over time). We have fabricated metal holders which allow the standards to be presented in a consistent manner to the probe so that spectra can be acquired. The probe tip itself is in the shape of a dome and the standards are flat. The holders are designed so the probe tip touches the standards. The problem I need to overcome is that, the reproducibility (in the absorbance axis) of spectra when the standards are applied and then re-applied is not currently acceptable. I have found that the method of presentation of the standards is definitely the source of the non-reproducibility through experiment. Has anyone had experience with presenting standards to probes on process instruments that could give an indication of what I can try to remove the non-reproducibility. Alternatively, does anyone have experience of "off the shelf" methods for NIR external calibration/verification using the sample measuring device (e.g probe). | ||
| Tony Davies (Td) |
Dear Stephen, I have been trying to find a cheap, robust and useful "standard" for the last 20 years! I have not heard of people having trouble with Spectralon (except the cost). What does the noise spectra look like? Is it just noise or does it have some character? Are you protected against stray light? Best wishes, Tony Davies | ||
| Stephen Robert Holland (Steve) |
Tony, the problem does not manifest itself as noise, rather a total displacement in the spectra in the y-axis. We think this is due to the poor reproducibility of placing the probe tip in exactly the same position each time. We have heard of others getting decent reproducibility of spectra from flat windowed probe tips - could this be something due to the hemispherical design of the probe tip? Would it be better to have the tip some fixed distance from the standard in the jig, or is "just touching" (as it is now) preferable? | ||
| Art Springsteen |
Dear Steve, Not to horn in on your conversation with Tony but the problem you're seeing, a Y-axis shift, is indicative of a number of potential problems. These may stem from a) the distance of the probe from the standard/sample b) the etendue of the fibre (I'm assuming this is a fiber optic probe) and c) the relative scattering of the standard/sample. The solution to parts a) and b) are fairly simple- avoid any bending of the fiber and set up a jig so that the probe is always the same differece from the standard/sample. The solution to part c) is not so easy- you want a standard or set of standards that all have the same scattering (brdf) characteristics. Tony has mentioned Spectralon (sintered PTFE), of which I am the proud father, But even Spectralon has different scattering characteristics as you change from the white material to the black- the white is almost perfectly lambertian, the black much less so. Not to be long winded, I now almost think a glossy sample may well be more consistent. If you're interested, I'll give my reasoning at a later date. Hope this hasn't all muddied the waters. Best regards, Art Springsteen, Avian Technologies LLC | ||
| Magaly Arocho Serrano (Mas) |
I need information about the NIR process contol for my graduate program. Thanks University of PR mayaguez Campus | ||
| David Hopkins (Hopkins) |
Steve, I don't know whether you are still interested in more responses. Assuming yes, I'd suggest that you never want to let anything touch the surface of the spectralon reference, especially not the round tip of your probe. If the probe presses on the sample, you may be making a continuously changing dimple that would give variable results. If you could back off the distance of the probe to the sample, and make a fixture to provide a fixed distance, you should achieve better results. For starters, you might try the distance that the probe is above your process sample. A longer distance would mean that small fluctuations would mean relatively less variation in the readings. You may want to try a series of distances and choose the optimum for reproducibility. Hope this helps. Regards, Dave | ||
| Art Springsteen |
Steve and Dave, I think I answered this is a previous posting- oh, yeah I did- it's above... Has to do with the scattering characteristics of sintered PTFE (aka Spectralon). Unless you control the distance with tremendous precision, you will get Y-axis shifts. The bigger the displacement, the bigger the shifts. In a way, I disagree with David. I think that you make a 'pit' and stay in it. That way you always are in the same spot. Best regards, Art Springsteen |