| Author | Message | ||
     Tony Davies (td) Moderator Username: td Post Number: 146 Registered: 1-2001 |
Hi Hendra, Unscrambler 5.5 is quite old! ( I threw out my old manuals only last month!) In 7.6 in "Modify" there is a sub-topic of "Spectroscopic" and (if you have it in 5.5) you can convert to reflectance. If it doesn't have it then you need to get the spectra into Excel and calculate it there. By the way, (Plea to all new users) this should have been posted as a new question because then it will be found more easily in the future. Best wishes Tony | ||
| Hendra Herdian (hendravit) New member Username: hendravit Post Number: 1 Registered: 2-2007 |
Hi, all Im newly handling an Infratec 1255, how can i interpret it transmitance result on some material to log 1/T curve absorbance with wavelength, since the result only send an absorbance and constituent variable values (im using unscramble ver 5.5 as chemometric software), thanks | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 56 Registered: 9-2001 |
Gabi - that seems pretty consistent with what I remembered, that the DTGS covers the mid-IR - if you replace "mm" with "um" (microns)! When I was using the DTGS detectors, I wasn't too interested in their out-of-band performance, and once I started working in NIR at Technicon, PbS was the "cat's meow" for detectors, so I lost interest in DTGS; anyway our optical engineer knew all about them both. I never had occasion to put the two together, to realize that the DTGS would work at wavelengths as short as 1.7 um, fairly well into the NIR. I suspect that it would have worked as well, though, as the PbS works at 600 nm - which, basically, is not all that well! Thanks for the info, though. It's always nice to keep updated on old technology friends. \o/ /_\ | ||
| Gavriel Levin (levin) Advanced Member Username: levin Post Number: 21 Registered: 1-2006 |
Hi Howard, here is an excerpt from a web site on these detectors: DTGS Detectors are sensitive from 6,000 to 350 cm-1 (1.7 to 28 mm), the usable range of the KBr beam splitter. This detector exhibits large, spontaneous electrical polarization effects. Incident radiation alters the polarization which generates the electrical signal. We optimize this detector for our 80007 SiC Source; fine gain adjustments can be made via an internal potentiometer. DTGS Detectors are sensitive from 6,000 to 350 cm-1 (1.7 to 28 mm), the usable range of the KBr beam splitter. This detector exhibits large, spontaneous electrical polarization effects. Incident radiation alters the polarization which generates the electrical signal. We optimize this detector for our 80007 SiC Source; fine gain adjustments can be made via an internal potentiometer. Thus, the detector puts itself out of a very important fraction of the NIR region. The important aspect is that the pyroelectric effect is an effect that needs to be "erased" before new reading can be taken, and since it has a fairly flat and very wide spectral response, it is very useful in the lab, but not so suitable for process. In fact, from my days in thermal imaging I recall that pyroelectric cameras were limited by the need to "erase" the picture before a new one could be taken. The current trend in low cost, medium performance, uncooled thermal imaging is the silicon wafer based bolometric detector arrays. For NIR spectrometry it is probably of lesser interest, although one needs to examine the pros and cons on the basis of his specific goals. | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 55 Registered: 9-2001 |
Gabi - we're starting to get into the engineering aspects of using detectors, which most of us won't have to deal with directly unless we're going to be designing our own instruments. But DTGS detectors are basically thermal detectors, i.e., they respond to the temperature changes of the detector, and therefore are speed-limited by how fast the detector element can warm up (and especially, cool down). My usage of them probably goes back ever farther than yours, They were used in the early FTIR's that I used (Digilab FTS-14's). At that time, the highest scan speed was limited by the speed response of the detector, which was 5 kHz. I don't know if the detector manufacturers have been able to speed that up appreciably. Their big advantage was that they could operate at room temperature. Anyone who needed a faster detector had to go to some of the really exotic ones: Au-doped germanium, or Pt-doped germanium, or the like, that had nanosecond response speeds. Those detectors were very nice: besides the high speed, they had extremely low noise levels, high sensitivity and wide wavelength range. The problem with them was that, besides their cost, they needed to be cooled by liquid helium, putting them out of reach of any sort of "routine" usage. As far as I know, only the military used them. The big breakthrough in mid-IR detectors was the development of CdHgTe detectors, which were also fast and sensitive and covered the mid-IR range, and "only" had to be cooled with liquid N2 - that brought them into the realm of laboratory usage. \o/ /_\ | ||
| Gavriel Levin (levin) Intermediate Member Username: levin Post Number: 20 Registered: 1-2006 |
One more word - regarding the detctors question - one should also consider the response time - which comes of great importance when you work in the mode where you have a single detctor and you scan the wavelength range - in this mode the detector is exposed to different wavelengths the scanning progresses. The speed at which you can scan depends on the speed at which the detector responds to cahnges in the intensity of light shined on it. The InGaAs detectors are known for very high speed of response - allowing very fast scans. The speed is usually reported in frequency parmeter 10MHz - 100 MHz, etc. Sometime people report the PV (photovolatic) type detctors in values of capacitance - so many picofards etc. The capcitance affects the speed through the RC value. The speed of detectors also relates to their size - same InGaAs will have higher speed at 0.5mm diameter, than at 1mm and then at 3mm. This is because the larger area also means higher capcitance. DTGS is not my cup of tea, but from my older days in thermal imaging I remember that the cameras based on this technology required "resetting" before new picture could be taken, does any body have data on the speed of response of these detctors? Thanks, Gabi Levin Brimrose | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 54 Registered: 9-2001 |
Aeronton - Ideally, both the sensitivity of the detector and the throughput of the optical system should be maximized (within budgetary constraints, of course). If you run into problems with detector saturation, as Jim mentions, it's much easier to back off from having too much signal than it is to improve the signal level if your S/N value is not satisfactory. Of course, this whole discussion is ignoring a very critical parameter of the measurement: the wavelength range. Normally you would not be choosing between DTGS and Si, for example, because they are not commonly used for the same wavelength ranges. Si responds only to wavelengths shorter than 1100 nm in the NIR region, while DTGS is best used in the mid-IR region, at wavelengths longer than, say, 3000 nm. While it's true that there is some overlap and detectors can be used outside their specified ranges (I've used PbS dectectors at wavelengths as low as 600 nm, for example, even though they're not recommended for use below 1100 nm) your best advice is to choose the best detector for the wavelength range you want to cover. Jim summed it up very well: choice of a detector should be matched to the application. | ||
| James Malone (jmalone) New member Username: jmalone Post Number: 3 Registered: 8-2006 |
Aerenton: Different types of detectors respond to changes in light levels in different ways. Detectors such as DTGS, MCT, InSb, and others are very good at detecting small changes in very large numbers of photons. In short, they require high light levels to function properly, hence they require a high throughput of the optical system. They are often used in instruments such as FT-IR or FT-NIR instruments which provide the high throughput needed. Other types of detectors, CCDs, InGaAs, and various arrays only respond well to small numbers of photons. Too many photons saturates the detector and then it ceases to respond to changes in number of photons. These detectors are best used in instruments with low throughput, such as dispersive instruments. Many of the newer low cost Raman systems use this type of detector because of the very low light levels that are emitted by the sample. Neither type of detector is inherently "better" or "worse" than the other, each needs to be matched with an optical system that provides the conditions under which they can respond properly. Hope this helps. Jim Malone | ||
| Aerenton Ferreira Bueno (aerenton) New member Username: aerenton Post Number: 2 Registered: 4-2006 |
Howard Thank you for the explanation. I think that the figure (where I saw it) means that InGaAs may be applied in low throughput systems and DTGS in high throughput optical systems, because of their relative sensivities. | ||
| Howard Mark (hlmark) Senior Member Username: hlmark Post Number: 53 Registered: 9-2001 |
Aerenton - I don't really know what that means, either. It might help if we knew where you saw it, or could at least send an extract containing the text. Detectors generally are not characterized by "throughput"; throughput is normally a characteristic of an entire optical system, not any single device in it. Detectors are characterized by their sensitivity, meaning the electrical output resulting from a specified amount of input optical energy. Other important characteristics are the noise level, wavelength range, and other characteristics. In normal usage, detectors are characterized by a quantity called d* (read "d-star"), this folds in the sensitivity, noise level and a couple of other optical parameters that lets an optical engineer calculate the maximum signal-to-noise value that detector is capable of, under the specified operating conditions. Howard \o/ /_\ | ||
| Aerenton Ferreira Bueno (aerenton) New member Username: aerenton Post Number: 1 Registered: 4-2006 |
I read somewhere that InGaAs and Si (photo diode) detectors have low throughput and DTGS has high throughput. I wonder what that means. Can somebody explain me? Thanks. |