| Author | Message | ||
     prathap (Prathap) |
hi, please help, how to determine straylight in my nir spectrophotometer | ||
| Tony Davies (Td) |
Hello Prathap, Stray light limits the maximum level of absorption that you can measure. This is only a problem in dispersive instruments; FT-NIR instruments do not have a stray light problem. To get an estimate of the straylight in your instrument you need to measure samples of increasing absorption. If you can measure liquid samples in transmission it is a fairly simple experiment. However, I expect you have a reflectance set-up for sold samples. You need to make mixtures of carbon black with sulphur (KCl or NaCl are reasonable alternatives). If you plot the absorption of these samples against the concentration of carbon black, the plot should be linear at the start and then curve away. If you continue you will find an upper absorption limit. Absorption observed = A(obs)=-log[I + Is)/(I0 + Is)] I = transmitted (reflected) energy from the sample. Is = stray light I0 = intensity of energy incident on the sample. If I= 0 then A(obs)=-log[Is/(I0 + Is)] Hope this helps! I expect you will see some different answers! Best wishes, Tony | ||
| David W. Hopkins (Dhopkins) |
Hi Prathap and Tony, Some comments on Tony's good start at answering your very good question. First, it is probably not accurate to say that there is no stray light in a FT spectrometer, but that the data treatment effectively eliminates the effect of stray light on the Absorbance measurement. Would you agree, Tony? Second, some non-FT spectrometers may include a stray light correction in their calculations, so that your measurements as outlined by Tony may not give the true stray light. You may have to dig a bit or inquire of the manufacturer to find out this detail. Third, it is very messy working with carbon black, and you need to be vitally interested in the details of your measurements to do the experiments Tony suggests. Also, I find that NaCl or KCl are not effective diluents in the NIR, because they are non-absorbing. Their effects on the absorption of light by absorbing materials is extremely non-linear. I think sulfur or powdered spectralon (tm) or equivalent might be better. Do you know of a reference reporting the use of KCl or NaCl with Carbon Black in the NIR, Tony? A quick determination that may indicate the level of stray light would be to measure a highly absorbing sample, say 5mm of water or more in transmission, or a very absorbing sample in reflection, and note where the peaks are flattened and appear "saturated". That is the limit Tony refers to, and the percent stray light is given approximately by the conversion from the log function, as the light measured is essentially all stray light. If the peak "saturates" at 2 AU, the stray light is 1% of the NIR at that region. The stray light you thus determine at 1940 or 2130 nm (for example) is also present at other measurement wavelengths. Best wishes, Dave | ||
| Tony Davies (Td) |
Dave and Prathap, I agree with everything you say Dave! No I have no reference to give. I wondered about "No stray light in FT-NIR" when I wrote it. I suppose if you cannot observe it then it is almost as good as it not being there! One question we should ask is "Why do you want to know". Best wishes, Tony | ||
| Tony Davies (Td) |
Dave and Prathap, PS! I had an e-mail exchange with Howard (Mark), which reminded me that I use to use a piece of black anodized aluminium as a "zero" reflectance sample. Not a true zero but it was quite useful for spotting instruments with a stray light problem. Tony | ||
| Edward Stark |
Dave, Tony, and Prathap Several comments: 1) Black anodized aluminum is far from black in the NIR. I have used 3M black conductive foam with some success although it still has a couple of percent reflectance. 2) In any reflectance setup there are many opportunities for reflection of energy that has not entered the sample. This has the same effect as stray light. 3) Doing very careful work with an FT spectrometer I have observed extraneous "stray" light, such the 1.06 micrometer plasma radiation from the HeNe laser and low level harmonics from residual non-linearity in the electronics. Ed Stark |