| Author | Message | ||
     Michel Coene (Michel) |
Purely academic: Osram is making a lot of commercials for lamps with ceramic elements instead of Wolfram. They claim it is best suited for photography because the colours remain stable throughout the lifetime of the lamp. Would this be usefull as a NIR source? Also, what is the best power supply for a NIR source: constant voltage, constant current, or constant power (Watts)? Is there an lamp-age-independent relationship between I,U, and filament temperature (considering filament gets thinner with age)? | ||
| Howard Mark (Hlmark) |
Michel - well, you've asked a lot of complicated questions - or maybe simple questions with complicated answers! I'll try to keep my answers manageable. Ceramic vs tungsten: Ceramic sources have been used for a long time in the mid-IR, because the envelope needed around a tungsten filament (or any metallic filament, needed to keep it from oxidizing) absorbed all the mid-IR radiation. In the near-IR we can use quartz for the envelope since quartz is transparent to NIR radiation, and allows us to use tungsten, which generally runs at a higher temperature than the (bare) ceramic sources do. Filament temperature is key: the higher the temperature, the greater the output (due tot he blackbody curve) of any given thermal source. One of the limitations of tungsten is that it has a low emissivity, and it is possible that a cersmic source might have equal or greater emissivity that would make up for running at a lower temperature. You would have to either measure the emissivity, or do a completely empirical comparison of a given ceramic source compared to a tungsten source, each running at its own optimum conditions, to know which is better for NIR usage. Stability - It does seem likely that a ceramic source might be more stable. On the other hand, they haven't been in use for very long, and maybe haven't been studied as much as tungsten sources have been. I've used ceramic in mid-IR spectrometers, but a long time ago; they tended to fail suddenly through cracking, similar to a filament "burning out", but I never had occasion to measure the output over long time periods to determine the stability, we simply used double-beaming (or Sample/Reference) to compensate, as we do today. There are known mechanisms for changes to tungsten (see below), and I don't know what mechanisms might affect ceramic. Power supply - constant current is generally preferred, for reasons of simplicity. If you use a constant voltage supply (or constant power) you have a problem knowing what the actual voltage supplied at the lamp is, the reason/problem is the lamp contacts. With time, heat and oxidation, contact resistance can develop at the contacts, which drop some of the supply voltage and therefore leave less for the lamp itself. The effect is small but measureable, and affects the lamp output measurably. What's worse is that it can be erratic, and therefore defeat compensation vis double-beaming or sample/reference measurements. There are ways to keep the actual lamp voltage (or power) constant, but those involve what are called "four-point measurements" and involve complications at the lamp socket as well as the circuitry. Constant-current supplies bypass all the complications by automatically supplying enough voltage to overcome the contact resistance as well as the lamp requirements. What is the relationship? - depends on whether you use constant current, voltage or power. For constant current we can expect the temeprature to go up as the filament thins (but see below), since the total power = I^2 R, which will increase with R. For constant voltage we can expect the temperature to go down, since power = V^2/R, so as R increases the power will decrease. For constant power it could go either way. A hidden fallacy in your question is that it implicitely assumes that filaments thin evenly as they age, and that is not the case. If a section develops a thinning, then that section runs hotter than the rest of the filament, and therefore tends to thin out even faster. Therefore filaments develop "hot spots", until one of the hot spots breaks through. So any attempt to describe it analytically becomes very difficult. Howard |