| Author | Message | ||
     Jonas Claeson (Jonasc) |
Hello, I have tried using the ceram material AD96 from Coorstek as calibration reference. When I measure the reflectance from pieces of the material I notice differences in reflectance. Probably I am using material from two different production batches since the reflectance characteristics of the pieces belong to two groups. The differences are: - Some pieces have about 5% higher reflection. - The reflectance curves do not only have an offset, but also different shapes (variations of about +/-2% between 1000nm and 2000nm). - The pieces with higher reflectance appear 'whiter' to the eye. Does anyone know what can be causing this effect? Contamination, different material structure? Are there cerams with less individual variations? Regards, Jonas | ||
| hlmark |
Jonas - as far as I'm aware, Coors makes these ceramics mainly for their own purposes, which largely is filtering beer. I don't know that they pay any attention at all to the optical properties. We in the NIR world seem to have almost "lucked out" in finding a material which itself has high reflectance, essentally no absorbance in the NIR and mininal affinity to other absorbing substances such as moisture. While there are other materials (gold deposited on sandblasted aluminum, for example), as far as I know, the Coors ceramics are the most uniform (both spectrally and batch-to-batch) of any that we know of, and, of course, is available off-the-shelf at reasonable prices. If you succeed in finding something better by all means let the rest of us know. What you might try doing to improve the reflectance properties and uniformity, is to use more than one thickness of the ceramic. Also try putting something highly reflective behind it, such as a mirror. Could you also report the results of these experiments - it would be useful and of interest to the rest of the group, I think. Howard \o/ /_\ | ||
| David W. Hopkins (Dhopkins) |
Jonas, I assume that there are slight differences in the composition of the sand from which they make the ceramic (perhaps Fe2O3 that causes yellow color?). I have had fair luck with Coors catalog numbers 65633 (2-in disks 3mm thick), 65638 (5-in diam x 5mm) and 65631 (1-in x 2mm). There are still differences between pieces, but if you order pieces at the same time and request the same lot, you may get better match. Sorry, I don't know which ceramic these part numbers correspond to. Differences between pieces of Spectralon (perfluorocarbon)from Labsphere or similar product from Avian Technologies give more reproducible scans, in my experience. The material is softer, but using a quartz window is acceptable for many applications. Hope this helps. Best regards, Dave Hope this helps. | ||
| Jonas Claeson (Jonasc) |
I received the following answer from the Ceramic Engineer at CoorsTek: "I would think that surface finish changes, whether from the forming process itself, or from our cleaning process could change the refectance. The actual surface roughness could have an effect. The amount of polishing/burnishing during cleaning can vary, which could cause changes in refectance. Also any surface irregularities such as dents, localized camber might cause the refectance to change. There can also be some variation in material density which would change light transmission/reflectance." So it seems that there could be many explanations for the reflectance variations. Regarding the thickness of the material, I have noticed that when I use only one piece (2.54mm), light penetrates all the way through. Two pieces changes the shape of the reflectance curve, and gives a little higher reflectance. Right now I use 3x2.54mm. I have asked CoorsTek if the penetration depth is less in a material with higher concentration of alumina, (e.g. 99.5% instead of the 96% that I use now). They answer that the less dense the material, the better the reflective properties - so the AD96 should give the highest reflectance. Dave - the materials you mention are 99.8% alumina. Have you compared reflectance with materials with other alumina concentration? Has anybody measured any variations or degradation in reflectance of the CoorsTek ceramic material? //Jonas |