| The SOX lamp needs to be hot (i.e. be at high temperature) in order to work. The temperature itself does not represent power, but power in the form of heating must be continuously provided in order to compensate for heat lost out of the lamp due to radiation, conduction and convection
Lets say we don't have any other limitations on the efficacy of the discharge itself, and we can make it as efficient as we want to. But if we make it too efficient, we will reach the point where the lamp generates less heat than the heat lost to the environment, so the lamp simply won't work
How can we minimize heat loss to the environment :
- Radiation : The IR coating of SOX lamps does attempt to reflect as much as the radiated heat back into the lamp. I imagine that there is a compromise to be done between reflecting the IR more effectively vs. not blocking too much of the actual light output. Since blocking the actual light has the most direct impact on efficacy, the balance is towards blocking as little light as possible, even if this would mean letting some of the IR out
- Conduction : The conduction of heat from the arctube to the outer environment through lamp components in contact is fairly minimal, in the form of 2 mica or small metallic holders near the arctube ends, and through the connecting wires. I imagine that the heat lost there is minimal as is. The exact shape of the holders can be optimized to make it touch the arctube only in a few small points and not along a complete line or so, but in the end it will hit practical limits of how fragile it can be made before it breaks in normal use
- Convection : The vacuum in the outer envelope effectively eliminates convection
Now that the heat loss is reduced to minimum, we can try to make the discharge as efficient as possible
Discharge is more efficient when the electrode loss is as low as possible. It can be zero in an induction lamp (but induction lamps have other mechanisms of losses), or otherwise it can be minimized by making the discharge as high voltage & low current as possible
With magnetic ballasts, making the discharge voltage more than approx 60% of the ballast Voc would make it hard to maintain a discharge
As long as we suffice with 230 Voc (for 230V line voltage), the ballast can be a simple series choke, which can be made very efficient (over 90% easily achievable with fairly standard size ballast, higher % achievable but with bigger and heavier ballast)
If we want to go higher, the ballast will have to be a transformer type, which makes it significantly less efficient for the same size. As long as we stay with magnetic ballasts, it makes sense to compromise in the lamp design to avoid a transformer
With electronic ballasts the limits are a bit less strict. It is possible to raise the voltage limits a bit before we get significant efficiency impact
The discharge itself is a little more efficient with HF driving. The efficiency of the ballast with the best electronic ballasts is approx. on par with the best magnetic ballasts - The efficiency advantage of HF comes from the discharge. (And some additional claims for efficiency advantage come when average or above-average HF ballasts are compared to the worst magnetic ones)
The discharge may be more efficient with different buffer gases. I am no expert on the matter
Some of the light emitted by the discharge is blocked by the vapors in the arctube in which the discharge takes place. This have been addressed in SLI/H lamps :
- The linear construction means that there are no 2 tubes side by side, which block some light from each other
- There have been arctubes made with a cross section shape that makes the arctube design flat - a shape which resembles an asterisk (Thorn) and a PG FL lamp (GEC)
However, linear lamp means larger outer lamp surface area for the same discharge power density, which may increase the heat loss from radiation, so is a compromise. The surface may be minimized also by increasing the power density (higher power in a shorter lamp) but this then means compromise on other things like fill pressure and electrode losses
With the as high efficiency as possible in the discharge and in the avoidance of light blocking, the heat generation is reduced. Ideally it must be reduced to the point where it just barely overcomes the heat loss from the lamp to the environment
This "Barely" implies that :
- The lamp will not work at reduced power. So the ballast must provide precise power level. In case of magnetic ballast, this will also mean a compromise vs. the required tolerances for line voltage
- The lamp will take ages to warm up, or the ballast must be designed to increase the power during warm up and switch to normal power level when the lamp reached working temperature
Both issues can be overcome with an electronic ballast. The 2nd can also be done with a magnetic ballast with a tap and a control module
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